Charge correlations and magnetoelastic coupling in intercalated transition metal dichalcogenides
Kar A., Basak R., Li X., Korshunov A., Subires D., Phillips J., Lim C., Zhou F., Song L., Wang W., Lau Y.-C., Garbarino G., Gargiani P., Plueckthun C., Francoual S., Jana A., Vobornik I., Valla T., Rajapitamahuni A., Analytis J.G., Birgeneau R.J., Vescovo E., Bosak A., Dai J., Tallarida M., Frano A., Pardo V., Wu S., Blanco-Canosa S.
Article, Physical Review Materials, 2026, DOI Link
View abstract ⏷
The large van der Waals gap in transition metal dichalcogenides (TMDs) offers an avenue to tune the ground state of 2D materials through the intercalation of magnetic atoms. Here, we investigate the charge correlations in Fe1/3TaS2, Co1/3TaS2, and Fe0.35NbS2 by combining angle-resolved photoemission spectroscopy (ARPES), x-ray scattering, magnetometry, and density functional theory (DFT). We find that, while short-range charge fluctuations develop in Ta-based compounds, Fe0.35NbS2 exhibits long-range charge order which is strongly coupled with magnetic order and tunable by external magnetic field. Our electronic structure analysis reveals that intercalation reconstructs the Fermi surface via charge transfer and band renormalization, yet does not generate the nesting conditions compatible with the observed ordering vectors. Complementary phonon calculations further exclude a conventional electron-phonon origin of charge order. Together, these results establish magnetoelastic coupling as the dominant mechanism behind charge ordering in Fe0.35NbS2 and highlight the contrasting role of Nb and Ta hosts in stabilizing correlated ground states in intercalated TMDs.
Electronic coherence evolution at the nearly commensurate-incommensurate CDW boundary of 1T-TaS2
Yilmaz T., Ng Y.S., Jain M., Tong X., Wongpinij T., Photongkam P., Rajapitamahuni A., Kundu A.K., Zheng J.-C., Vescovo E.
Article, Physical Review Materials, 2026, DOI Link
View abstract ⏷
Transition-metal dichalcogenides host a variety of charge-density-wave phases that couple lattice, charge, and correlation effects. In 1T-TaS2, the commensurate and nearly commensurate states are well characterized, yet the transition near 350 K into the incommensurate phase has lacked direct momentum-resolved insight. Here, we use temperature-dependent angle-resolved photoemission spectroscopy to track the electronic structure across this transition. We observe a suppression of quasiparticle spectral weight at the Brillouin-zone center, coincident with the transport anomaly, but without clear evidence of a full band-gap opening. The transition appears to involve momentum-dependent redistribution of spectral weight, consistent with a loss of coherence that reshapes the Fermi surface while leaving conduction dispersions largely intact. These results suggest that the nearly commensurate–incommensurate transition may not align with a conventional metal-insulator transition picture, but rather as an electronic reconstruction driven by loss of coherence. Our work provides new microscopic insight into the resistivity anomaly near room temperature and may guide design principles for collective electronic switching in transition-metal dichalcogenides.
Intrinsic vs. extrinsic magnetic transitions in Sr3Ru2O7 films
Choudhary R., Rajapitamahuni A., Guo S., Jiang Q., Chu J.-H., Mkhoyan K.A., Jalan B.
Article, Journal of Materials Research, 2025, DOI Link
View abstract ⏷
In scientific research, both positive and negative results play crucial role in advancing the field. Negative results provide valuable insights that can guide future experiments and prevent repeated failures. Here we present our growth attempts of Sr3Ru2O7 thin films using the hybrid molecular beam epitaxy. X-ray diffraction suggests nominally phase-pure films. A combination of magnetoresistance and magnetization measurements exhibits an onset of ferromagnetism at 170 K and 100 K along with a metamagnetic-like transition at 40 K. These results could initially be interpreted as intrinsic behavior of strain-engineered Sr3Ru2O7 films. However, detailed microstructural analysis reveals intergrowths of Sr2RuO4, Sr4Ru3O10, and SrRuO3 phases, dispersed throughout the film. Our findings suggest that the Sr3Ru2O7 films are likely paramagnetic, with the observed ferromagnetism arising from the Sr4Ru3O10 and SrRuO3 phases. Angle-dependent magnetoresistance measurements indicate that the transition below 40 K is consistent with the metamagnetic behavior reported in bulk single crystals of Sr3Ru2O7, but with no accompanying in-plane anisotropy, suggesting the absence of electronic nematicity. Our results highlight the need for detailed microstructural analysis when interpreting strain-induced emergent properties in material systems susceptible to intergrowth.
Topological excitonic insulator with tunable momentum order
Hossain M.S., Cheng Z.-J., Jiang Y.-X., Cochran T.A., Zhang S.-B., Wu H., Liu X., Zheng X., Cheng G., Kim B., Zhang Q., Litskevich M., Zhang J., Liu J., Yin J.-X., Yang X.P., Denlinger J.D., Tallarida M., Dai J., Vescovo E., Rajapitamahuni A., Yao N., Keselman A., Peng Y., Yao Y., Wang Z., Balicas L., Neupert T., Hasan M.Z.
Article, Nature Physics, 2025, DOI Link
View abstract ⏷
Correlated topological materials often maintain a delicate balance among physical symmetries. Many topological orders are symmetry protected, whereas most correlated phenomena arise from spontaneous symmetry breaking. Cases where symmetry breaking induces a non-trivial topological phase are rare. Here we demonstrate the presence of two such phases in Ta2Pd3Te5, where Coulomb interactions form excitons that condense below 100 K, one with zero and the other with finite momentum. We observed a full spectral bulk gap, which stems from exciton condensation. This topological excitonic insulator state spontaneously breaks mirror symmetries but involves a weak structural coupling. Scanning tunnelling microscopy shows gapless boundary modes in the bulk insulating phase. Their magnetic field response, together with theoretical modelling, indicates a topological origin. These observations establish Ta2Pd3Te5 as a topological excitonic insulator in a three-dimensional crystal. Thus, our results manifest a unique sequence of topological exciton condensations in a bulk crystal, offering exciting opportunities to study critical behaviour and excitations.
Frustrated electron hopping from the orbital configuration in a two-dimensional lattice
Devarakonda A., Koay C.S., Chica D.G., Thinel M., Kundu A.K., Lin Z., Georgescu A.B., Rossi S., Han S.Y., Ziebel M.E., Holbrook M.A., Rajapitamahuni A., Vescovo E., Watanabe K., Taniguchi T., Delor M., Zhu X., Pasupathy A.N., Queiroz R., Dean C.R., Roy X.
Article, Nature Physics, 2025, DOI Link
View abstract ⏷
Electron hopping on spatially periodic lattices gives rise to intriguing electronic behaviour. For example, hopping on the geometrically frustrated two-dimensional kagome, dice and Lieb lattices yields electronic band structures with both massless Dirac-like and perfectly dispersion-less, flat bands. As materials featuring the dice and Lieb lattice structures are scarce, an alternative approach proposes to leverage atomic orbitals to realize the characteristic electron hopping of geometrically frustrated lattices. This strategy promises to expand the list of candidate materials with frustrated electron hopping, but is yet to be shown in experiments. Here we demonstrate frustrated hopping in the van der Waals intermetallic Pd5AlI2, emerging from the arrangement of atomic orbitals in a primitive square lattice. Using angle-resolved photoemission spectroscopy and quantum oscillation measurements, we reveal that the band structure of Pd5AlI2 includes linear Dirac-like bands intersected at their crossing point by a locally flat band—an essential characteristic of frustrated hopping in Lieb and dice lattices. Moreover, this compound shows exceptional chemical stability, with its unusual bulk band structure and metallicity persisting in ambient conditions down to the monolayer limit. Hence, our results showcase a way to realize electronic structures characteristic of geometrically frustrated lattices in non-frustrated systems.
Intercalation induced quasi-freestanding layer in TiSe2
Yilmaz T., Ng Y.S., Rajapitamahuni A., Kundu A., Wang H.-Q., Zheng J.-C., Vescovo E.
Review, Physical Review B, 2025, DOI Link
View abstract ⏷
Angle-resolved photoemission spectroscopy is employed to study the electronic structure of bulk TiSe2 before and after doping with potassium impurities. A splitting in the conduction band into two branches is observed after room-temperature deposition. The splitting energy increases to approximately 130 meV when the sample is cooled to 40 K. One branch exhibits a nondispersive two-dimensional feature, while the other shows the characteristics of three-dimensional bulk band dispersion. Core-level spectroscopy suggests that the K impurities predominantly occupy the intercalated sites within the van der Waals gap. The results indicate the formation of a quasi-freestanding TiSe2 layer. Additionally, doping completely suppresses the periodic lattice distortion in the surface region. These findings are further supported by density functional theory calculations, which compare the band structure of monolayer and bulk TiSe2 with experimental data. Thus, the dimensional and intrinsic electronic properties of 1T -TiSe2 can be controlled through the intercalation procedure used in this work.
Single-crystalline orthorhombic GdAlGe as a rare-earth magnetic Dirac nodal-line metal
Laha A., Yao J., Kundu A.K., Aryal N., Rajapitamahuni A., Vescovo E., Camino F., Kisslinger K., Zhang L., Nykypanchuk D., Sears J., Tranquada J.M., Yin W., Li Q.
Article, Physical Review B, 2025, DOI Link
View abstract ⏷
Crystal engineering is a method for discovering new quantum materials and phases, which may be achieved using external pressure or strain. Chemical pressure is unique in that it generates internal pressure perpetually to the lattice. As an example, GdAlSi from the rare-earth (R) RAlX (X = Si or Ge) family of Weyl semimetals is considered. Replacing Si with the larger isovalent element Ge creates sufficiently large chemical pressure to induce a structural transition from the tetragonal structure of GdAlSi, compatible with a Weyl semimetallic state, to an orthorhombic phase in GdAlGe, resulting in an inversion-symmetry-protected nodal-line metal. We find that GdAlGe hosts an antiferromagnetic ground state with two successive orderings, at TN1 = 35 K and TN2 = 30 K. In-plane isothermal magnetization shows a magnetic field induced metamagnetic transition at 6.2 T for 2 K. Furthermore, electron-hole compensation gives rise to a large magnetoresistance of ~100% at 2 K and 14 T. Angle-resolved photoemission spectroscopy measurements and density functional theory calculations reveal a Dirac-like linear band dispersion over an exceptionally large energy range of ~1.5 eV with a high Fermi velocity of ~106 m/s, a rare feature not observed in any magnetic topological materials.
Overcoming the challenges of accessing topological hallmarks in Sb(112)
Asland A.C., Bakkelund J., Cooil S.P., Rost H.I., Hu J., Vescovo E., Rajapitamahuni A., Mazzola F., Wells J.W.
Article, Electronic Structure, 2025, DOI Link
View abstract ⏷
Sb is topologically non-trivial and semi-metallic, but differs from many topological semi-metals because of its continuous band gap. By measuring its (112) surface using angle- and spin-resolved photoemission spectroscopy, Sb(112) was shown to have 1D spin-polarised surface states resembling those on vicinal Bi surfaces and many topological insulators and topological semi-metals. The shape and spin-polarisation of the measured features and the calculated bands agreed. However, the measured features had a slightly steeper energy dispersion and different Fermi-momenta than the calculated bands. Both theoretical and experimental methods were necessary when determining the topology of Sb(112). The presence of projected bulk states near the Fermi-level and varying surface localisation of the electronic states meant it was challenging to deduce the topology of Sb(112) from the number of bands crossing the Fermi-level or a continuous contour in the bulk band gap. Ultimately, the calculations and measurements suggest that there are topological surface states on the Sb(112) surface.
Hund’s flat band in a frustrated spinel oxide
Oh D., Hampel A., Wakefield J.P., Moen P.C., Smit S., Luo X., Zonno M., Gorovikov S., Leandersson M., Polley C., Kundu A.K., Rajapitamahuni A., Vescovo E., Jozwiak C., Bostwick A., Rotenberg E., Isobe M., Verma M., Crispino M., Grundner M., Kugler F.B., Parcollet O., Schollwock U., Takagi H., Damascelli A., Sangiovanni G., Checkelsky J.G., Georges A., Comin R.
Article, Proceedings of the National Academy of Sciences of the United States of America, 2025, DOI Link
View abstract ⏷
Electronic flat bands associated with quenched kinetic energy and heavy electron mass have attracted great interest for promoting strong electronic correlations and emergent phenomena such as high-temperature charge fractionalization and superconductivity. Intense experimental and theoretical research has been devoted to establishing the rich nontrivial metallic and heavy fermion phases intertwined with such localized electronic states. Here, we investigate the transition metal oxide spinel LiV2O4, an enigmatic heavy fermion compound lacking localized f orbital states. We use angle-resolved photoemission spectroscopy and dynamical mean-field theory to reveal a kind of correlation-induced flat band with suppressed interatomic electron hopping arising from intra-atomic Hund’s coupling. The appearance of heavy quasiparticles is ascribed to a proximate orbital-selective Mott state characterized by fluctuating local moments as evidenced by complementary magnetotransport measurements. The spectroscopic fingerprints of long-lived quasiparticles and their disappearance with increasing temperature further support the emergence of a high-temperature “bad” metal state observed in transport data. This work resolves a long-standing puzzle on the origin of heavy fermion behavior and unconventional transport in LiV2O4. Simultaneously, it opens a path to achieving flat bands through electronic interactions in d-orbital systems with geometrical frustration, potentially enabling the realization of exotic phases of matter such as the fractionalized Fermi liquids.
Kramers nodal lines in intercalated TaS2 superconductors
Zhang Y., Gao Y., Pulkkinen A., Guo X., Huang J., Guo Y., Yue Z., Oh J.S., Moon A., Oudah M., Gao X.-J., Marmodoro A., Fedorov A., Mo S.-K., Hashimoto M., Lu D., Rajapitamahuni A., Vescovo E., Kono J., Hallas A.M., Birgeneau R.J., Balicas L., Minar J., Hosur P., Law K.T., Morosan E., Yi M.
Article, Nature Communications, 2025, DOI Link
View abstract ⏷
Kramers degeneracy is one fundamental embodiment of the quantum mechanical nature of particles with half-integer spin under time reversal symmetry. Under the chiral and noncentrosymmetric achiral crystalline symmetries, Kramers degeneracy emerges respectively as topological quasiparticles of Weyl fermions and Kramers nodal lines (KNLs), anchoring the Berry phase-related physics of electrons. However, an experimental demonstration for ideal KNLs well isolated at the Fermi level is lacking. Here, we establish a class of noncentrosymmetric achiral intercalated transition metal dichalcogenide superconductors with large Ising-type spin-orbit coupling, represented by InxTaS2, to host an ideal KNL phase. We provide evidence from angle-resolved photoemission spectroscopy with spin resolution, angle-dependent quantum oscillation measurements, and ab-initio calculations. Our work not only provides a realistic platform for realizing and tuning KNLs in layered materials, but also paves the way for exploring the interplay between KNLs and superconductivity, as well as applications pertaining to spintronics, valleytronics, and nonlinear transport.
Signature of magnetoelectric coupling driven finite momentum pairing in 3D ising superconductor
Yang F.Z., Zhang H.D., Mandal S., Meng F.Y., Fabbris G., Said A.H., Mercado Lozano P., Rajapitamahuni A., Vescovo E., Nelson C., Lin S., Park Y., Clements E.M., Ward T.Z., Lee H.-N., Lei H.C., Liu C.X., Miao H.
Article, Nature Communications, 2025, DOI Link
View abstract ⏷
The finite momentum superconducting paring states (FMPs) represent a forefront of condensed matter physics. Here we report experimental evidence of FMP in a locally noncentrosymmetric bulk superconductor 4Hb-TaS2. Using hard X-ray diffraction and angle-resolved photoemission spectroscopy, we reveal unusual 2D ferro-rotational charge density wave (CDW) and weak interlayer hopping in 4Hb-TaS2. The superconducting upper critical field, Hc2, linearly increases via decreasing temperature, and well exceeds the Pauli limit, suggesting the dominant orbital pair-breaking mechanism. Remarkably, we observed evidence of field-induced superconductivity-to-superconductivity transition that breaks continuous rotational symmetry of the s-wave uniform pairing in the Bardeen-Cooper-Schrieffer theory down to the six-fold rotation symmetry. Ginzburg-Landau free energy analysis shows that magnetoelectric coupling, induced by 2D ferro-rotational CDW, stabilizes FMP that provides an explanation of the lowering rotation symmetry. Our results provide a new understanding of unconventional superconducting behaviors of the bulk quantum heterostructure 4Hb-TaS2.
Orbital selective band re-normalization induced Lifshitz transition in TiSe2
Yilmaz T., Ng Y.S., Ideta S., Shimada K., Zheng J.-C., Wang H.-Q., Rajapitamahuni A., Kundu A.K., Vescovo E.
Article, Communications Physics, 2025, DOI Link
View abstract ⏷
A Lifshitz transition is a sudden change in Fermi surface topology, often linked to quantum phenomena, with major impact on transport and sometimes superconductivity. Here we demonstrate a Lifshitz transition in TiSe2 using micro–angle-resolved photoemission spectroscopy. At low temperatures an electron pocket appears at the Brillouin-zone center, whereas at higher temperatures spectral weight from a hole-like valence band dominates the Fermi level. Unlike previously reported cases typically driven by rigid band shifts, in TiSe2 an orbital-selective strong band renormalization induces the crossover near 160 K. This mechanism naturally explains the longstanding resistivity anomaly of TiSe2, which peaks around 160 K. Our results clarify its puzzling transport behavior and open avenues to investigate how periodic lattice distortions interact with strong electronic correlations.
Correlation-driven topological band inversion in VSe2
Ng Y.S., Yilmaz T., Rajapitamahuni A., Kundu A., Vescovo E., Sinkovic B., Wang H.-Q., Zheng J.-C.
Article, Physical Review B, 2025, DOI Link
View abstract ⏷
Recent spectroscopic studies have uncovered topological surface states and band inversion in 1T-VSe2, positioning this material at the intersection of correlated electron physics and nontrivial band topology. While previous interpretations attribute these features to surface strain, the microscopic origin of the topological band structure remains unresolved. Here, we present an alternative explanation based on electronic correlations, showing that a negative effective Hubbard interaction (Ueff < 0) applied to the Se 4p orbitals can reproduce the experimentally observed band inversion at both the Γ and M points. Using density functional theory (DFT) calculations with orbital-selective interactions, we demonstrate that this approach naturally gives rise to topological surface states without invoking structural distortions. Our results highlight the crucial role of ligand orbital correlations in shaping band topology and provide a novel framework for understanding and engineering topological phases in chalcogenide-based quantum materials.
Fermi-surface driven frustrations in charge ordered kagome metal LuNb6Sn6
Yang F.Z., Huang X., Tan H., Kundu A., Kim S., Thinel M., Ingham J., Rajapitamahuni A., Cai Y.Q., Nelson C., Vescovo E., Meier W.R., Mandrus D., Ortiz B.R., Pasupathy A.N., Yan B., Miao H.
Article, Physical Review B, 2025, DOI Link
View abstract ⏷
The charge density wave (CDW), a translational symmetry breaking electronic liquid, plays a pivotal role in correlated quantum materials, such as high-Tc superconductors and topological semimetals. Recently, CDWs that possibly intertwine with superconductivity and magnetism are observed in various kagome metals. However, the nature of CDWs and the role of the Fermi-surface (FS) topology in these materials remain an unresolved challenge. In this paper, we reveal the formation of CDWs in the newly discovered kagome metal LuNb6Sn6. We observe a “yield sign”-like hollow triangular diffuse scattering pattern and nearly complete softening of a flat optical phonon band near QH = (1/3, 1/3, 1/2). The scattering intensity of the diffuse scattering displays divergent behavior as decreasing temperature until TCDW = 70 K, where a competing CDW at QCDW = (1/3, 1/3, 1/3) emerges. Combined with scanning tunneling microscopy/spectroscopy and first-principles calculations, our observations support a Fermi-surface driven frustration that suppresses the leading CDW instability at QH. This frustration is relieved by the emergence of a subleading CDW at QCDW. These results provide insights into the interplay between the Fermi surface, strong electron-phonon coupling, and charge density wave formation in quantum materials.
Charge density waves and the effects of uniaxial strain on the electronic structure of 2H-NbSe2
Kundu A.K., Rajapitamahuni A., Vescovo E., Klimovskikh I.I., Berger H., Valla T.
Article, Communications Materials, 2024, DOI Link
View abstract ⏷
Interplay of superconductivity and density wave orders has been at the forefront of research of correlated electronic phases for a long time. 2H-NbSe2 is considered to be a prototype system for studying this interplay, where the balance between the two orders was proven to be sensitive to band filling and pressure. However, the origin of charge density wave in this material is still unresolved. Here, by using angle-resolved photoemission spectroscopy, we revisit the charge density wave order and study the effects of uniaxial strain on the electronic structure of 2H-NbSe2. Our results indicate previously undetected signatures of charge density waves on the Fermi surface. The application of small amount of uniaxial strain induces substantial changes in the electronic structure and lowers its symmetry. This, and the altered lattice should affect both the charge density wave phase and superconductivity and should be observable in the macroscopic properties.
Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film
Ren Z., Huang J., Tan H., Biswas A., Pulkkinen A., Zhang Y., Xie Y., Yue Z., Chen L., Xie F., Allen K., Wu H., Ren Q., Rajapitamahuni A., Kundu A.K., Vescovo E., Kono J., Morosan E., Dai P., Zhu J.-X., Si Q., Minar J., Yan B., Yi M.
Article, Nature Communications, 2024, DOI Link
View abstract ⏷
Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X = Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe dxy+dx2−y2 spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems.
Thickness-dependent insulator-to-metal transition in epitaxial Ru O2 films
Rajapitamahuni A.K., Nair S., Yang Z., Manjeshwar A.K., Jeong S.G., Nunn W., Jalan B.
Article, Physical Review Materials, 2024, DOI Link
View abstract ⏷
Epitaxially grown RuO2 films on TiO2 (110) exhibit significant in-plane strain anisotropy, with a compressive strain of -4.7% along the [001] crystalline direction and a tensile strain of +2.3% along [11¯0]. As the film thickness increases, anisotropic strain relaxation is expected. By fabricating Hall bar devices with current channels along two in-plane directions, (001) and (11¯0), we reveal anisotropic in-plane transport in RuO2/TiO2 (110) films grown via the solid-source metal-organic molecular beam epitaxy approach. For film thicknesses (tfilm)≤3.6 nm, the resistivity along (001) exceeds that along the (11¯0) direction at all temperatures. With further decrease in film thickness, we uncover a transition from metallic to insulating behavior at tfilm≤2.1 nm. Our combined temperature- and magnetic field-dependent electrical transport measurements reveal that this transition from metallic to insulating behavior is driven by electron-electron interactions.
Electronic structure and magnetic and transport properties of antiferromagnetic Weyl semimetal GdAlSi
Laha A., Kundu A.K., Aryal N., Bozin E.S., Yao J., Paone S., Rajapitamahuni A., Vescovo E., Valla T., Abeykoon M., Jing R., Yin W., Pasupathy A.N., Liu M., Li Q.
Article, Physical Review B, 2024, DOI Link
View abstract ⏷
We report the topological electronic structure and magnetic and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at TN=32 K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric under positive and negative magnetic fields. First-principles calculations were performed on various magnetic configurations, revealing that the antiferromagnetic state is the ground state, and the spiral antiferromagnetic state is a close competing state. The calculations also reveal that GdAlSi hosts multiple Weyl points near the Fermi energy. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) shows relatively good agreement with the theory, with the possibility of Weyl nodes slightly above the Fermi energy. Within the magnetic ordered state, we observe an exceptionally large anomalous Hall conductivity (AHC) of ∼1310 ω-1cm-1 at 2 K. Interestingly, the anomalous Hall effect persists up to room temperature with a significant value of AHC (∼155 ω-1cm-1). Our analysis indicates that the large AHC originates from the Berry curvature associated with the multiple pairs of Weyl points near Fermi energy.
The Electron Spectro-Microscopy (ESM) Beamline at NSLS-II
Rajapitamahuni A., Yilmaz T., Kaznatcheev K., Kundu A.K., Vescovo E., Al-Mahboob A., Sadowski J.T.
Article, Synchrotron Radiation News, 2024, DOI Link
Softening of a flat phonon mode in the kagome ScV6Sn6
Korshunov A., Hu H., Subires D., Jiang Y., Calugaru D., Feng X., Rajapitamahuni A., Yi C., Roychowdhury S., Vergniory M.G., Strempfer J., Shekhar C., Vescovo E., Chernyshov D., Said A.H., Bosak A., Felser C., Bernevig B.A., Blanco-Canosa S.
Article, Nature Communications, 2023, DOI Link
View abstract ⏷
Geometrically frustrated kagome lattices are raising as novel platforms to engineer correlated topological electron flat bands that are prominent to electronic instabilities. Here, we demonstrate a phonon softening at the kz = π plane in ScV6Sn6. The low energy longitudinal phonon collapses at ~98 K and q = 131312 due to the electron-phonon interaction, without the emergence of long-range charge order which sets in at a different propagation vector q CDW = 131313 . Theoretical calculations corroborate the experimental finding to indicate that the leading instability is located at 131312 of a rather flat mode. We relate the phonon renormalization to the orbital-resolved susceptibility of the trigonal Sn atoms and explain the approximately flat phonon dispersion. Our data report the first example of the collapse of a kagome bosonic mode and promote the 166 compounds of kagomes as primary candidates to explore correlated flat phonon-topological flat electron physics.
Three-dimensional flat bands in pyrochlore metal CaNi2
Wakefield J.P., Kang M., Neves P.M., Oh D., Fang S., McTigue R., Frank Zhao S.Y., Lamichhane T.N., Chen A., Lee S., Park S., Park J.-H., Jozwiak C., Bostwick A., Rotenberg E., Rajapitamahuni A., Vescovo E., McChesney J.L., Graf D., Palmstrom J.C., Suzuki T., Li M., Comin R., Checkelsky J.G.
Article, Nature, 2023, DOI Link
View abstract ⏷
Electronic flat-band materials host quantum states characterized by a quenched kinetic energy. These flat bands are often conducive to enhanced electron correlation effects and emergent quantum phases of matter 1. Long studied in theoretical models 2–4, these systems have received renewed interest after their experimental realization in van der Waals heterostructures 5,6 and quasi-two-dimensional (2D) crystalline materials 7,8. An outstanding experimental question is if such flat bands can be realized in three-dimensional (3D) networks, potentially enabling new materials platforms 9,10 and phenomena 11–13. Here we investigate the C15 Laves phase metal CaNi2, which contains a nickel pyrochlore lattice predicted at a model network level to host a doubly-degenerate, topological flat band arising from 3D destructive interference of electronic hopping 14,15. Using angle-resolved photoemission spectroscopy, we observe a band with vanishing dispersion across the full 3D Brillouin zone that we identify with the pyrochlore flat band as well as two additional flat bands that we show arise from multi-orbital interference of Ni d-electrons. Furthermore, we demonstrate chemical tuning of the flat-band manifold to the Fermi level that coincides with enhanced electronic correlations and the appearance of superconductivity. Extending the notion of intrinsic band flatness from 2D to 3D, this provides a potential pathway to correlated behaviour predicted for higher-dimensional flat-band systems ranging from tunable topological 15 to fractionalized phases 16.
Evolution of Highly Anisotropic Magnetism in the Titanium-Based Kagome Metals LnTi3Bi4 (Ln: La···Gd3+, Eu2+, Yb2+)
Ortiz B.R., Miao H., Parker D.S., Yang F., Samolyuk G.D., Clements E.M., Rajapitamahuni A., Yilmaz T., Vescovo E., Yan J., May A.F., McGuire M.A.
Article, Chemistry of Materials, 2023, DOI Link
View abstract ⏷
Here, we present a family of titanium-based kagome metals of the form LnTi3Bi4 (Ln: La···Gd3+, Eu2+, Yb2+). Four previously unreported compounds are presented: YbTi3Bi4, GdTi3Bi4, NdTi3Bi4, and PrTi3Bi4. Single-crystal growth methods are provided alongside detailed magnetic and thermodynamic measurements across the entire series. The LnTi3Bi4 family of compounds are orthorhombic (Fmmm), layered compounds that exhibit slightly distorted titanium-based kagome nets interwoven with zigzag lanthanide-based (Ln) chains. Crystals are easily exfoliated parallel to the kagome sheets, and angular resolved photoemission (ARPES) measurements highlight the intricacy of the electronic structure in these compounds. Density functional theory (DFT) and ARPES studies find Dirac points near the Fermi level, consistent with the kagome-derived band structure. The magnetic properties and the associated anisotropy emerge from the quasi-1D zigzag chains of Ln and impart a wide array of magnetic ground states ranging from anisotropic ferromagnetism to complex antiferromagnetism with a cascade of metamagnetic transitions. The combination of the kagome-based electronic structure and highly anisotropic Ln-based magnetism on an exfoliatable platform cements the LnTi3Bi4 family as an interesting addition to the ever-expanding suite of kagome metals.
Adsorption-Controlled Growth and Magnetism in Epitaxial SrRuO3 Films
Manjeshwar A.K., Nair S., Rajapitamahuni A.K., James R.D., Jalan B.
Article, ACS Nano, 2023, DOI Link
View abstract ⏷
Controlling defect densities in SrRuO3 films is the cornerstone for probing the intricate relationship among its structural, electrical, and magnetic properties. We combine film growth, electrical transport, and magnetometry to demonstrate the adsorption-controlled growth of phase-pure, epitaxial, and stoichiometric SrRuO3 films on SrTiO3 (001) substrates using solid source metal-organic molecular beam epitaxy. Across the growth window, we show that the anomalous Hall curves arise from two distinct magnetic domains. Domains with similar anomalous Hall polarities generate the stepped feature observed within the growth window, and those with opposite polarities produce the hump-like feature present exclusively in the highly Ru-poor film. We achieve a residual resistivity ratio (RRR = ρ300K/ρ2K) of 87 in a 50 nm-thick, coherently strained, and stoichiometric SrRuO3 film, the highest reported value to date on SrTiO3 (001) substrates. We hypothesize further improvements in the RRR through strain engineering to control the tetragonal-to-orthorhombic phase transformation and the domain structure of SrRuO3 films.
Publisher Correction: Combined experimental-theoretical study of electron mobility-limiting mechanisms in SrSnO3 (Communications Physics, (2021), 4, 1, (241), 10.1038/s42005-021-00742-w)
Truttmann T.K., Zhou J.-J., Lu I.-T., Rajapitamahuni A.K., Liu F., Mates T.E., Bernardi M., Jalan B.
Erratum, Communications Physics, 2022, DOI Link
View abstract ⏷
Some of the notations and equations of this article contained errors. The list of corrections is as follows.
Remote surface optical phonon scattering in ferroelectric Ba0.6Sr0.4TiO3gated graphene
Chen H., Li T., Hao Y., Rajapitamahuni A., Xiao Z., Schoeche S., Schubert M., Hong X.
Article, Journal of Applied Physics, 2022, DOI Link
View abstract ⏷
We report the effect of remote surface optical (RSO) phonon scattering on carrier mobility in monolayer graphene gated by ferroelectric oxide. We fabricate monolayer graphene transistors back-gated by epitaxial (001) Ba0.6Sr0.4TiO3 films, with field effect mobility up to 23 000 cm2 V-1 s-1 achieved. Switching ferroelectric polarization induces nonvolatile modulation of resistance and quantum Hall effect in graphene at low temperatures. Ellipsometry spectroscopy studies reveal four pairs of optical phonon modes in Ba0.6Sr0.4TiO3, from which we extract RSO phonon frequencies. The temperature dependence of resistivity in graphene can be well accounted for by considering the scattering from the intrinsic longitudinal acoustic phonon and the RSO phonon, with the latter dominated by the mode at 35.8 meV. Our study reveals the room temperature mobility limit of ferroelectric-gated graphene transistors imposed by RSO phonon scattering.
Plasmon-Phonon Coupling in Electrostatically Gated β-Ga2O3Films with Mobility Exceeding 200 cm2V-1s-1
Rajapitamahuni A.K., Manjeshwar A.K., Kumar A., Datta A., Ranga P., Thoutam L.R., Krishnamoorthy S., Singisetti U., Jalan B.
Article, ACS Nano, 2022, DOI Link
View abstract ⏷
Monoclinic β-Ga2O3, an ultra-wide bandgap semiconductor, has seen enormous activity in recent years. However, the fundamental study of the plasmon-phonon coupling that dictates electron transport properties has not been possible due to the difficulty in achieving higher carrier density (without introducing chemical disorder). Here, we report a highly reversible, electrostatic doping of β-Ga2O3 films with tunable carrier densities using ion-gel-gated electric double-layer transistor configuration. Combining temperature-dependent Hall effect measurements, transport modeling, and comprehensive mobility calculations using ab initio based electron-phonon scattering rates, we demonstrate an increase in the room-temperature mobility to 201 cm2 V-1 s-1 followed by a surprising decrease with an increasing carrier density due to the plasmon-phonon coupling. The modeling and experimental data further reveal an important "antiscreening"(of electron-phonon interaction) effect arising from dynamic screening from the hybrid plasmon-phonon modes. Our calculations show that a significantly higher room-temperature mobility of 300 cm2 V-1 s-1 is possible if high electron densities (>1020 cm-3) with plasmon energies surpassing the highest energy LO mode can be realized. As Ga2O3 and other polar semiconductors play an important role in several device applications, the fundamental understanding of the plasmon-phonon coupling can lead to the enhancement of mobility by harnessing the dynamic screening of the electron-phonon interactions.
Impurity band conduction in Si-doped β -Ga2O3films
Rajapitamahuni A.K., Thoutam L.R., Ranga P., Krishnamoorthy S., Jalan B.
Article, Applied Physics Letters, 2021, DOI Link
View abstract ⏷
By combining temperature-dependent resistivity and Hall effect measurements, we investigate donor state energy in Si-doped β-Ga2O3 films grown using metal-organic vapor phase epitaxy. High-magnetic field (H) Hall effect measurements (-90 kOe ≤ H ≤ +90 kOe) showed non-linear Hall resistance for T < 150 K, revealing two-band conduction. Further analyses revealed carrier freeze out characteristics in both bands yielding donor state energies of ∼33.7 and ∼45.6 meV. The former is consistent with the donor energy of Si in β-Ga2O3, whereas the latter suggests a residual donor state. This study provides critical insight into the impurity band conduction and the defect energy states in β-Ga2O3 using high-field magnetotransport measurements.
Novel synthesis approach for “stubborn” metals and metal oxides
Nunn W., Manjeshwar A.K., Yue J., Rajapitamahuni A., Truttmann T.K., Jalan B.
Article, Proceedings of the National Academy of Sciences of the United States of America, 2021, DOI Link
View abstract ⏷
Advances in physical vapor deposition techniques have led to a myriad of quantum materials and technological breakthroughs, affecting all areas of nanoscience and nanotechnology which rely on the innovation in synthesis. Despite this, one area that remains challenging is the synthesis of atomically precise complex metal oxide thin films and heterostructures containing "stubborn" elements that are not only nontrivial to evaporate/sublimate but also hard to oxidize. Here, we report a simple yet atomically controlled synthesis approach that bridges this gap. Using platinum and ruthenium as examples, we show that both the low vapor pressure and the difficulty in oxidizing a "stubborn" element can be addressed by using a solid metal-organic compound with significantly higher vapor pressure and with the added benefits of being in a preoxidized state along with excellent thermal and air stability. We demonstrate the synthesis of high-quality single crystalline, epitaxial Pt, and RuO2films, resulting in a record high residual resistivity ratio (=27) in Pt films and low residual resistivity, ∼6 μΩ·cm, in RuO2films. We further demonstrate, using SrRuO3as an example, the viability of this approach for more complex materials with the same ease and control that has been largely responsible for the success of the molecular beam epitaxy of III-V semiconductors. Our approach is a major step forward in the synthesis science of "stubborn" materials, which have been of significant interest to the materials science and the condensed matter physics community.
Solid-source metal-organic molecular beam epitaxy of epitaxial RuO2
Nunn W., Nair S., Yun H., Kamath Manjeshwar A., Rajapitamahuni A., Lee D., Mkhoyan K.A., Jalan B.
Article, APL Materials, 2021, DOI Link
View abstract ⏷
A seemingly simple oxide with a rutile structure, RuO2, has been shown to possess several intriguing properties ranging from strain-stabilized superconductivity to a strong catalytic activity. Much interest has arisen surrounding the controlled synthesis of RuO2 films, but unfortunately, utilizing atomically controlled deposition techniques, such as molecular beam epitaxy (MBE), has been difficult due to the ultra-low vapor pressure and low oxidation potential of Ru. Here, we demonstrate the growth of epitaxial, single crystalline RuO2 films on different substrate orientations using the novel solid-source metal-organic (MO) MBE. This approach circumvents these issues by supplying Ru using a “pre-oxidized” solid MO precursor containing Ru. High-quality epitaxial RuO2 films with a bulk-like room-temperature resistivity of 55 μΩ cm were obtained at a substrate temperature as low as 300 °C. By combining x-ray diffraction, transmission electron microscopy, and electrical measurements, we discuss the effect of substrate temperature, orientation, film thickness, and strain on the structure and electrical properties of these films. Our results illustrating the use of a novel solid-source metal-organic MBE approach pave the way to the atomic-layer controlled synthesis of complex oxides of “stubborn” metals, which are not only difficult to evaporate but also hard to oxidize.
Combined experimental-theoretical study of electron mobility-limiting mechanisms in SrSnO3
Truttmann T.K., Zhou J.-J., Lu I.-T., Rajapitamahuni A.K., Liu F., Mates T.E., Bernardi M., Jalan B.
Article, Communications Physics, 2021, DOI Link
View abstract ⏷
The discovery and development of ultra-wide bandgap (UWBG) semiconductors is crucial to accelerate the adoption of renewable power sources. This necessitates an UWBG semiconductor that exhibits robust doping with high carrier mobility over a wide range of carrier concentrations. Here we demonstrate that epitaxial thin films of the perovskite oxide NdxSr1−xSnO3 (SSO) do exactly this. Nd is used as a donor to successfully modulate the carrier concentration over nearly two orders of magnitude, from 3.7 × 1018 cm−3 to 2.0 × 1020 cm−3. Despite being grown on lattice-mismatched substrates and thus having relatively high structural disorder, SSO films exhibited the highest room-temperature mobility, ~70 cm2 V−1 s−1, among all known UWBG semiconductors in the range of carrier concentrations studied. The phonon-limited mobility is calculated from first principles and supplemented with a model to treat ionized impurity and Kondo scattering. This produces excellent agreement with experiment over a wide range of temperatures and carrier concentrations, and predicts the room-temperature phonon-limited mobility to be 76–99 cm2 V−1 s−1 depending on carrier concentration. This work establishes a perovskite oxide as an emerging UWBG semiconductor candidate with potential for applications in power electronics.
Hysteretic Magnetoresistance in a Non-Magnetic SrSnO3Film via Thermal Coupling to Dynamic Substrate Behavior
Thoutam L.R., Truttmann T.K., Rajapitamahuni A.K., Jalan B.
Article, Nano Letters, 2021, DOI Link
View abstract ⏷
Hysteretic magnetoresistance (MR) is often used as a signature of ferromagnetism in conducting oxide films and heterostructures. Here, magnetotransport is investigated in a nonmagnetic La-doped SrSnO3 film. A 12 nm La:SrSnO3/2 nm SrSnO3/GdScO3 (110) film with insulating behavior exhibited a robust hysteresis loop in the MR at T < 5 K accompanied by an anomaly at ∼±3 T at T < 2.5 K. Furthermore, MR with the field in-plane yielded a value exceeding 100% at 1.8 K. Using detailed temperature-, angle- and magnetic field-dependent resistance measurements, we illustrate the origin of hysteresis is not due to magnetism in the film but rather is associated with the magnetocaloric effect of the substrate. Given GdScO3 and similar substrates are commonly used, this work highlights the importance of thermal coupling to processes in the substrates which must be carefully accounted for in the data interpretation for heterostructures utilizing these substrates.
Ferroelectric polarization control of magnetic anisotropy in PbZ r0.2 T i0.8 O3 / L a0.8 S r0.2Mn O3 heterostructures
Rajapitamahuni A., Tao L.L., Hao Y., Song J., Xu X., Tsymbal E.Y., Hong X.
Article, Physical Review Materials, 2019, DOI Link
View abstract ⏷
The interfacial coupling between the switchable polarization and neighboring magnetic order makes ferroelectric/ferromagnetic composite structures a versatile platform to realize voltage control of magnetic anisotropy. We report the nonvolatile ferroelectric field effect modulation of the magnetocrystalline anisotropy (MCA) in epitaxial PbZr0.2Ti0.8O3 (PZT)/La0.8Sr0.2MnO3 (LSMO) heterostructures grown on (001) SrTiO3 substrates. Planar Hall effect measurements show that the in-plane magnetic anisotropy energy in LSMO is enhanced by about 22% in the hole accumulation state compared to the depletion state, in quantitative agreement with our first-principles density functional theory calculations. Modeling the spin-orbit coupling effect with second-order perturbation theory points to the critical role of the d-orbital occupancy in controlling MCA. Our work provides insights into the effect of ferroelectric polarization on the magnetic anisotropy at the composite multiferroic interfaces, paving the path for their implementation into high-performance, low-power spintronic applications.
Enhanced Piezoelectric Response in Hybrid Lead Halide Perovskite Thin Films via Interfacing with Ferroelectric PbZr0.2Ti0.8O3
Song J., Xiao Z., Chen B., Prockish S., Chen X., Rajapitamahuni A., Zhang L., Huang J., Hong X.
Article, ACS Applied Materials and Interfaces, 2018, DOI Link
View abstract ⏷
We report a more than 10-fold enhancement of the piezoelectric coefficient d33 of polycrystalline CH3NH3PbI3 (MAPbI3) films when interfacing them with ferroelectric PbZr0.2Ti0.8O3 (PZT). Piezoresponse force microscopy (PFM) studies reveal d33MAPbI3 values of 0.3-0.4 pm/V for MAPbI3 deposited on Au, indium tin oxide, and SrTiO3 surfaces, with small phase angle fluctuating at length scales smaller than the grain size. In sharp contrast, on samples prepared on epitaxial PZT films, we observe large-scale polar domains exhibiting clear, close to 180° PFM phase contrasts, pointing to polar axes along the film normal. By separating the piezoresponse contributions from the MAPbI3 and PZT layers, we extract a significantly higher d33MAPbI3 of ∼4 pm/V, which is attributed to the enhanced alignment of the MA molecular dipoles promoted by the unbalanced surface potential of PZT. We also discuss the effect of the interfacial screening layer on the preferred polar direction.
Probing magnetic anisotropy in epitaxial La0.67Sr0.33MnO3thin films and nanostructures via planar Hall effect
Conference paper, Proceedings of SPIE - The International Society for Optical Engineering, 2018, DOI Link
View abstract ⏷
The ability to control and manipulate magnetic anisotropy in the colossal magnetoresistive (CMR) oxide (La,Sr)MnO3 (LSMO) is critical for its implementation in magnetic memory applications. In this work, we employ the planar Hall effect (PHE) as a powerful tool to probe the magnetic anisotropy in LSMO thin films and nanostructures, where the magnetization is too small to be detected by conventional magnetometry techniques. By analyzing the angular- A nd magnetic field-dependences of the PHE, we deduced an in-plane biaxial magnetocrystalline anisotropy (MCA) energy of ∼1.2x105erg/cm2 in LSMO thin films fully strained on (001) SrTiO3 substrates. Creating nanoscale periodic depth modulation in LSMO establishes a uniaxial anisotropy with substantially enhanced MCA energy density, which is attributed to a high strain gradient sustained in the nanostructure. The energy competition between the biaxial and uniaxial MCA leads to multi-level resistance switching behavior in properly engineered LSMO nanostructures, which can be utilized to design the switching dynamics in magnetic memory devices. Our work points to the critical role of epitaxial strain in determining the MCA in CMR oxides, and provides an effective material strategy for engineering the magnetic properties of LSMO for novel spintronic applications with high thermal stability and high density data storage.
Nanodomain Engineering in Ferroelectric Capacitors with Graphene Electrodes
Lu H., Wang B., Li T., Lipatov A., Lee H., Rajapitamahuni A., Xu R., Hong X., Farokhipoor S., Martin L.W., Eom C.-B., Chen L.-Q., Sinitskii A., Gruverman A.
Article, Nano Letters, 2016, DOI Link
View abstract ⏷
Polarization switching in ferroelectric capacitors is typically realized by application of an electrical bias to the capacitor electrodes and occurs via a complex process of domain structure reorganization. As the domain evolution in real devices is governed by the distribution of the nucleation centers, obtaining a domain structure of a desired configuration by electrical pulsing is challenging, if not impossible. Recent discovery of polarization reversal via the flexoelectric effect has opened a possibility for deterministic control of polarization in ferroelectric capacitors. In this paper, we demonstrate mechanical writing of arbitrary-shaped nanoscale domains in thin-film ferroelectric capacitors with graphene electrodes facilitated by a strain gradient induced by a tip of an atomic force microscope (AFM). A phase-field modeling prediction of a strong effect of graphene thickness on the threshold load required to initiate mechanical switching has been confirmed experimentally. Deliberate voltage-free domain writing represents a viable approach for development of functional devices based on domain topology and electronic properties of the domains and domain walls.
Giant Enhancement of Magnetic Anisotropy in Ultrathin Manganite Films via Nanoscale 1D Periodic Depth Modulation
Rajapitamahuni A., Zhang L., Koten M.A., Singh V.R., Burton J.D., Tsymbal E.Y., Shield J.E., Hong X.
Article, Physical Review Letters, 2016, DOI Link
View abstract ⏷
The relatively low magnetocrystalline anisotropy (MCA) in strongly correlated manganites (La,Sr)MnO3 has been a major hurdle for implementing them in spintronic applications. Here we report an unusual, giant enhancement of in-plane MCA in 6 nm La0.67Sr0.33MnO3 (LSMO) films grown on (001) SrTiO3 substrates when the top 2 nm is patterned into periodic stripes of 100 or 200 nm width. Planar Hall effect measurements reveal an emergent uniaxial anisotropy superimposed on one of the original biaxial easy axes for unpatterned LSMO along 110 directions, with a 50-fold enhanced anisotropy energy density of 5.6×106 erg/cm3 within the nanostripes, comparable to the value for cobalt. The magnitude and direction of the uniaxial anisotropy exclude shape anisotropy and the step edge effect as its origin. High resolution transmission electron microscopy studies reveal a nonequilibrium strain distribution and drastic suppression in the c-axis lattice constant within the nanostructures, which is the driving mechanism for the enhanced uniaxial MCA, as suggested by first-principles density functional calculations.
Nonlinear transport in nanoscale phase separated colossal magnetoresistive oxide thin films
Singh V.R., Zhang L., Rajapitamahuni A.K., Devries N., Hong X.
Article, Journal of Applied Physics, 2014, DOI Link
View abstract ⏷
We report a study of the I-V characteristics of 2.5-5.4 nm epitaxial La1-xSrxMnO3 (x = 0.33 and 0.5) and La0.7Ca0.3MnO3 thin films. While La 0.67Sr0.33MnO3 films exhibit linear conduction over the entire temperature and magnetic field ranges investigated, we observe a strong correlation between the linearity of the I-V relation and the metal-insulator transition in highly phase separated La0.5Sr 0.5MnO3 and La0.7Ca0.3MnO 3 films. Linear I-V behavior has been observed in the high temperature paramagnetic insulating phase, and an additional current term proportional to Vα (α = 1.5-2.8) starts to develop below the metal-insulator transition temperature TMI, with the onset temperature of the nonlinearity increasing in magnetic field as TMI increases. The exponent α increases with decreasing temperature and increasing magnetic field and is significantly enhanced in ultrathin films with thicknesses close to that of the electrically dead layer. We attribute the origin of the nonlinearity to transport through the nanoscale coexisting metallic and insulating regions. Our results suggest that phase separation is not fully quenched even at low temperatures and high magnetic fields. © 2014 AIP Publishing LLC.
Examining graphene field effect sensors for ferroelectric thin film studies
Article, Nano Letters, 2013, DOI Link
View abstract ⏷
We examine a prototype graphene field effect sensor for the study of the dielectric constant, pyroelectric coefficient, and ferroelectric polarization of 100-300 nm epitaxial (Ba,Sr)TiO3 thin films. Ferroelectric switching induces hysteresis in the resistivity and carrier density of n-layer graphene (n = 1-5) below 100 K, which competes with an antihysteresis behavior activated by the combined effects of electric field and temperature. We also discuss how the polarization asymmetry and interface charge dynamics affect the electronic properties of graphene. © 2013 American Chemical Society.
The local structure and I-V characteristics of chromium doped semiconducting boron carbide
Liu J., Dowben P.A., Luo G., Mei W.-N., Rajapitamahuni A.K., Sokolov A., Karki S., Caruso A.N.
Conference paper, Materials Research Society Symposium Proceedings, 2011, DOI Link
View abstract ⏷
The local spin configuration and band structure of chromium doped boron carbide calculated by density functional theory suggests local magnetic ordering. While the long range dopant position appears random in the boron carbide semiconductor, the local position and initial empirical/computational results suggest the promise of large magneto-resistive effects. The chromium doped boron carbide thin films, fabricated by boron carbide-chromium co-deposition, were studied by current-voltage (I-V) characteristics measurements. The results provide some reason to believe that magneto-resistive effects are indeed present at room temperature. © 2011 Materials Research Society.
