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At least 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Spin injection and spin transport in paramagnetic insulators

We investigate the spin injection and the spin transport in paramagnetic insulators described by simple Heisenberg interactions using auxiliary particle methods. Some of these methods allow access to both paramagnetic states above magnetic transition temperatures and magnetic states at low temperatures. It is predicted that the spin injection at an interface with a normal metal is rather insensitive to temperatures above the magnetic transition temperature. On the other hand below the transition temperature, it decreases monotonically and disappears at zero temperature. We also analyze the bulk spin conductance. We show that the conductance becomes zero at zero temperature as predicted by linear spin wave theory but increases with temperature and is maximized around the magnetic transition temperature. These findings suggest that the compromise between the two effects determines the optimal temperature for spintronics applications utilizing magnetic insulators.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Non percolative nature of the metal-insulator transition and persistence of local Jahn-Teller distortions in the rhombohedral regime of La 1-x Ca x MnO 3

Evolution of the average and local crystal structure of Ca-doped LaMnO 3 has been studied across the metal to insulator (MI) and the orthorhombic to rhombohedral (OR) structural phase transitions over a broad temperature range for two Ca concentrations (x = 0.18,0.22). Combined Rietveld and high real space resolution atomic pair distribution function (PDF) analysis of neutron total scattering data was carried out with aims of exploring the possibility of nanoscale phase separation (PS) in relation to MI transition, and charting the evolution of local Jahn-Teller (JT) distortion of MnO 6 octahedra across the OR transition at T S ~720 K. The study utilized explicit two-phase PDF structural modeling, revealing that away from T MI there is no evidence for nanoscale phase coexistence. The local JT distortions disappear abruptly upon crossing into the metallic regime both with doping and temperature, with only a small temperature-independent signature of quenched disorder being observable at low temperature as compared to CaMnO 3 . The results hence do not support the percolative scenario for the MI transition in La 1–x Ca x MnO 3 based on PS, and question its ubiquity in the manganites. In contrast to LaMnO 3 that exhibits long-range orbital correlations and sizable octahedral distortions at low temperature, the doped samples with compositions straddling the MI boundary exhibit correlations (in the insulating regime) limited to only ~1 nm with observably smaller distortions. In the x = 0.22 sample local JT distortions are found to persist across the OR transition and deep into the R phase (up to ~1050 K), where they are crystallographically prohibited. As a result, their magnitude and subnanometer spatial extent remain unchanged.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Spin-Josephson effects in exchange coupled antiferromagnetic insulators

The spin superfluid analogy can be extended to include Josephson-like oscillations of the spin current. In a system of two antiferromagnetic insulators (AFMIs) separated by a thin metallic spacer, a threshold spin chemical potential established perpendicular to the direction of the N&#233el vector field drives terahertz oscillations of the spin current. This spin current also has a nonlinear, time-averaged component which provides a “smoking gun” signature of spin superfluidity. The time-averaged spin current can be detected via the inverse spin Hall effect in a metallic spacer with large spin-orbit coupling. The physics illustrated here with AFMIs also applies to easy-plane ferromagnetic insulators. We suggest these findings may provide a new approach for experimental verification of spin superfluidity and realization of a terahertz spin oscillator.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

B a 2 NiOs O 6 : A Dirac-Mott insulator with ferromagnetism near 100 K

In this study, the ferromagnetic semiconductor Ba 2 NiOsO 6 ( T mag ~ 100 K ) was synthesized at 6 GPa and 1500 °C. It crystallizes into a double perovskite structure [Fm - 3m ; a = 8.0428 ( 1 ) Å], where the Ni 2+ and Os 6+ ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os 6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te ( T mag < 180 K ), the spin-gapless semiconductor Mn 2 CoAl ( T mag ~ 720 K ), and the ferromagnetic insulators EuO ( T mag ~ 70 K ) and Bi 3 Cr 3 O 11 ( T mag ~ 220 K ). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba 2 NiOsO 6 should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Theory of the spin-Seebeck effect at a topological-insulator/ferromagnetic-insulator interface

The spin-Seebeck effect refers to voltage signals induced in metals by thermally driven spin currents in adjacent magnetic systems. We present a theory of the spin-Seebeck signal in the case where the conductor that supports the voltage signal is the topologically protected two-dimensional surface-state system at the interface between a ferromagnetic insulator (FI) and a topological insulator (TI). Our theory uses a Dirac model for the TI surface states and assumes Heisenberg exchange coupling between the TI quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by the TI surface states scattering off the nonequilibrium magnon population at the surface of the semi-infinite thermally driven FI. Our theory is readily generalized to spin-Seebeck voltages in any two-dimensional conductor that is exchange-coupled to the surface of a FI. Surface-state carrier-density-dependent signal strengths calculated using Bi 2 Te 3 and yttrium iron garnet material parameters are consistent with recent experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Metal-insulator transition in the dimerized organic conductor κ - ( BEDT-TTF ) 2 Hg ( SCN ) 2 Br

The organic charge-transfer salt κ-(BEDT-TTF) 2 Hg(SCN) 2 Br is a quasi-two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below T = 80 K, it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition, we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized κ-(BEDT-TTF) salts, in particular the Cl analog, where a charge-order transition takes place at T CO = 30 K.

Ivek, Tomislav↗
DOE OSTI2017

Ordering mechanism and quantum anomalous Hall effect of magnetically doped topological insulators

We investigate magnetic ordering and the quantum anomalous Hall effect (QAHE) in Cr-doped topological insulators using systematic first-principles calculations, explaining the mechanism responsible for ferromagnetic order and the reason why Sb 2 Te 3 is a better QAHE host than Bi 2 Se 3 or Bi 2 Te 3 . In this work, we conclude that these magnetic topological insulators have relatively long-range exchange interactions within quintuple layers and weak interactions between quintuple layers. Our analyses for the spin splitting of the topological surface states suggest that the temperature at which the QAHE occurs in these materials can be enhanced significantly by Mo-Cr co-doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Producing coherent excitations in pumped Mott antiferromagnetic insulators

Nonequilibrium dynamics in correlated materials has attracted attention due to the possibility of characterizing, tuning, and creating complex ordered states. To understand the photoinduced microscopic dynamics, especially the linkage under realistic pump conditions between transient states and remnant elementary excitations, we performed nonperturbative simulations of various time-resolved spectroscopies. We used the Mott antiferromagnetic insulator as a model platform. The transient dynamics of multi-particle excitations can be attributed to the interplay between Floquet virtual states and a modification of the density of states, in which interactions induce a spectral weight transfer. Using an autocorrelation of the time-dependent spectral function, we show that resonance of the virtual states with the upper Hubbard band in the Mott insulator provides the route towards manipulating the electronic distribution and modifying charge and spin excitations. In conclusion, our results link transient dynamics to the nature of many-body excitations and provide an opportunity to design nonequilibrium states of matter via tuned laser pulses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Chiral topological insulator of magnons

Here, we propose a magnon realization of 3D topological insulator in the AIII (chiral symmetry) topological class. The topological magnon gap opens due to the presence of Dzyaloshinskii-Moriya interactions. The existence of the topological invariant is established by calculating the bulk winding number of the system. Within our model, the surface magnon Dirac cone is protected by the sublattice chiral symmetry. By analyzing the magnon surface modes, we confirm that the backscattering is prohibited. By weakly breaking the chiral symmetry, we observe the magnon Hall response on the surface due to opening of the gap. Finally, we show that by changing certain parameters, the system can be tuned between the chiral topological insulator, three-dimensional magnon anomalous Hall, and Weyl magnon phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Crossover from itinerant to localized magnetic excitations through the metal-insulator transition in NaOsO 3

NaOsO 3 undergoes a metal-insulator transition (MIT) at 410 K, concomitant with the onset of antiferromagnetic order. The excitation spectra have been investigated through the MIT by resonant inelastic x-ray scattering (RIXS) at the Os L 3 edge. Low resolution (ΔE ~ 300 meV) measurements over a wide range of energies reveal that local electronic excitations do not change appreciably through the MIT. This is consistent with a picture in which structural distortions do not drive the MIT. In contrast, high resolution (ΔE ~ 56 meV) measurements show that the well-defined, low-energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in NaOsO 3 . In particular, the presence of weak correlations in the paramagnetic phase implies a degree of departure from the ideal Slater limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Photoinduced Hund excitons in the breakdown of a two-orbital Mott insulator

We study the photoinduced breakdown of a two-orbital Mott insulator and resulting metallic state. Using time-dependent density matrix renormalization group, we scrutinize the real-time dynamics of the half-filled two-orbital Hubbard model interacting with a resonant radiation field pulse. The breakdown, caused by production of doublon-holon pairs, is enhanced by Hund's exchange, which dynamically activates large orbital fluctuations. The melting of the Mott insulator is accompanied by a high to low spin transition with a concomitant reduction of antiferromagnetic spin fluctuations. Most notably, the overall time response is driven by the photogeneration of excitons with orbital character that are stabilized by Hund's coupling. These unconventional “Hund excitons” correspond to bound spin-singlet orbital-triplet doublon-holon pairs. We study exciton properties such as bandwidth, binding potential, and size within a semiclassical approach. In conclusion, the photometallic state results from a coexistence of Hund excitons and doublon-holon plasma.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Doping-induced antiferromagnetic bicollinear insulating state and superconducting temperature of monolayer FeSe systems

First-principles calculations of ionic-liquid-gated (Li,Fe)OHFeSe suggest a metal-insulator transition at a nominal Li/Fe ratio of 75/25 in the (Li,Fe)OH layer. While doping increases Fermi energy, the formation of an antiferromagnetic bicollinear phase is the key for the transition. It is expected that this insulating phase also exists in other FeSe systems upon heavy electron doping, and its presence can hinder the increase of superconducting temperature. Here these results offer clues on how to optimize superconductivity amid its interplay with magnetic properties in FeSe systems.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Spin-Nernst effect in the paramagnetic regime of an antiferromagnetic insulator

In this study, we theoretically investigate a pure spin Hall current driven by a longitudinal temperature gradient, i.e., the spin Nernst effect (SNE), in a paramagnetic state of a collinear antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction. The SNE in a magnetic ordered state in such an insulator was proposed by Cheng et al. [R. Cheng, S. Okamoto, and D. Xiao, Phys. Rev. Lett. 117, 217202 (2016)]. Here we show that the Dzyaloshinskii-Moriya interaction can generate a pure spin Hall current even without magnetic ordering. By using a Schwinger boson mean-field theory, we calculate the temperature dependence of SNE in a disordered phase. Lastly, we also discuss the implication of our results to experimental realizations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Observation of cyclotron antiresonance in the topological insulator Bi 2 Te 3

Here, we report on the experimental observation of a cyclotron antiresonance in a canonical three-dimensional topological insulator Bi 2 Te 3 . Magnetoreflectance response of single-crystal Bi 2 Te 3 was studied in 18-T magnetic field, and compared to other topological insulators studied before, the main spectral feature is inverted. We refer to it as an antiresonance. In order to describe this unconventional behavior we propose the idea of an imaginary cyclotron resonance frequency, which on the other hand indicates that the form of the Lorentz force that magnetic field exerts on charge carriers takes an unconventional form.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Honeycomb lattice Na 2 IrO 3 at high pressures: A robust spin-orbit Mott insulator

The honeycomb iridate Na 2 IrO 3 has received much attention as a candidate to realize a quantum spin liquid state, but the nature of its insulating state remains controversial. We found that the material exhibits structural transitions at 3 and 10 GPa. The former is accompanied by 166-meV suppression of the activation gap, but the energies for the low-lying interband transitions change by less than 10 meV. This can be reconciled in a picture in which the application of high pressure barely shifts the electronic bands, but rather merely broadens them. First-principles calculations uncover a strong correlation between the band gap and the β angle of the monoclinic structure, indicating non-negligible interlayer coupling. Furthermore, these results offer clear evidence for a spin-orbit Mott insulating state in Na 2 IrO 3 and are inconsistent with the quasimolecular orbital model.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Voltage-induced metal-insulator transition in a one-dimensional charge density wave

Here, we present a theoretical investigation of the voltage-driven metal insulator transition based on solving coupled Boltzmann and Hartree-Fock equations to determine the insulating gap and the electron distribution in a model system – a one dimensional charge density wave. Electric fields that are parametrically small relative to energy gaps can shift the electron distribution away from the momentum-space region where interband relaxation is efficient, leading to a highly non-equilibrium quasiparticle distribution even in the absence of Zener tunneling. The gap equation is found to have regions of multistability; a non-equilibrium analog of the free energy is constructed and used to determine which phase is preferred.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Topological crystalline insulator states in the Ca 2 As family

Topological crystalline insulators (TCIs) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have proposed new TCI states protected by rotational symmetries and provided powerful guidelines to search for TCIs in real materials. Building upon recent theoretical works, we demonstrate a feasible method to identify new TCI states based on first-principles calculations. We systematically unveil the topological properties of the TCI states in Ca 2 As. Here, on both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high-symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including Ca 2 Sb, Ca 2 Bi, and Sr 2 Sb, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudobinary systems such as (Ca 1–x Sr x ) 2 As and Ca 2 As x Sb 1–x . Our work reveals rich and exotic TCI physics across the Ca 2 As family of materials and demonstrates a complete roadmap for uncovering TCIs topological nature based on first-principles calculations. Such a method can be broadly applied in searching for new TCIs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Bandwidth controlled insulator-metal transition in BaFe 2 S 3 : A Mössbauer study under pressure

BaFe 2 S 3 is a quasi one-dimensional Mott insulator that orders antiferromagnetically below 125 K. The application of pressure induces a transition to a metallic state, and superconductivity emerges.The evolution of the magnetic behavior on increasing pressure has up to now been either studied indirectly by means of transport measurements, or by using local magnetic probes only in the low pressure region. Here, we investigate the magnetic properties of BaFe 2 S 3 up to 9.9GPa by means of synchrotron 57 Fe Mossbauer spectroscopy experiments, providing the First local magnetic phase diagram. The magnetic ordering temperature increases up to 185(5) K at 7.5GPa, and is fully suppressed at 9.9GPa. The low-temperature magnetic hyperne field is continuously reduced from 12.9 to 10.3T between 1.4 and 9.1GPa, followed by a sudden drop to zero at 9.9GPa indicating a First-order phase transition. The pressure dependence of the magnetic order in BaFe2S3 can be qualitatively explained by a combination of a bandwidth-controlled insulator-metal transition as well as a pressure enhanced exchange interaction between Fe-atoms and Fe 3d-S 3p hybridization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗