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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

The reverse quantum limit and its implications for unconventional quantum oscillations in YbB12

Abstract The quantum limit in a Fermi liquid, realized when a single Landau level is occupied in strong magnetic fields, gives rise to unconventional states, including the fractional quantum Hall effect and excitonic insulators. Stronger interactions in metals with nearly localized f -electron degrees of freedom increase the likelihood of these unconventional states. However, access to the quantum limit is typically impeded by the tendency of f -electrons to polarize in a strong magnetic field, consequently weakening the interactions. In this study, we propose that the quantum limit in such systems must be approached in reverse, starting from an insulating state at zero magnetic field. In this scenario, Landau levels fill in the reverse order compared to regular metals and are closely linked to a field-induced insulator-to-metal transition. We identify YbB 12 as a prime candidate for observing this effect and propose the presence of an excitonic insulator state near this transition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Synthesis and Magnetic Properties of Soft Magnetic Composites Based on Silicone Resin-Coated Iron Powders

Iron-based soft magnetic composites with silicone resin coating have been designed to reduce the core loss and improve the magnetic properties of cores. The silicone resin serving as electrical insulating layer was modified by 3-glycidoxypropyltrimethoxy silane, and the effect of silicone resin content, coupling agent, annealing treatment, and thermal cycling on the magnetic properties of composites was investigated in the frequency range from DC to 150 kHz. The coated silicone resin insulating layer was characterized by scanning electron microscopy and energy dispersive spectroscopy. The results showed that the optimum coating content of silicone resin to attain the acceptable permeability and the minimum magnetic loss for the prepared composites is 4 wt%. The sample with silicone resin coat exhibited better DC-bias superposition characteristics compared to the uncoated sample, 6 wt% silicone resin coating sample have excellent percentage effective permeability (84.8%) under 50 Oe applied field. AC magnetic properties evaluated at 1 and 100 kHz do not vary with a 95% confidence interval with the number of thermal cycles; therefore, the properties of the silicone resin-insulating composites are stable under thermal cycling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Robust Surface States and Coherence Phenomena in Magnetically Alloyed SmB 6

Samarium hexaboride is a candidate for the topological Kondo insulator state, in which Kondo coherence is predicted to give rise to an insulating gap spanned by topological surface states. Here we investigate the surface and bulk electronic properties of magnetically alloyed Sm 1-x M x B 6 (M = Ce, Eu), using angle-resolved photoemission spectroscopy and complementary characterization techniques. Remarkably, topologically nontrivial bulk and surface band structures are found to persist in highly modified samples with up to 30% Sm substitution and with an antiferromagnetic ground state in the case of Eu doping. The results are interpreted in terms of a hierarchy of energy scales, in which surface state emergence is linked to the formation of a direct Kondo gap, while low-temperature transport trends depend on the indirect gap.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Magnetization switching using topological surface states

Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi 2 Se 3 and an insulating ferromagnet BaFe 12 O 19 . A charge current in Bi 2 Se 3 can switch the magnetization in BaFe 12 O 19 up and down. When the magnetization is switched by a field, a current in Bi 2 Se 3 can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe 12 O 19 . These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Enhancement of chiral edge currents in ($d$+1)-dimensional atomic Mott-band hybrid insulators

We consider the effect of a local interatomic repulsion on synthetic heterostructures where a discrete synthetic dimension is created by Raman processes on top of SU(N) S U ( N ) -symmetric two-dimensional lattice systems. At a filling of one fermion per site, increasing the interaction strength, the system is driven towards a Mott state which is adiabatically connected to a band insulator. The chiral currents associated with the synthetic magnetic field increase all the way to the Mott transition, where they reach the maximum value, and they remain finite in the whole insulating state. The transition towards the Mott-band insulator is associated with the opening of a gap within the low-energy quasiparticle peak, while a mean-field picture is recovered deep in the insulating state.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Quantum oscillations in strong magnetic fields, berry phase, and superconductivity in three-dimensional topological Bi{sub 2–x}Cu{sub x}Se{sub 3} insulators

Two-dimensional (2D) Shubnikov–de Haas oscillations and 2D Hall oscillations are observed in 3D copper-doped Bi{sub 2}Se{sub 3} single crystals in magnetic fields up to 19.5 T at temperatures down to 0.3 K. Three samples with a high bulk carrier concentration (n ≈ 10{sup 19}–10{sup 20} cm{sup –3}) are studied. The rotation of the samples in a magnetic field shows that these oscillations are related to numerous parallel 2D conducting channels 1–5 nm thick. Their basic kinetic parameters are found. Quantized Hall resistance R{sub xy} is detected in 1-nm-thick 2D conducting channels at high fields. The distance Δ(1/R{sub xy}) between the steps in the field dependence of 1/R{sub xy} is found to be constant for different Landau levels, 1.3e{sup 2}/h per 1-nm-thick layer. The constructed fan diagrams of 2D Landau levels for various angles of sample inclination with respect to the magnetic field direction allowed us to conclude that the Berry phase in the 2D conducting channels is γ ≈ π and independent of the magnetic field direction. When studying the angular dependence of upper resistive critical magnetic field H{sub c2} in one of the superconducting samples, we showed that it can be considered as a bulk superconductor consisting of superconducting layers with an effective thickness of about 50 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

The reverse quantum limit and its implications for unconventional quantum oscillations in YbB 12

The quantum limit in a Fermi liquid, realized when a single Landau level is occupied in strong magnetic fields, gives rise to unconventional states, including the fractional quantum Hall effect and excitonic insulators. Stronger interactions in metals with nearly localized $f$-electron degrees of freedom increase the likelihood of these unconventional states. However, access to the quantum limit is typically impeded by the tendency of $f$-electrons to polarize in a strong magnetic field, consequently weakening the interactions. In this study, we propose that the quantum limit in such systems must be approached in reverse, starting from an insulating state at zero magnetic field. In this scenario, Landau levels fill in the reverse order compared to regular metals and are closely linked to a field-induced insulator-to-metal transition. We identify YbB 12 as a prime candidate for observing this effect and propose the presence of an excitonic insulator state near this transition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Energy Storage via Topological Spin Textures

We formulate an energy-storage concept based on the free energy associated with metastable magnetic configurations. Despite the active magnetic region of the battery being electrically insulating, it can sustain effective hydrodynamics of spin textures, whose conservation law is governed by topology. To illustrate the key physics and potential functionality, we focus here on the simplest quasi-one-dimensional case of planar winding of the magnetic order parameter. The energy is stored in the metastable winding number, which can be injected electrically by an appropriately tailored spin torque. Because of the nonvolatility and the endurance of magnetic systems, the injected energy can be stored essentially indefinitely, with the topological charge cycles that do not degrade over time.

25 ENERGY STORAGE↗
DOE OSTI2017

Topological crystalline magnets: Symmetry-protected topological phases of fermions

Here, we introduce a novel class of interaction-enabled topological crystalline insulators in two- and three-dimensional electronic systems, which we call “topological crystalline magnet.” It is protected by the product of the time-reversal symmetry T and a mirror symmetry or a rotation symmetry R . A topological crystalline magnet exhibits two intriguing features: (i) it cannot be adiabatically connected to any Slater insulator and (ii) the edge state is robust against coupling electrons to the edge. These features are protected by the anomalous symmetry transformation property ( RT ) 2 = -1 of the edge state. Finally, an anisotropic response to the external magnetic field can be an experimental signature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Coulomb blockade-tuned indirect exchange in ferromagnetic nanostructures

Here, we develop a theory of the reversible switching of the magnetic state of the ferromagnet-insulator-normal metalferromagnet (FINF) nanostructure. The switching is controlled by tuning the Coulomb blockade strength via the gate voltage on the normal metal granule. The proposed mechanism allows for realizing the switching without passing a dissipative current through the structure.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Non-Centrosymmetric Sr 2 IrO 4 Obtained Under High Pressure

Sr 2 IrO 4 with strong spin-orbit coupling (SOC) and Hubbard repulsion (U) hosts Mott insulating states. The similar crystal structure, magnetic and electronic properties, particularly the d-wave gap observed in Sr 2 IrO 4 enhanced the analogies to cuprate high-$T_c$ superconductor, La 2 CuO 4 . The incomplete analogy was due to the lack of broken inversion symmetry phases observed in Sr 2 IrO 4 . Here, under high pressure and high temperature conditions, we report a non-centrosymmetric Sr 2 IrO 4 . The crystal structure and its noncentrosymmetric character were determined by single crystal X-ray diffraction and high-resolution scanning transmission electron microscopy (HR-STEM). The magnetic characterization confirms the Ir 4+ with $\textit{S}$ = 1/2 at low temperature in Sr 2 IrO 4 with magnetic ordering occurred at around 86 K, where a larger moment is observed than the ambient pressure Sr 2 IrO 4 . Moreover, the resistivity measurement shows three-dimensional Mott variable-range hopping existed in the system. Further, this non-centrosymmetric Sr 2 IrO 4 phase appears to be a unique material to offer further understanding of high-$T_c$ superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

A Semiconducting Conjugated Radical Polymer: Ambipolar Redox Activity and Faraday Effect

Investigations of magnetism in electronically coupled polyradicals have largely focused on applications in photonic and magnetic devices, wherein radical polymers were found to possess molecularly tunable and cooperative magnetic properties. Radical polymers with nonconjugated insulating backbones have been intensively investigated previously; however the integration of radical species into conducting polymer backbones is at an early stage. We report herein 1,3-bisdiphenylene-2-phenylallyl (BDPA)-based conjugated radical polymers that display ambipolar redox activities and conductivities. Moreover, these radical polymers were demonstrated to be promising magneto-optic (MO) materials with Faraday rotations wherein the sign is modulated by the radical character and display absolute Verdet constants up to (2.80 ± 0.84) × 10 4 deg T –1 m –1 at 532 nm. These values rival the performance of the present-day commercial inorganic MO materials (e.g., terbium gallium garnet, V = –1.0 × 10 4 deg T –1 m –1 at 532 nm). Lastly, the structure property studies detailed herein reveal the promise of multifunctional conjugated radical polymers as responsive MO materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Magnetoelectric transport and quantum interference effect in ultrathin manganite films

The magnetoelectric transport behavior with respect to the thicknesses of ultrathin La{sub 0.9}Sr{sub 0.1}MnO{sub 3} films is investigated in detail. The metal-insulator phase transition, which has never been observed in bulk La{sub 0.9}Sr{sub 0.1}MnO{sub 3}, is found in ultrathin films with thicknesses larger than 6 unit cells. Low-temperature resistivity minima appeared in films with thicknesses less than 10 unit cells. This is attributed to the presence of quantum interference effects. These data suggest that the influence of the weak localization becomes much pronounced as the film thickness decreases from 16 to 8 unit cells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

New type of Weyl semimetal with quadratic double Weyl fermions

Significance We predict a new Weyl semimetal candidate. This is critically needed for this rapidly developing field as TaAs is the only known Weyl semimetal in nature. We show that SrSi 2 has many new and novel properties not possible in TaAs. Our prediction provides a new route to studying the elusive Weyl fermion particles originally considered in high-energy physics by tabletop experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Magnetotransport of single crystalline YSb

Here, we report magnetic field dependent transport measurements on a single crystal of cubic YSb together with first principles calculations of its electronic structure. The transverse magnetoresistance does not saturate up to 9 T and attains a value of 75 000% at 1.8 K. The Hall coefficient is electron-like at high temperature, changes sign to hole-like between 110 and 50 K, and again becomes electron-like below 50 K. First principles calculations show that YSb is a compensated semimetal with a qualitatively similar electronic structure to that of isostructural LaSb and LaBi, but with larger Fermi surface volume. The measured electron carrier density and Hall mobility calculated at 1.8 K, based on a single band approximation, are $6.5\times {{10}^{20}}$ cm –3 and $6.2\times {{10}^{4}}$ cm 2 Vs –1 , respectively. These values are comparable with those reported for LaBi and LaSb. Like LaBi and LaSb, YSb undergoes a magnetic field-induced metal-insulator-like transition below a characteristic temperature T m , with resistivity saturation below 13 K. Thickness dependent electrical resistance measurements show a deviation of the resistance behavior from that expected for a normal metal; however, they do not unambiguously establish surface conduction as the mechanism for the resistivity plateau.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Higher Chern numbers in multilayer Lieb lattices ( N ≥ 2 ): Topological transitions and quadratic band crossing lines

In this work, we consider a hitherto unexplored setting of a stacked multilayer (N) Lieb lattice which undergoes an unusual topological transition in the presence of intralayer spin-orbit coupling (SOC). The specific stacking configuration induces an effective nonsymmorphic two-dimensional lattice structure, even though the constituent monolayer Lieb lattice is characterized by a symmorphic space group. This emergent nonsymmorphicity leads to multiple doubly degenerate bands extending over the edge of the Brillouin zone (i.e., quadratic band crossing lines). In the presence of intralayer SOC, these doubly degenerate bands typically form three N-band subspaces, mutually separated by two band gaps. We analyze the topological properties of these multiband subspaces, using specially devised Wilson loop operators to compute non-Abelian Berry phases in order to show that they carry a higher Chern number N.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Magnetic impurity as a local probe of the $U(1)$ quantum spin liquid with spinon Fermi surface

We solve the problem of a magnetic impurity coupled to a U(1) quantum spin liquid with a spinon Fermi surface, and we compute the impurity spectral function. Using the slave rotor mean-field approach combined with gauge field fluctuations, we find that a peak located at the top of the lower Hubbard band and one at the bottom of the upper Hubbard band can emerge in the impurity spectral function. The peaks at the Hubbard band edges arise from the gauge field fluctuation-induced spinon-chargon binding inside the spinon Kondo screening cloud of the magnetic impurity. For a magnetic impurity embedded in a Mott insulator, our findings suggest that the emergence of a pair of peaks at the Hubbard band edges in the impurity density of states spectra provides strong evidence that the host Mott insulator is a U(1) quantum spin liquid with a spinon Fermi surface.

36 MATERIALS SCIENCE↗