We investigate the emergence of long-range electron hopping mediated by cavity vacuum fields in disordered quantum Hall systems. We show that the counter-rotating (antiresonant) light-matter interaction produces an effective hopping between disordered eigenstates within the last occupied Landau band. The process involves a number of intermediate states equal to the Landau degeneracy: each of these states consists of a virtual cavity photon and an electron excited in the next Landau band with the same spin. We study such a cavity-mediated hopping mechanism in the dual presence of a random disordered potential and a wall potential near the edges, accounting for both paramagnetic coupling and diamagnetic renormalization. We determine the cavity-mediated scattering rates, showing the impact on both bulk and edge states. The effect for edge states is shown to increase when their energy approaches the disordered bulk band, while for higher energy, the edge states become asymptotically free. We determine the scaling properties while increasing the Landau band degeneracy. Consequences on the quantum Hall physics and future perspectives are discussed.

We perform a quantum transport study of Hall viscous liquid in multiterminal narrow Hallbar devices of a high-mobility two-dimensional electron system (2DES) in GaAs/AlGaAs heterostructure. In the nonlocal transport measurements of vicinity geometry under magnetic fields $\left(B\right)$, we observe that the absolute negative magnetoresistance $\left(R_{xx}\right)$ value $(R_{\text{min}}<0)$ and the corresponding magnetic field $\left(B_{\text{min}}\right)$ are both inversely proportional to the adjacent Hallbar arm distance $\left(L\right)$. The occurrence of negative resistance is dependent on the characteristic lengths of the devices and the electron flow direction under $B$-fields. The minimal resistance $R_{\text{min}}$ occurs when the cyclotron radius $R_C$ approximates $L$. Multiples of high-order ($n\mathrm{th}$-order) $R_{\text{min},n}$'s persist from low to high magnetic fields in a large sample size of $L=5\mu \mathrm{m}$. Our experimental study reveals the transport behaviors in the vicinity regime, where the negative resistances depict viscous electronic flows in the high-mobility 2DES. In addition, the negative and high-order minimal resistances expand to a magnetic field of several kGs.

The nitrogen-vacancy (NV) center in diamond is an ideal candidate for quantum sensing because of its excellent spin coherence property as well as the possibility for optical initialization and readout. Previous studies, on the other hand, have typically been conducted at low or room temperature. The inability to fully understand the coherence properties of the NV center at high temperatures limits NV's further applications. We systematically investigate the coherence properties of NV center ensemble at temperatures ranging from 300 K to 600 K in this paper. Coherence time $T_2$ decreases rapidly from $184\mu \text{s}$ at 300 K to $30\mu \text{s}$ at 600 K due to the interaction with paramagnetic impurities. At all experiment temperatures, both single-quantum and double-quantum transitions exhibit a $T^5$ relaxation rate, which is attributed to the two-phonon Raman process. Nonetheless, the inhomogeneous dephasing time $T_2^{*}$ and thermal-echo decoherence time $T_{\text{TE}}$ remain almost unchanged at temperature up to 600 K. A thermal-echo-based thermometer is demonstrated to have a sensitivity of $41\text{mK}/\sqrt{\mathrm{Hz}}$ at 450 K. These findings will pave the way for NV-based high-temperature sensing and provide a more comprehensive understanding of solid-state qubit decoherence.

The fluctuation behavior of triple quantum dots (TQDs) has, so far, largely focused on current cumulants in the long-time limit via full counting statistics. Given that TQDs are nontrivial open quantum systems with many interesting features, such as Aharonov-Bohm interference and coherent population blocking, new fluctuating-time statistics, such as the waiting time distribution (WTD), may provide more information than just the current cumulants alone. In this paper, we use a Born-Markov master equation to calculate the standard and higher-order WTDs for coherently coupled TQDs arrayed in triangular ring geometries for several transport regimes. In all cases, we find that the WTD displays coherent oscillations that correspond directly to individual time-dependent dot occupation probabilities, a result also reported recently [G. Engelhardt and J. Cao, Phys. Rev. B 99, 075436 (2019)]. Our analysis, however, goes beyond the single-occupancy and single waiting time regimes, investigating waiting time behavior for TQDs occupied by multiple electrons and with finite electron-electron interactions. We demonstrate that in these regimes of higher occupancy, quantum coherent effects introduce correlations between successive waiting times, which we can tune via an applied magnetic field. We also show that correlations can be used to distinguish between TQD configurations that have identical full counting statistics and that dark states can be tuned with Aharonov-Bohm interference for more complicated regimes than single occupancy.

Recently, kagome materials have become an engrossing platform to study the interplay among symmetry, magnetism, topology, and electron correlation. The latest works on $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ ($R$ = rare-earth metal) compounds have illustrated that this family could be intriguing to investigate various physical phenomena due to large spin-orbit coupling and strong magnetic ordering. However, combined transport and spectroscopic studies in $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ materials are still limited. Here, we report magnetic, magnetotransport, and angle-resolved photoemission spectroscopy measurements of a kagome magnet ${\mathrm{ErMn}}_6{\mathrm{Sn}}_6$ that undergoes antiferromagnetic ($T_N=345\mathrm{K}$) to ferrimagnetic ($T_C=68\mathrm{K}$) phase transitions in the presence of a field. We observe large anomalous and topological Hall effects serving as transport signatures of the nontrivial Berry curvature. The isothermal magnetization exhibits strong anisotropic nature and the topological Hall effect of the compound depends on the critical field of metamagnetic transition. Our spectroscopic results complemented by theoretical calculations show the multiorbital kagome fermiology. This Letter provides new insight into the tunability and interplay of topology and magnetism in a kagome magnet.

The understanding and control of charge carrier interactions with defects at buried insulator/semiconductor interfaces is essential for achieving optimum performance in modern electronics. Here, we report on the use of scanning ultrafast electron microscopy (SUEM) to remotely probe the dynamics of excited carriers at a Si surface buried below a thick thermal oxide. Our measurements illustrate a previously unidentified SUEM contrast mechanism, whereby optical modulation of the space-charge field in the semiconductor modulates the electric field in the thick oxide, thus affecting its secondary electron yield. By analyzing the SUEM contrast as a function of time and laser fluence we demonstrate the diffusion mediated capture of excited carriers by interfacial traps.

The ability to tailor a material's electronic properties using density driven disordering has emerged as a powerful route to materials design. The observation of anomalous structural and electronic behavior in the rutile to ${\mathrm{CaCl}}_2$ phase transition in ${\mathrm{SnO}}_2$ led to the prediction that such behavior is inherent to all oxides experiencing such a phase transition sequence [Smith et al., J. Phys. Chem. Lett. 10, 5351 (2019)]. Here, the ultrawide band gap semiconductor ${\mathrm{GeO}}_2$ is confirmed to exhibit anomalous behavior during the rutile to ${\mathrm{CaCl}}_2$ phase transition. A phase pure rutile ${\mathrm{GeO}}_2$ sample synthesized under high-pressure, high-temperature conditions is probed using synchrotron diffraction and x-ray and optical spectroscopy under high pressure conditions. Density functional theory calculations show that the enthalpic barrier to displacing an oxygen along the $B_{1g}$ librational mode decreases with pressure leading up to the rutile to ${\mathrm{CaCl}}_2$ phase transition. The band structure of the distorted state shows that such oxygen displacements form small polarons.

We studied structure and dynamics in liquid Ge and ${\text{Si}}_{20}{\text{Ge}}_{80}$. Quasielastic neutron scattering (QNS) was employed to accurately determine the Ge self-diffusion coefficient. Static structure factors were measured using neutron and x-ray diffraction. Containerless processing via electromagnetic levitation (EML) enabled to obtain structure factors over a broad temperature range from the metastable regime of the undercooled melt to several hundred Kelvin above the melting point. Moreover, isotopic substitution and combination with x-ray diffraction allowed to determine the partial structure factors $S_{\mathrm{NN}}\left(q\right), S_{\mathrm{NC}}\left(q\right), S_{\mathrm{GeGe}}\left(q\right)$, and $S_{\mathrm{GeSi}}\left(q\right)$ for liquid ${\text{Si}}_{20}{\text{Ge}}_{80}$. The topological structures of liquid Ge and ${\text{Si}}_{20}{\text{Ge}}_{80}$ are very similar. In addition, the Ge self-diffusion coefficients in liquid Ge and ${\text{Si}}_{20}{\text{Ge}}_{80}$ are equal within error limits.

Spin angular momentum and mechanical rotation both contribute to the total angular momentum of rigid bodies, leading to spin-rotational coupling via the Einstein–de Haas and Barnett effects. Here, we show that the revolutions of symmetric nanorotors can be strongly affected by a small number of intrinsic spins. The resulting dynamics are observable with freely rotating nanodiamonds with embedded nitrogen-vacancy centers and persist for realistically shaped near-symmetric particles, opening the door to torque-free schemes to control their rotations at the quantum level.

Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used $\mathrm{DFT}+\mathrm{U}$ calculations with antiferromagnetic supercells of ${\mathrm{La}}_{\text{2}}{\mathrm{CuO}}_{\text{4}}$ to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct optical gap and magnetic moments. We demonstrate that using distorted supercells within $\mathrm{DFT}+\mathrm{U}$ is a promising framework to model phonons in undoped cuprates and other perovskite oxides with complex, interrelated structural and electronic degrees of freedom.

We explored the variations of the topological magnetic textures of vortices, skyrmions, and skyrmioniums in magnetic elements of hemispherical-shell shape with respect to surface-normal uniaxial magnetic anisotropy constant $K_u$, Dzyaloshinskii-Moriya interaction (DMI) constant $D_{\mathrm{int}}$, and shell diameter $2R$. For given values of $2R$, the combination of $K_u$ and $D_{\mathrm{int}}$ plays a crucial role in the stabilization of those different spin textures. With decreasing $2R$, the geometrical confinement of hemispherical shells more significantly affects the stabilization of skyrmions owing to curvature-induced DM-like interaction. This effect is contrastingly dependent on the sign of $D_{\mathrm{int}}$: skyrmion formation is more favorable for positive $D_{\mathrm{int}}$ values, whereas it is less favorable for negative ones. A quite promising feature is that skyrmions can be stabilized even in the absence of intrinsic DMI for $2R<25\mathrm{nm}$. We also explored characteristic dynamic properties of skyrmions excited by in-plane and out-of-plane oscillating magnetic fields. Similarly to the fundamental dynamic modes found in planar dots, in-plane gyration and azimuthal spin-wave modes as well as out-of-plane breathing modes were found, but additional higher-frequency hybrid modes also appeared due to coupling between radially quantized and azimuthal spin-wave modes. Finally, we found a switching behavior of skyrmion polarity through a transient skyrmionium state using very-low-strength AC magnetic fields. This work provides further physical insight into the static and dynamic properties of skyrmions in curved-geometry nanodots and suggests potential applications to low-power-consumption and ultra-high-density information-storage devices.

We report the observation of the spin Hall magnetoresistance (SMR) in a paramagnetic insulator. By measuring the transverse resistance in a $\mathrm{Pt}/{\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG) system at low temperatures, paramagnetic SMR is found to appear with an intensity that increases with the magnetic field aligning GGG's spins. The observed effect is well supported by a microscopic SMR theory, which provides the parameters governing the spin transport at the interface. Our findings clarify the mechanism of spin exchange at a Pt/GGG interface, and demonstrate tunable spin-transfer torque through the field-induced magnetization of GGG. In this regard, paramagnetic insulators offer a key property for future spintronic devices.

The emergent concept of the magnetic charge quasiparticle provides a new realm to study the evolution of magnetic properties in two-dimensional artificially frustrated magnets. We report on the exploration of magnetic phases due to various magnetic charge correlation using the complementary numerical techniques of micromagnetic and distorted-wave Born approximation simulations in artificial permalloy honeycomb lattice. The honeycomb element length varies between 10 nm and 100 nm, while the width and thickness are kept within the single domain limit. In addition to the charge ordered loop state, we observe disordered charge arrangement, characterized by the random distribution of $\pm \mathrm{Q}$ charges, in single domain size honeycomb lattice. As the length of the honeycomb element increases, low multiplicity magnetic charges tend to form contiguous bands in thinner lattice. Thin honeycomb lattice with 100 nm element length exhibits a perfect spin ice pattern, which remains unaffected by the modest increase in the width of element size. We simulate scattering profiles under the pretext of distorted-wave Born approximation formalism for the micromagnetic phases. The results are expected to provide useful guidance in the experimental investigation of magnetic phases in an artificial honeycomb magnet.

We study the Josephson effect in spin-triplet superconductor−quantum anomalous Hall insulator−spin-triplet superconductor junctions using the nonequilibrium Green's function method. The current-phase difference relations show strong dependence on the orientations of the $\mathbf{d}$ vectors in superconductors. We focus on two $\mathbf{d}$-vector configurations, a parallel one with the left and right $\mathbf{d}$ vectors being in the same direction and a nonparallel one with the left $\mathbf{d}$ vector fixed at the $z$ axis. For the parallel configuration, the $0\text{−}\pi$ transition can be realized when one rotates the $\mathbf{d}$ vectors from the parallel to the junction plane to the perpendicular direction. The ${\varphi }_0$ phase with nonzero Josephson current at zero phase difference can be obtained as long as $d_x{d}_z\ne 0$. For the nonparallel configuration, the $0\text{−}\pi$ transition and the ${\varphi }_0$ phase still exist. The condition for the formation of the ${\varphi }_0$ phase becomes $d_{Rx}\ne 0$. The switch effects of the Josephson current are found in both configurations when the $\mathbf{d}$ vectors are rotated in the $xy$ plane. Furthermore, the symmetries satisfied by the current-phase difference relations are analyzed in detail by the operations of the time-reversal, mirror-reflections, the spin-rotation, and the gauge transformation, which can well explain the above selection rules for the ${\varphi }_0$ phase. Our results reveal the peculiar Josephson effect between spin-triplet superconductors and the quantum anomalous Hall insulator, which provide helpful phases and effects for device designs. The distinct current-phase difference relations for different orientations may be used to determine the direction of the $\mathbf{d}$ vector in the spin-triplet superconductor.

Recent experiments in quantum simulators have provided evidence for the many-body localized (MBL) phase in one-dimensional (1D) and two-dimensional (2D) bosonic quantum matter. The theoretical study of such a bosonic MBL phase, however, is a daunting task due to the unbounded nature of its Hilbert space. In this work, we introduce a method to compute the long-time real-time evolution of 1D and 2D bosonic systems in an MBL phase at strong disorder and weak interactions. We focus on local dynamical indicators that are able to distinguish an MBL phase from an Anderson localized one. In particular, we consider the temporal fluctuations of local observables, the spatiotemporal behavior of two-time correlators and out-of-time-correlators. We show that these few-body observables can be computed with a computational effort that depends only polynomially on the system size but is independent of the target time, by extending a recently proposed numerical method [G. De Tomasi, F. Pollmann, and M. Heyl, Phys. Rev. B 99, 241114(R) (2019)] to mixed states and bosons. Our method also allows us to surrogate our numerical study with analytical considerations of the time-dependent behavior of the studied quantities.

The physical properties and electronic structure of the layered SrAgAs single crystal have been systematically studied via transport measurements and first-principles calculations. The Landau level index derived from the low-temperature Shubnikov–de Haas oscillations suggests nontrivial Berry phase of the Fermi surface. In addition, the compound exhibits high transport mobility in the whole measured temperature range. First-principles calculations indicate that the stoichiometry compound is an ideal Dirac semimetal close to semimetal-topological-insulator transition. Due to its perfect Dirac semimetallic nature, the shape and size of the Fermi surfaces could be drastically tuned by charge doping.

We investigate a mechanism for phase transitions between topological magnon phases on the breathing kagome lattice ferromagnets with Dzyaloshinskii-Moriya interaction, which is induced by the lattice deformation. With the linear spin-wave theory, we calculate the magnonic Chern number, topological phase diagram, and magnon thermal Hall conductivity at low temperature with tunable modulated exchange interactions due to the lattice deformation. We show that the modulated exchange interactions and Dzyaloshinskii-Moriya interaction strength determine the band topology of magnons. We find a sign reversal of the thermal Hall conductivity during topological phase transitions explained in terms of topological edge modes and their propagation directions. We also discuss candidate materials to realize our ideas.

Spin-filter van der Waals Magnetic tunnel junctions (sf-vdW MTJs) formed by an interlayer antiferromagnetic (AFM) vdW semiconductor as a barrier have exhibited promising prospects in achieving a high tunneling magnetoresistance (TMR) ratio in MTJs. Here, using first-principles calculations, we investigate the spin-dependent transport and the TMR effect of sf-vdW MTJs formed by sandwiching bilayer and trilayer $\mathrm{Ni}{\mathrm{Br}}_2$ barriers between two graphite electrodes ($\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_2/\mathrm{Gr}$ sf-vdW MTJs). Similar to the experimental results of sf-vdW MTJs formed by a few-layer $\mathrm{Cr}{\mathrm{I}}_3$ barrier, the TMR ratios of the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_2/\mathrm{Gr}$ sf-vdW MTJs increase first with the increase of bias voltage and decrease with the further increase of bias voltage after reaching the highest points because the conduction bands of the interlayer ferromagnetic (FM) $\mathrm{Ni}{\mathrm{Br}}_2$ barrier at the $K$ points go into the bias window earlier than those of the interlayer AFM $\mathrm{Ni}{\mathrm{Br}}_2$ barrier with the increase of bias voltage. Compared to the TMR ratios of about 170% and 206% at zero bias voltage, the TMR ratios of the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_2/\mathrm{Gr}$ sf-vdW MTJs with bilayer and trilayer $\mathrm{Ni}{\mathrm{Br}}_2$ barriers are largely increased about 34 and 67 times by the optimized bias voltage, respectively. Correspondingly, a giant TMR ratio of about 6000% and 14 000% can be achieved in the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_2/\mathrm{Gr}$ sf-vdW MTJs with bilayer and trilayer $\mathrm{Ni}{\mathrm{Br}}_2$ barriers at 0.14 and 0.125 V bias voltage, respectively. Our results elucidate the mechanism of bias voltage induced giant TMR ratio in the $\mathrm{Gr}/\mathrm{Ni}{\mathrm{Br}}_2/\mathrm{Gr}$ sf-vdW MTJs and provide promising routes for developing MTJs with a high TMR ratio.