By means of the Lanczos method, a numerically efficient theoretical approach, we study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in a graphene bubble. We find that the RKKY interaction in a graphene bubble can be larger or smaller than the corresponding result in pristine graphene by a few orders of magnitude, depending on the sublattice attribution and pseudomagnetic field strength where two magnetic impurities are positioned, which is due to the sublattice polarization of the low-energy electronic states in a strong pseudomagnetic field. If two magnetic impurities are both in the bubble region, the $R^{-3}$ decay rate of the RKKY interaction found in pristine graphene breaks down. But it recovers when one magnetic impurity is far away from the bubble center no matter where another impurity is located. When the Fermi level deviates from the Dirac point by carrier doping, the antiferromagnetic RKKY interaction between two magnetic impurities located at the opposite sublattices can be inverted to be ferromagnetic by altering the bubble height properly.

We investigate if a sharp topological transition in a metal with a large Fermi surface may be detected in transport measurements. In particular, we address if a skew scattering and a side jump on elastic disorder in the bulk of such a metal masks signatures of the topological transition. We conclude that certain transport coefficients exhibit discontinuous changes across the transition. These discontinuities are not smeared or dwarfed by the bulk metallic transport in a broad range of parameters.

The Berry-phase mediated valley-selected skew scattering in $\alpha -T_3$ lattices is demonstrated. The interplay of Lorentz and Berry forces in position and momentum spaces is revealed and analyzed. Many-body screening of the electron-impurity interaction is taken into account to avoid overestimation of back and skew scattering of electrons in the system. Triplet peak from skew interactions at two valleys is found in near-vertical and near-horizontal forward- and backward-scattering directions for small Berry phases and low magnetic fields. Magnetic-field dependence in both nonequilibrium and thermal-equilibrium currents is also presented for valley-dependent longitudinal and transverse transports mediated by a Berry phase. Mathematically, two Boltzmann moment equations are employed for computing scattering-angle distributions of nonequilibrium skew currents by using microscopic inverse energy- and momentum-relaxation times. Meanwhile, a valley-dependent unbalanced thermal-equilibrium anomalous Hall current induced by the Berry force in momentum space, due to different mobilities for two valleys, is also computed for comparisons.

We investigate atomic collapse in pseudospin-1 Dirac material systems whose energy band structure constitutes a pair of Dirac cones and a flat band through the conic intersecting point. We obtain analytic solutions of the Dirac-Weyl equation for the three-component spinor in the presence of a Coulomb impurity and derive a general criterion for the occurrence of atomic collapse in terms of the normalized strength of Coulomb interaction and the angular momentum quantum number. In particular, for the lowest angular momentum state, the solution coincides with that for pseudospin-1/2 systems, but with a reduction in the density of resonance peaks. For higher angular momentum states, the underlying pseudospin-1 wave functions exhibit a singularity at the point of zero kinetic energy. Divergence of the local density of states associated with the flat band leads to an inverse square type of singularity in the conductivity. These results provide insights into the physics of the two-body problem for relativistic quantum pseudospin-1 quasiparticle systems.

We present results from an experimental technique used to estimate the strength of Ta at extreme pressures (150 GPa) and strain rates (${10}^7{\mathrm{s}}^{-1}$). A graded-density impactor (GDI) was fabricated using sputter deposition to produce an approximately $40-\mu \mathrm{m}$-thick film containing alternating layers of Al and Cu. The thicknesses of the respective layers are adjusted to give an effective density gradient through the film. The GDIs were launched with a 2-stage light gas gun, and shock-ramp-release velocity profiles were measured over timescales of $\sim 10$ ns. Results are presented for the direct impact of the film onto LiF windows, which allows for a dynamic characterization of the GDI, as well as from impact onto thin ($\sim 40\mu \mathrm{m}$) sputtered Ta samples backed by a LiF window. The measurements were coupled with mesoscale numerical simulations to infer the strength of Ta, and the results agree well with other high-pressure platforms, particularly when strain-rate, microstructural, and thermodynamic-path differences are considered.

We have studied the partially ordered double perovskite (PODP) and spin-glass phase in $\mathrm{Sr}\left(\mathrm{M}{\mathrm{n}}_{1-x}{\mathrm{W}}_x\right){\mathrm{O}}_3$ ($x=0.20$ to 0.40) using neutron powder diffraction (NPD), muon spin relaxation (μSR), magnetic susceptibility (χ), and x-ray photoelectron spectroscopy measurements. Structural studies reveal that $\mathrm{SrM}{\mathrm{n}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_3$ undergoes a quasicontinuous transformation from simple perovskite ($Pm-3m$) to PODP ($P{2}_1/n$) phase as $x$ increases. ${\chi }_{\mathrm{dc}}\left(T\right)$ and ${\chi }_{\mathrm{ac}}\left(T\right)$ measurements show a sharp cusplike peak at spin-glass transition $T_g$. The muon relaxation rate (λ) peaks at $T_g$ following a critical growth, given by $\lambda =\lambda \left(0\right){\tau }^{-w}\left[\tau =\left(T-T_g\right)/T_g\right]$. No long-range magnetic order is observed in NPD below $T_g$. These measurements confirm a tunable spin-glass state of $\mathrm{SrM}{\mathrm{n}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_3$ for $0.2<x<0.4$. The spin-glass phase appears with the onset of $\mathrm{M}{\mathrm{n}}^{2+}$ cations, which induce competing ferro-antiferro interactions leading to exchange frustration. The $T_g$ decreases as the W content and $\mathrm{M}{\mathrm{n}}^{2+}$ concentration increase. Our results suggest that the spin-glass phase can be tuned through the relative concentration of 2+, 3+, and 4+ Mn ions.

Information scrambling, characterized by the out-of-time-ordered correlator (OTOC), has attracted much attention, as it sheds new light on chaotic dynamics in quantum many-body systems. The scale invariance, which appears near the quantum critical region in condensed matter physics, is considered to be important for the fast decay of the OTOC. In this paper, we focus on the one-dimensional spin-1/2 XXZ model, which exhibits quantum criticality in a certain parameter region, and investigate the relationship between scrambling and the scale invariance. We quantify scrambling by the averaged OTOC over the Pauli operator basis, which is related to the operator space entanglement entropy (OSEE). Using the infinite time-evolving block decimation method, we numerically calculate time dependence of the OSEE in the early-time region in the thermodynamic limit. We show that the averaged OTOC decays faster in the gapless region than in the gapped region. In the gapless region, the averaged OTOC behaves in the same manner regardless of the anisotropy parameter. This result is consistent with the fact that the low-energy excitations of the gapless region belong to the same universality class as the Tomonaga-Luttinger liquid with the central charge $c=1$. Furthermore, we estimate $c$ by fitting the numerical data of the OSEE with an analytical result of the two-dimensional conformal field theory, and confirm that $c$ is close to unity. Thus, our numerical results suggest that the scale invariance leads to a universal behavior of the OTOC that is independent of the anisotropic parameter, which reflects the universality of the two-dimensional conformal field theory at low temperatures. Although the one-dimensional XXZ model is integrable, our results suggest such a universal behavior of generic nonintegrable systems, because the Tomonaga-Luttinger liquid serves as a low-energy effective theory for many nonintegrable and integrable systems. On the other hand, the OTOC in our numerical result does not exhibit the exponential decay, as our parameter regime is far from the semiclassical limit.

We report a series of measurements on a $<1$ mg sample of ${}^{248}\mathrm{Cm}$ metal. Magnetization measurements are similar to those reported earlier. X-ray absorption and magnetic circular dichroism (XMCD) measurements at the ESRF synchrotron at the $M_{4,5}$ absorption edges show that the element has an orbital moment of $\sim 6\%$ of the spin moment, and the two are parallel. This confirms the predictions of intermediate coupling, which has been suggested for Cm by other spectroscopies. In an XMCD measurement 2.5 years later, a field of 17 T was found to induce a larger induced signal (by a factor of $\sim 2$) than that found earlier, indicating a change in the sample. We attribute this to the gradual loss of crystallinity with time since preparation, as a result of the damaged caused by fragments from the spontaneous fission within the sample itself. The final measurements aimed to determine the antiferromagnetic structure with resonant x rays were unable to observe any diffraction peaks from the sample. The ground-state antiferromagnetic structure remains unresolved.

Spin-triplet superconductors with time-reversal symmetry breaking can naturally lead to chiral domain walls in their interior. We study the magnetic properties emerging at the interface of chiral domains with opposite winding by focusing on the effects of a superconducting phase drop across the wall and an applied electric gating. The local inversion symmetry breaking at the domain wall drives mixed singlet-triplet pairing configurations that allow a phase- and electric-controllable magnetization with resulting parallel or antiparallel orientations on the two sides of the domain wall. The magnetic switching is also generally accompanied by both spin and charge currents flowing along the edges, whose amplitudes depend on the achieved parallel or antiparallel magnetic phase. The specific magnetoelectric properties of chiral domains with zero or nonvanishing net magnetization near the wall may have several implications. They can be employed to detect the presence of unconventional pairing as well as to design information storage units based on spin-polarized states attached to topological defects.

We study symmetry-protected topological (SPT) phases of matter in $2\mathrm{D}$ protected by symmetries acting on fractal subsystems of a certain type. Despite the total symmetry group of such systems being subextensively large, we show that only a small number of phases are actually realizable by local Hamiltonians. Which phases are possible depends crucially on the spatial structure of the symmetries, and we show that in many cases, no nontrivial SPT phases are possible at all. In cases where nontrivial SPT phases do exist, we give an exhaustive enumeration of them in terms of their locality.

We discuss an extension of higher-order topological phases to include bosonic systems. We present two spin models for a second-order topological phase protected by a global ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry. One model is built from layers of an exactly solvable cluster model for a one-dimensional ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ topological phase, while the other is built from more conventional spin couplings (XY or Heisenberg). These models host gapped, symmetry-protected topological phases on their edges, and corner modes that fall into a projective representation of the symmetry. Using Jordan-Wigner transformations we show that both our models are related to a bilayer of free Majorana fermions that form a fermionic second-order topological phase. We also discuss how our models can be extended to three dimensions to form a third-order topological phase.

It is shown theoretically that the confinement of an electron at a repulsive potential can exist in nanostructures subjected to a strong high-frequency electromagnetic field. As a result of the confinement, the metastable bound electron state of the repulsive potential appears. This effect can lead, particularly, to electron pairing in nanostructures containing conduction electrons with different effective masses.

We report the coherent study of structural, transport, and ultrafast dynamic properties of ${\mathrm{V}}_{1-x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_2$ thin films. Films of a solid solution of ${\mathrm{V}}_{1-x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_2(0\le x\le 1)$ are prepared on (0001) sapphire substrates using reactive target bias ion beam deposition. We found that, at room temperature, the low-temperature lattice symmetry of pure ${\mathrm{VO}}_2$ (monoclinic, $P{2}_1/c$) and ${\mathrm{NbO}}_2$ (tetragonal, $I{4}_1/a$) is only maintained at low substituting levels ($x<0.1$ and $x>0.9$) and regular rutile lattices are adopted by films with higher substituting levels ($0.1<x<0.9$). We characterized the dc transport properties and the photoinduced ultrafast dynamics at the terahertz frequency range, both of which demonstrate strong dependence on the film composition. At $x\sim 0.5$, both the dc conductivity and ultrafast response diminish, which corresponds to a strong localization of free charge carriers in the system. The strong composition dependence of film transport, ultrafast dynamic, and insulator-metal transition properties promises approaches to improve the performance of devices based on transition metal oxides.

Hybrid superconducting devices based on high-mobility two-dimensional electron gases with strong spin-orbit coupling are considered to offer a flexible and scalable platform for topological quantum computation. Here, we report the realization and electrical characterization of hybrid devices based on high-quality InSb nanosheets and superconducting niobium (Nb) electrodes. In these hybrid devices, we observe gate-tunable proximity-induced supercurrent and multiple Andreev reflections, indicating a transparent Nb-InSb nanosheet interface. The high critical magnetic field of Nb combined with high-mobility InSb nanosheets allows us to exploit the transport properties in the exotic regime where the superconducting proximity effect coexists with the quantum Hall effect. Transport spectroscopy measurements in such a regime reveal an enhancement of the conductance at the quantum Hall plateaus, accompanied by a pronounced zero-bias peak in the differential conductance. We discuss that these features originate from the formation of Andreev edge states at the superconductor-InSb nanosheet interface in the quantum Hall regime. In addition to shedding light on the interplay between superconductivity and quantum Hall effect, our work opens a new possibility to develop hybrid superconducting devices based on 2D semiconductor nanosheets with strong spin-orbit coupling.

Two-photon transition rates are investigated in resonance to the ground state in wurtzite GaN/AlN quantum dots. The ground-state transition is two-photon allowed because of the electron-hole separation inherent to polar wurtzite III–nitride heterostructures. We show that this built-in parity-breaking mechanism can allow deterministic triggering of single-photon emission via coherent two-photon excitation. Radiative lifetimes obtained for single-photon relaxation are in good agreement with available time-resolved microphotoluminescence experiments, indicating the reliability of the employed computational framework based on eight-band $\mathit{k}·\mathit{p}$ wave functions. Two-photon singly induced emission is explored in terms of possible cavity and nondegeneracy enhancement of two-photon processes.

We study theoretically the interaction of topological Weyl semimetals with an ultrafast optical pulse. The electron dynamics in such material is coherent and highly anisotropic. For some directions of pulse polarization, the electric dynamics is irreversible, which means that the residual electron conduction-band population after the pulse is comparable to the maximum conduction-band population during the pulse. For other directions of polarization, the electron dynamics is highly reversible and, after the pulse, the electron system returns to its initial state with almost zero conduction-band population. Such high anisotropy in electron dynamics is related to anisotropy in interband dipole matrix elements. In the reciprocal space, the electron conduction-band population density shows hot spots near the Weyl points. The optical pulse also causes net charge transfer through the system. The direction of transfer is the same as the direction of the field maximum. The transferred charge has highly anisotropic dependence on polarization direction with almost zero transferred charge for some directions.

We study in this paper effects of Dzyaloshinskii-Moriya (DM) magnetoelectric coupling between ferroelectric and magnetic layers in a superlattice formed by alternate magnetic and ferroelectric films. Magnetic films are films of simple cubic lattice with Heisenberg spins interacting with each other via an exchange $J$ and a DM interaction with the ferroelectric interface. Electrical polarizations of $\pm 1$ are assigned at simple cubic lattice sites in the ferroelectric films. We determine the ground-state (GS) spin configuration in the magnetic film. In zero field, the GS is periodically noncollinear and in an applied field $\mathbf{H}$ perpendicular to the layers, it shows the existence of skyrmions at the interface. Using the Green's function method, we study the spin waves (SW) excited in a monolayer and also in a bilayer sandwiched between ferroelectric films, in zero field. We show that the DM interaction strongly affects the long-wavelength SW mode. We calculate also the magnetization at low temperature $T$. We use next Monte Carlo simulations to calculate various physical quantities at finite temperatures such as the critical temperature, the layer magnetization, and the layer polarization, as functions of the magnetoelectric DM coupling and the applied magnetic field. Phase transition to the disordered phase is studied in detail.

We propose to investigate the full counting statistics of nonequilibrium spin transport with an ultracold atomic quantum gas. The setup makes use of the spin control available in atomic systems to generate spin transport induced by an impurity atom immersed in a spin-imbalanced two-component Fermi gas. In contrast to solid-state realizations, in ultracold atoms spin relaxation and the decoherence from external sources is largely suppressed. As a consequence, once the spin current is turned off by manipulating the internal spin degrees of freedom of the Fermi system, the nonequilibrium spin population remains constant. Thus one can directly count the number of spins in each reservoir to investigate the full counting statistics of spin flips, which is notoriously challenging in solid-state devices. Moreover, using Ramsey interferometry, the dynamical impurity response can be measured. Since the impurity interacts with a many-body environment that is out of equilibrium, our setup provides a way to realize the nonequilibrium orthogonality catastrophe. Here, even for spin reservoirs initially prepared in a zero-temperature state, the Ramsey response exhibits an exponential decay, which is in contrast to the conventional power-law decay of Anderson's orthogonality catastrophe. By mapping our system to a multistep Fermi sea, we are able to derive analytical expressions for the impurity response at late times. This allows us to reveal an intimate connection of the decay rate of the Ramsey contrast and the full counting statistics of spin flips.

Inelastic neutron scattering was performed for an iron-based superconductor ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{D}}_{0.1}$, where most of D (deuterium) replaces oxygen, while a tiny amount goes into the interstitial sites. By first-principles calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature $T_{\mathrm{c}}=26\mathrm{K}$, excitations emerge in the range 5–15 meV, while they are absent in the reference system ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$. The strong excitations at 14.5 and 11.1 meV broaden rapidly around 15 and 20 K, respectively, where each energy becomes comparable to twice the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.

We study the ground state and low-lying excited states of the Kitaev-Heisenberg model on a two-leg ladder geometry using the density-matrix renormalization-group and Lanczos exact diagonalization methods. The Kitaev and Heisenberg interactions are parametrized as $K=sin\varphi$ and $J=cos\varphi$ with an angle parameter $\varphi$. Based on the results for several types of order parameters, excitation gaps, string order parameter, and entanglement spectra, the $\varphi$-dependent ground state phase diagram is determined. Remarkably, the phase diagram is quite similar to that of the Kitaev-Heisenberg model on a honeycomb lattice, exhibiting the same long-range-ordered states, namely rung singlet (analog to Néel in three dimensions), zigzag, ferromagnetic, and stripy, and the presence of gapped spin liquids around the exactly solvable Kitaev points $\varphi =\pm \pi /2$. We also calculate the expectation value of a plaquette operator corresponding to a $\pi$-flux state in order to establish how the gapped Kitaev spin liquid extends away from the $\varphi =\pm \pi /2$. Furthermore, we determine the dynamical spin structure factor and discuss the effect of the Kitaev interaction on the spin-triplet dispersion.