The ferromagnetic superconductor URhGe has been investigated by high field magnetic circular dichroism (XMCD) at the U $M_{4,5}$, Rh $L_{2,3}$, and Ge $K$ edges at 2.1 K and at applied fields up to 17 T. The XMCD performed at the $M_{4,5}$ absorption edges allows us to determine the spectroscopic branching ratio and the $5f$ electron contribution to the valence spin-orbit interaction. Combination with polarized neutron diffraction results allows us to derive the individual $\mathrm{U}$ orbital and spin moments and the magnetic-dipole contribution $\langle T_z\rangle$. There is no evidence for any change of the orbital-to-spin moment ratios across the spin reorientation transition at $H_R=12\mathrm{T}$, when the field is applied along the initial hard $b$ axis. We also confirm that the magnetism of URhGe is dominated by $\mathrm{U}$, with the contribution of Rh representing only about $10\%$ of the macroscopic moment. The orbital and spin moments at the Rh site are found to be parallel to each other and parallel to the macroscopic magnetization, but an unexpectedly large orbital-to-spin moment ratio is observed. The XMCD at the Ge $K$ edge reveals the presence of a small induced Ge $4p$ orbital moment, parallel to the macroscopic magnetization. The results are discussed against predictions of the electronic band structure calculations by the density functional theory plus Coulomb $U$, including spin-orbit coupling $(\mathrm{DFT}+U+\mathrm{SOC})$.

Raman scattering in the three-dimensional Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ shows an intricate interplay of electronic and phonon degrees of freedom. We observe resonant phonon scattering due to interband transitions, an anomalous anharmonicity of phonon frequency and intensity, as well as quasielastic $(E\sim 0)$ electronic scattering. The latter two effects are governed by a characteristic temperature scale $T^{*}\sim 100\mathrm{K}$ that is related to mutual fluctuations of lattice and electronic degrees of freedom. A refined analysis shows that this characteristic temperature corresponds to the energy of optical phonons which couple to interband transitions in the Dirac states of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$. As electron-phonon coupling in a topological semimetal is primarily related to phonons with finite momenta, the back action on the optical phonons is only observed as anharmonicities via multiphonon processes involving a broad range of momenta. The resulting energy density fluctuations of the coupled system have previously only been observed in low dimensional or frustrated spin systems with suppressed long range ordering.

We report a detailed study of the transport properties of a $\beta -\mathrm{B}{\mathrm{i}}_4{\mathrm{I}}_4$ quasi-one-dimensional topological insulator. Electrical resistivity, thermoelectric power, thermal conductivity, and Hall coefficient measurements are consistent with the possible appearance of a charge-density-wave order at low temperatures. Both electrons and holes contribute to the conduction in $\beta -\mathrm{B}{\mathrm{i}}_4{\mathrm{I}}_4$ and the dominant type of charge carrier changes with temperature as a consequence of temperature-dependent carrier densities and mobilities. Measurements of resistivity and Seebeck coefficient under hydrostatic pressure up to 2 GPa show a shift of the charge-density-wave order to higher temperatures suggesting a strongly one-dimensional character at ambient pressure. Surprisingly, superconductivity is induced in $\beta -\mathrm{B}{\mathrm{i}}_4{\mathrm{I}}_4$ above 10 GPa with $T_c$ of 4.0 K which is slightly decreasing upon increasing the pressure up to 20 GPa. Chemical characterization of the pressure-treated samples shows amorphization of $\beta -\mathrm{B}{\mathrm{i}}_4{\mathrm{I}}_4$ under pressure and rules out decomposition into Bi and $\mathrm{Bi}{\mathrm{I}}_3$ at room-temperature conditions.

We consider theoretically staggered honeycomb lattices for photons which can be viewed as photonic analogs of transitional metal dichalcogenide (TMD) monolayers. We propose a simple realization of a photonic quantum valley Hall effect (QVHE) at the interface between two inverted lattices. This results in the formation of valley-polarized propagating modes whose existence relies on the difference between the valley Chern numbers, an analog of the ${\mathbb{Z}}_2$ topological invariant. We show that the magnitude of the photonic spin-orbit coupling based on the energy splitting between TE and TM photonic modes allows to control the number and propagation direction of these interface modes. Finally, we consider the interface between a staggered and a regular honeycomb lattice subject to a nonzero Zeeman field, therefore showing quantum anomalous Hall effect (QAHE). In such a case, the topologically protected one-way modes of the QAHE become valley-polarized and the system behaves as a perfect valley filter.

We investigate Klein tunneling for the $\alpha \text{−}{T}_3$ model, which interpolates between graphene and the dice lattice via parameter $\alpha$. We study transmission across two types of electrostatic interfaces: sharp potential steps and sharp potential barriers. We find both interfaces to be perfectly transparent for normal incidence for the full range of the parameter $\alpha$ for both interfaces. For other angles of incidence, we find that transmission is enhanced with increasing $\alpha$. For the dice lattice, we find perfect, all-angle transmission across a potential step for incoming electrons with energy equal to half of the height of the potential step. This is analogous to the “super”, all-angle transmission reported for the dice lattice for Klein tunneling across a potential barrier.

Recently discovered Dirac and Weyl semimetals display unusual magnetoresistance phenomena, including a large, nonsaturating, linear transverse magnetoresistance and a negative longitudinal magnetoresistance. The latter is often considered as evidence of fermions that have a defined chirality. Classical mechanisms, due to disorder or nonuniform current injection, can, however, also produce negative longitudinal magnetoresistance. Here, we report on magnetotransport measurements performed on epitaxial thin films of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$, a three-dimensional Dirac semimetal. Quasilinear positive transverse magnetoresistance and negative longitudinal magnetoresistance are observed. By evaluating films of different thickness and by correlating the temperature dependence of the carrier density and mobility with the magnetoresistance characteristics, we demonstrate that both the quasilinear positive and the negative magnetoresistance are caused by conductivity fluctuations. Chiral anomaly is not needed to explain the observed features.

Excitation of a semiconductor quantum dot with a chirped laser pulse allows excitons to be created by rapid adiabatic passage. In quantum dots this process can be greatly hindered by the coupling to phonons. Here we add a high chirp rate to ultrashort laser pulses and use these pulses to excite a single quantum dot. We demonstrate that we enter a regime where the exciton-phonon coupling is effective for small pulse areas, while for higher pulse areas a decoupling of the exciton from the phonons occurs. We thus discover a reappearance of rapid adiabatic passage, in analogy to the predicted reappearance of Rabi rotations at high pulse areas. The measured results are in good agreement with theoretical calculations.

The spin noise in singly charged self-assembled quantum dots is studied theoretically and experimentally under the influence of a perturbation, provided by additional photoexcited charge carriers. The theoretical description takes into account generation and relaxation of charge carriers in the quantum dot system. The spin noise is measured under application of above barrier excitation for which the data are well reproduced by the developed model. Our analysis demonstrates a strong difference of the recharging dynamics for holes and electrons in quantum dots.

Transport measurements on the two-dimensional electron system in $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures indicate significant noncrystalline anisotropic behavior below $T\approx 30\mathrm{K}$. Lattice dislocations in $\mathrm{SrTi}{\mathrm{O}}_3$ and interfacial steps are suggested to be the main sources for electronic anisotropy. Anisotropic defect scattering likewise alters magnetoresistance at low temperature remarkably and influences spin-orbit coupling significantly by the Elliot-Yafet mechanism of spin relaxation, resulting in anisotropic weak localization. Applying a magnetic field parallel to the interface results in an additional field-induced anisotropy of the conductance, which can be attributed to Rashba spin-orbit interaction. Compared to $\mathrm{LaAl}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$, Rashba coupling seems to be reduced, indicating a weaker polarity in $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures.

The SU(4)-SU(2) crossover, driven by an external magnetic field $h$, is analyzed in a capacitively coupled double quantum dot device connected to independent leads. As one continuously charges the dots from empty to quarter filled, by varying the gate potential $V_g$, the crossover starts when the magnitude of the spin polarization of the double quantum dot, as measured by $\langle n_{\uparrow }\rangle -\langle n_{\downarrow }\rangle$, becomes finite. Although the external magnetic field breaks the SU(4) symmetry of the Hamiltonian, the ground state preserves it in a region of $V_g$, where $\langle n_{\uparrow }\rangle -\langle n_{\downarrow }\rangle =0$. Once the spin polarization becomes finite, it initially increases slowly until a sudden change occurs, in which $\langle n_{\downarrow }\rangle$ (polarization direction opposite to the magnetic field) reaches a maximum and then decreases to negligible values abruptly, at which point an orbital SU(2) ground state is fully established. This crossover from one Kondo state, with emergent SU(4) symmetry, where spin and orbital degrees of freedom all play a role, to another, with SU(2) symmetry, where only orbital degrees of freedom participate, is triggered by a competition between $g{\mu }_{B}h$, the energy gain by the Zeeman-split polarized state and the Kondo temperature $T_K^{SU\left(4\right)}$, the gain provided by the SU(4) unpolarized Kondo-singlet state. At fixed magnetic field, the knob that controls the crossover is the gate potential, which changes the quantum dots occupancies. If one characterizes the occurrence of the crossover by $V_g^{max}$, the value of $V_g$ where $\langle n_{\downarrow }\rangle$ reaches a maximum, one finds that the function $f$ relating the Zeeman splitting, $B_{max}$, which corresponds to $V_g^{max}$, i.e., $B_{max}=f\left(V_g^{max}\right)$, has a similar universal behavior to that of the function relating the Kondo temperature to $V_g$. In addition, our numerical results show that near the SU(4) Kondo temperature and for relatively small magnetic fields the device has a ground state that restricts the electronic population at the dots to be spin polarized along the magnetic field. These two facts introduce very efficient spin-filter properties to the device, also discussed in detail in the paper. This phenomenology is studied adopting two different formalisms: the mean-field slave bosons approximation, which allows an approximate analysis of the dynamical properties of the system, and a projection operator approach, which has been shown to describe very accurately the physics associated to the ground state of Kondo systems.

We show that a simple artificial neural network trained on entanglement spectra of individual states of a many-body quantum system can be used to determine the transition between a many-body localized and a thermalizing regime. Specifically, we study the Heisenberg spin-1/2 chain in a random external field. We employ a multilayer perceptron with a single hidden layer, which is trained on labeled entanglement spectra pertaining to the fully localized and fully thermal regimes. We then apply this network to classify spectra belonging to states in the transition region. For training, we use a cost function that contains, in addition to the usual error and regularization parts, a term that favors a confident classification of the transition region states. The resulting phase diagram is in good agreement with the one obtained by more conventional methods and can be computed for small systems. In particular, the neural network outperforms conventional methods in classifying individual eigenstates pertaining to a single disorder realization. It allows us to map out the structure of these eigenstates across the transition with spatial resolution. Furthermore, we analyze the network operation using the dreaming technique to show that the neural network correctly learns by itself the power-law structure of the entanglement spectra in the many-body localized regime.

In underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$, there is evidence of a small Fermi surface pocket subject to substantial mass enhancement in the doping regime $0.12<p<0.16$. This mass enhancement may vary substantially over the Fermi surface, due to “hot spot” or other relevant physics. We therefore examine the magnetotransport of an electronlike Fermi pocket with large effective mass anisotropy. Within the relaxation time approximation, we show that even for a pocket with a fixed shape, the magnitude and sign of the Hall effect may change as the mass anisotropy changes (except at very large, likely inaccessible magnetic fields). We discuss implications for recent Hall measurements in near optimally doped cuprates in high fields. In addition we identify a novel intermediate asymptotic regime of magnetic field, characterized by B-linear magnetoresistance. Similar phenomena should occur in a variety of other experimental systems with anisotropic mass enhancement.

We theoretically study the effect of the Berry curvature on the transport properties of Weyl semimetals using a semiclassical Boltzmann transport theory, which results in nonlinear optical responses. In the adiabatic process, the Berry curvature, which involves the time derivative of the Bloch states, contributes to the transport properties such as the adiabatic Thouless pump. Although this effect is very weak in usual solids, it is enhanced in Weyl semimetals, where the Berry curvature contributes to observable nonlinear optical responses due to its nodal structure. In this paper, using the semiclassical Boltzmann theory, we show that a dc photocurrent induced by the Berry curvature robustly persists even in the limit of short scattering time. We also show that the photocurrent is well explained as a consequence of the electromagnetic induction in momentum space. The results indicate that the electromagnetic induction gives rise to a nondissipative photocurrent that is robust against decoherence within a time scale shorter than the periodicity of the incident electromagnetic field. We also discuss the second harmonic response of an ac current when the electron distribution is displaced from the ground state by an external field.

We report an efficient method to observe single photon emissions in monolayer $\mathrm{WS}{\mathrm{e}}_2$ by applying hydrostatic pressure. The photoluminescence peaks of typical two-dimensional excitons show a nearly identical pressure-induced blueshift, whereas the energy of pressure-induced discrete emission lines (quantum emitters) demonstrates a pressure insensitive behavior. The decay time of these discrete line emissions is approximately 10 ns, which is at least one order longer than the lifetime of the broad localized ($L$) excitons. These characteristics lead to a conclusion that the excitons bound to deep-level defects can be responsible for the observed single photon emissions.

We model quantum Hall skyrmions in graphene monolayer at quarter filling by a theory of ${\mathrm{CP}}^3$ fields and study the energy minimizing skyrmions in the presence of valley pseudospin anisotropy and Zeeman coupling. We present a diagram of all types of skyrmions in a wide range of the anisotropy parameters. For each type of skyrmion, we visualize it on three Bloch spheres, and present the profiles of its texture on the graphene honeycomb lattice, thus providing references for the scanning-tunneling microscopy and spectroscopy imaging of spin-pseudospin textures in graphene monolayer in the quantum Hall regime. Besides the spin and pseudospin skyrmions for the corresponding degrees of freedom of an electron in the $N=0$ Landau level, we discuss two unusual types—the “entanglement skyrmion”, the texture of which lies in the space of the entanglement of spin and pseudospin, as well as the “deflated pseudospin skyrmion” with partial entanglement. For all skyrmion types, we study the dependence of the energy and the size of a skyrmion on the anisotropy parameters and perpendicular magnetic field. We also propose three ways to modify the anisotropy energy, namely, the sample tilting, the substrate anisotropy, and the valley pseudospin analog of Zeeman coupling.

We measure subnanometer displacements of thin samples vibrated by piezotransducer. Samples contain ${}^{57}\mathrm{Fe}$ nuclei, which are exposed to 14.4-keV $\gamma$ radiation. Vibration produces sidebands from a single absorption line of the sample. The sideband intensities depend on the vibration amplitude and its distribution along the sample. We developed a model of this distribution, which adequately describes the spectra of powder and stainless steel (SS) absorbers. We propose to filter $\gamma$ radiation through a small round hole in the lead mask, placed before the absorber. In this case only a small spot of the vibrated absorber is observed. We found for SS foil that nuclei, exposed to $\gamma$ radiation in this small spot, vibrate with almost the same amplitudes whose difference does not exceed a few picometers within the irradiated area.

Magnetic skyrmions are topological spin configurations in materials with chiral Dzyaloshinskii-Moriya interaction (DMI), that are potentially useful for storing or processing information. To date, DMI has been found in few bulk materials, but can also be induced in atomically thin magnetic films in contact with surfaces with large spin-orbit interactions. Recent experiments have reported that isolated magnetic skyrmions can be stabilized even near room temperature in few-atom-thick magnetic layers sandwiched between materials that provide asymmetric spin-orbit coupling. Here we present the minimum-energy path analysis of three distinct mechanisms for the skyrmion collapse, based on ab initio input and the performed atomic-spin simulations. We focus on the stability of a skyrmion in three atomic layers of Co, either epitaxial on the Pt(111) surface or within a hybrid multilayer where DMI nontrivially varies per monolayer due to competition between different symmetry breaking from two sides of the Co film. In laterally finite systems, their constrained geometry causes poor thermal stability of the skyrmion toward collapse at the boundary, which we show to be resolved by designing the high-DMI structure within an extended film with lower or no DMI.

We study the time evolution of a one-dimensional interacting fermion system described by the Luttinger model starting from a nonequilibrium state defined by a smooth temperature profile $T\left(x\right)$. As a specific example we consider the case when $T\left(x\right)$ is equal to $T_L$ $\left(T_R\right)$ far to the left (right). Using a series expansion in $\epsilon =2(T_R-T_L)/(T_L+T_R)$, we compute the energy density, the heat current density, and the fermion two-point correlation function for all times $t\ge 0$. For local (delta-function) interactions, the first two are computed to all orders, giving simple exact expressions involving the Schwarzian derivative of the integral of $T\left(x\right)$. For nonlocal interactions, breaking scale invariance, we compute the nonequilibrium steady state (NESS) to all orders and the evolution to first order in $\epsilon$. The heat current in the NESS is universal even when conformal invariance is broken by the interactions, and its dependence on $T_{L,R}$ agrees with numerical results for the $XXZ$ spin chain. Moreover, our analytical formulas predict peaks at short times in the transition region between different temperatures and show dispersion effects that, even if nonuniversal, are qualitatively similar to ones observed in numerical simulations for related models, such as spin chains and interacting lattice fermions.