In this work, we address the ground-state properties of the anisotropic spin-1/2 Heisenberg XYZ chain under the interplay of magnetic fields and the Dzyaloshinskii-Moriya (DM) interaction which we interpret as an electric field. The identification of the regions of enhanced sensitivity determines criticality in this model. We calculate the Wigner-Yanase skew information (WYSI) as a coherence witness of an arbitrary two-qubit state under specific measurement bases. The WYSI is demonstrated to be a good indicator for detecting the quantum phase transitions. The finite-size scaling of coherence susceptibility is investigated. We find that the factorization line in the antiferromagnetic phase becomes the factorization volume in the gapless chiral phase induced by DM interactions, implied by the vanishing concurrence for a wide range of field. We also present the phase diagram of the model with three phases—antiferromagnetic, paramagnetic, and chiral—and point out a few common mistakes in deriving the correlation functions for the systems with broken reflection symmetry.

We use the rotation-invariant Green's function method (RGM) and the high-temperature expansion to study the thermodynamic properties of the Heisenberg antiferromagnet on the pyrochlore lattice. We discuss the excitation spectra as well as various thermodynamic quantities, such as spin correlations, uniform susceptibility, specific heat, and static and dynamical structure factors. For the ground state we present RGM data for arbitrary spin quantum numbers $S$. At finite temperatures we focus on the extreme quantum cases $S=\frac{1}{2}$ and 1. We do not find indications for magnetic long-range order for any value of $S$. We discuss the width of the pinch point in the static structure factor in dependence on temperature and spin quantum number. We compare our data with experimental results for the pyrochlore compound ${\mathrm{NaCaNi}}_2{\mathrm{F}}_7$ ($S=1$). Thus, our results for the dynamical structure factor agree well with the experimentally observed features at 3 ...8 meV for ${\mathrm{NaCaNi}}_2{\mathrm{F}}_7$. We analyze the static structure factor $S_{\mathbf{q}}$ to find regions of maximal $S_{\mathbf{q}}$. The high-temperature series of the $S_{\mathbf{q}}$ provide a fingerprint of weak order by disorder selection of a collinear spin structure, where (classically) the total spin vanishes on each tetrahedron and neighboring tetrahedra are dephased by $\pi$.

By conducting helicity-dependent ultrafast magnetization dynamics in a CoTb ferrimagnetic alloy, we are able to quantitatively determine the magnetic circular dichroism (MCD) and resolve its role in the helicity-dependent all-optical switching (AOS). Unequivocal interpretation of the sign of the dichroism is provided by performing AOS and femtosecond laser-induced domain wall motion experiments. We demonstrate that AOS occurs when the magnetization is initially in the most absorbent state, according to the light helicity. Moreover, we evidence that the MCD creates a thermal gradient that drives a domain wall toward hotter regions. Our experimental results are in agreement with the purely thermal models of AOS.

An electric current flowing in Pt, a material with strong spin-orbit coupling, leads to spins accumulating at the interfaces by virtue of the spin Hall effect and interfacial charge-spin conversion. We measure the influence of these interfacial magnetic moments onto adjacent $3d$ transition-metal layers by x-ray absorption spectroscopy and x-ray magnetic circular dichroism in a quantitative and element-selective way, with sensitivity below ${10}^{-5}{\mu }_B$ per atom. In Pt(6 nm)/Co(2.5 nm), the accumulated spins cause a deviation of the Co magnetization direction, which corresponds to an effective spin Hall angle of 0.08. The spin and orbital magnetic moments of Co are affected in equal proportion by the absorption of the spin current, showing that the transfer of orbital momentum from the recently predicted orbital Hall effect is either below our detection limit, or not directed to the $3d$ states of Co. For Pt/NM (NM = Ti, Cr, Cu), we find upper limits for the amount of injected spins corresponding to about $3\times {10}^{-6}{\mu }_B$ per atom.

We introduce the functional field integral approach to study the statistics of quantum work under nonequilibrium conditions and derive the general formalism for a bilinear Hamiltonian with arbitrary time dependence. The method is then examined in three models. For the transverse Ising chain, it yields the correct quantum critical scaling and dynamical quantum phase transitions for single and double quench protocols, respectively. For the Su-Schrieffer-Heeger (SSH) model, we observe nonuniversal quantum critical scaling with anomalous $1/N$ correction due to its topological nature. Dynamical quantum phase transitions are observed for three different time evolution protocols but their time periodicity only appears in the double quench case. We then extend our method to the Bardeen-Cooper-Schrieffer (BCS) model for superconductivity and discuss the possibility of its application for general correlated models in combination with either the mean-field approximation or exact Monte Carlo simulations on classical (auxiliary) fields or disorders. Our method has the advantage of numerical simplicity, in the cost of explicit state evolution, and provides a promising way for exploring the physics of quantum work under general conditions.

For materials that exhibit dynamic changes in the electronic quasiparticle states after ultrafast optical excitation, it is extremely difficult to unambiguously separate quasiparticle band dynamics and carrier redistribution processes in state-of-the-art photoemission experiments. We study theoretically the interplay of band and carrier dynamics for a model system of a quasi-two-dimensional material with a small band gap and investigate the consequences for electronic distribution curves. Our model system contains photo-induced band-gap narrowing by a coherent phonon mechanism, which mimics the quenching of an insulator phase. We discuss the importance of impact ionization in the ultrafast response and investigate the interplay between carrier and band dynamics. Our model allows us to compare with recent experiments and identify signatures of carrier multiplication in typical electronic distribution curves as measured by time-resolved photoemission spectroscopy. We also investigate the influence of the shape of the bands on the carrier multiplication.

The lacunar spinel ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ is a material whose properties are dominated by tetrahedral clusters of V atoms. The compound is known to undergo a polar distortion to a ground state structure in the $R3m$ space group, and orders ferromagnetically with a relatively small magnetic moment. We develop an understanding into the relationship between crystal structure and magnetic order in this material, and the influence of electron correlations in establishing the observed ground state using first-principles density functional theory (DFT) electronic structure calculations. Because electrons are delocalized within ${\mathrm{V}}_4$ clusters but localized between them, the usual approaches to simulate electron correlations—such as the use of the Hubbard $U$ in $\mathrm{DFT}+U$ schemes—do not adequately recreate the experimental ground state. We find instead that the experimental ground state of ${\mathrm{GaV}}_4{\mathrm{Se}}_8$ is well represented by the random-phase approximation to the correlation energy. Additionally, we find that magnetism and crystal structure are strongly coupled in this material, and only certain arrangements of magnetic moment within a ${\mathrm{V}}_4$ cluster can stabilize the observed structural distortion. In combination with the anisotropic, polar nature of the material, the strength of magnetostructural coupling indicates that application of strain could be used to tune the magnetic properties of ${\mathrm{GaV}}_4{\mathrm{Se}}_8$.

While many post-transition metals could form mixed valence oxides with high conductivities leading to various interesting applications, oxides of tellurium are usually insulators. The only known mixed valence tellurium oxides that show appreciable electronic conductivities are the $\mathrm{Cs}{(M,\mathrm{Te})}_2{\mathrm{O}}_6$ series. However, experimental and theoretical investigations of the compounds are scarce. To study the effects of $M$ metal substitution of tellurium on the structural and electronic properties of $\mathrm{Cs}{(M,\mathrm{Te})}_2{\mathrm{O}}_6$ mixed valence compounds, a series of oxides with the general formula $\mathrm{CsM}{\mathrm{n}}_x\mathrm{T}{\mathrm{e}}_{2\text{−}x}{\mathrm{O}}_6$ have been prepared and characterized. Substituting Te in $\mathrm{CsT}{\mathrm{e}}_2{\mathrm{O}}_6$ with as small as 2.5% Mn $(x=0.05)$ changes the structure from rhombohedral to cubic symmetry. Based on the x-ray photoelectron spectroscopy and x-ray absorption near edge structure studies, we found that Mn in the samples with $x\le 0.33$ is in +3 states, while samples with $x>0.33$ contain a mixture of $\mathrm{M}{{\mathrm{n}}^3}^{+}$ and $\mathrm{M}{{\mathrm{n}}^4}^{+}$. Detailed studies indicate that there are two substitution schemes depending on the $x$ values. The switching between these two schemes occurs at around $x=0.30-0.33$, which results in a change in the trend of electronic conductivity. To describe the electronic properties, simple schematic band structure diagrams of the samples are proposed based on experimental results and density functional theory calculations. ${\mathrm{CsMn}}_{\mathrm{x}}{\mathrm{Te}}_{2\text{−}x}{\mathrm{O}}_6(x<0.33)$ contains $\mathrm{T}{{\mathrm{e}}^4}^{+}/\mathrm{T}{{\mathrm{e}}^6}^{+}$ mixed valency where both species are in the same crystallographic sites. Yet, the Te $5s$ band is split into two parts with $\mathrm{T}{{\mathrm{e}}^4}^{+}$ contributing to the valence band maximum and $\mathrm{T}{{\mathrm{e}}^6}^{+}$ contributing to the conduction band minimum. Such a splitting indicates that they are distinguishable because of a local distortion in the lattice. We found that energy levels of $\mathrm{T}{{\mathrm{e}}^4}^{+}5s$ states relative to the conduction band exhibit a strong correlation with the unit cell parameters. When the cell parameter is small, $\mathrm{T}{{\mathrm{e}}^4}^{+}$ is destabilized in the lattice causing its energy level to increase. While metallic conduction has not been achieved in this series of compounds, the detailed investigation in this work deepens the understanding about their electronic structures and sheds light on the possibility of designing type III mixed valence tellurium oxides.

The semimetallic behavior of the so-called “failed Kondo insulator” CeNiSn has been ascribed to a nodal line in the Kondo hybridization derived from a particular symmetry of the Ce $4f$ orbitals ground state. Here we investigate the geometry of the CeNiSn conduction band by combined angle-resolved photoemission spectroscopy (ARPES) in the high-temperature regime and Open core generalized gradient approximation plus spin-orbit coupling calculations, in order to determine how the nodal hybridization takes place. We identify the Fermi sheet involved in the semimetallic regime from its locus and its shape, respectively, in agreement with the expected nodal line and with quantum oscillations. We further extrapolate and discuss the low-temperature Fermi surface in terms of the expected nodal hybridization with a localized $f$-level. The obtained hypothetical low-temperature Fermi surface is compatible with the description from quantum oscillations, and with both the highly anisotropic magnetoresistance and the isotropic Nernst effect. This work offers an overview of the conduction band of CeNiSn before hybridization, and it paves the way to a definitive understanding of its low-temperature state. In addition, this work serves as a basis for more challenging low-temperature ARPES measurements.

We have measured a helicity-dependent photocurrent at zero external magnetic field in a device based on a semiconductor quantum well embedded in a p-i-n junction. The device is excited under vertical incidence with circularly polarized light. The spin filtering effect is evidenced in the temperature range 77–300 K owing to a CoFeB/MgO spin filter with out-of-plane magnetization in remanence. The helicity-dependent photocurrent is explored as a function of the temperature and bias. These characteristics are compared with those of a spin photocurrent device with in-plane magnetized CoFeB/MgO spin filter, excited under oblique incidence with circularly polarized light. In contrast to the in-plane spin filter device, the circularly polarized light asymmetry of the photocurrent in the out-of-plane device depends weakly on the external bias. The two devices are sensitive to the spin filtering of either the in-plane $\left(S_x\right)$ or out-of-plane $\left(S_z\right)$ photogenerated electron spin in the semiconductor quantum well. The helicity-dependent photocurrent results can be explained by the Dyakonov-Perel electron spin-relaxation mechanism. Our study reveals the giant spin relaxation anisotropy in III-V zinc-blende quantum wells in the presence of a vertical electric field.

Recently, emission linewidths of single defect centers in layered hexagonal boron nitride down to 50 MHz were reported, close to the lifetime limit. The linewidths were extracted from time traces of laser scanning photoluminescence excitation experiments. At the same time, temporal blinking was affecting the measured time traces even in the absence of a frequency scanning the excitation laser. I argue here that the reported temporal blinking timescale is consistent with the observed transients, implying that the interpretation as spectral shapes and thus the extracted linewidths are invalid.

In this Reply, we answer to the comments by Langbein [Phys. Rev. B 100, 047401 (2019)]. We disagree with the argument that our measured spectral shapes and the extracted linewidths are caused by temporal blinking. We give detailed information on our evaluation process to exclude blinking events. Beyond the question raised in the Comment, we analyze the influence of spectral diffusion. Although spectral diffusion is an ongoing limitation for defect centers in hexagonal boron nitride, we prove that it is not influencing our extracted linewidths.

We derive the system of equations that allows to include nonequilibrium correlations of filling numbers into the theory of the hopping transport. The system includes the correlations of arbitrary order in a universal way and can be cut at any place relevant to a specific problem to achieve the balance between rigor and computation possibilities. In the linear-response approximation, it can be represented as an equivalent electric circuit that generalizes the Miller-Abrahams resistor network. With our approach, we show that nonequilibrium correlations are essential to calculate conductivity and distribution of currents in certain disordered systems. Different types of disorder affect the correlations in different applied fields. The effect of energy disorder is most important at weak electric fields while the position disorder by itself leads to nonzero correlations only in strong fields.

We investigate the growth of the entanglement entropy $S_{\mathrm{ent}}$ following global quenches in two-dimensional free fermion models with potential and bond disorder. For the potential disorder case, we show that an intermediate weak localization regime exists in which $S_{\mathrm{ent}}\left(t\right)$ grows logarithmically in time $t$ before Anderson localization sets in. For the case of binary bond disorder near the percolation transition, we find additive logarithmic corrections to area and volume laws as well as a scaling at long times, which is consistent with an infinite randomness fixed point.

Dipolar interactions are crucial in the modeling of many complex magnetic systems, such as the pyrochlores and artificial spin systems. Remarkably, many classical dipolar-coupled spin systems exhibit a continuous ground-state degeneracy, which is unexpected as the Hamiltonian does not possess a continuous symmetry. In this paper, we explain how such a finite point symmetry leads to a continuous ground-state degeneracy of specific classical dipolar-coupled systems. This work therefore provides new insight into the theory of classical dipolar-coupled spin systems and opens the way to understand more complex dipolar-coupled systems.

${\left(\mathrm{LaCo}{\mathrm{O}}_3/\mathrm{LaMn}{\mathrm{O}}_3\right)}_5$ multilayers with fixed $\mathrm{LaMn}{\mathrm{O}}_3$ thickness and varying $\mathrm{LaCo}{\mathrm{O}}_3$ thickness grown on $\mathrm{SrTi}{\mathrm{O}}_3$ (001) substrates are studied by scanning transmission electron microscopy at the atomic scale. Utilizing peak pairs analysis on annular bright field images, the precise atomic positions and the degree of octahedral rotation are obtained. Our study shows that the $c$ axis of $\mathrm{LaMn}{\mathrm{O}}_3$ sandwiched by $\mathrm{LaCo}{\mathrm{O}}_3$ layers experiences a transition from the in-plane direction to the out-of-plane direction as the $\mathrm{LaCo}{\mathrm{O}}_3$ layer thickness increases, due to the oxygen octahedral coupling with $\mathrm{LaCo}{\mathrm{O}}_3$ layers. An abnormal suppression in saturation magnetization with increasing the thickness of the $\mathrm{LaCo}{\mathrm{O}}_3$ is observed and ascribed to the enhanced $\mathrm{Mn}{\mathrm{O}}_6$ octahedral rotation. Our work provides a way to tune the magnetic properties of epitaxially grown thin films via interfacial octahedral engineering.

Monte Carlo (MC) simulations are essential computational approaches with widespread use throughout all areas of science. We present a method for accelerating lattice MC simulations using fully connected and convolutional artificial neural networks that are trained to perform local and global moves in configuration space, respectively. Both networks take local spacetime MC configurations as input features and can, therefore, be trained using samples generated by conventional MC runs on smaller lattices before being utilized for simulations on larger systems. This approach is benchmarked for the case of determinant quantum Monte Carlo (DQMC) studies of the two-dimensional Holstein model. We find that both artificial neural networks are capable of learning an unspecified effective model that accurately reproduces the MC configuration weights of the original Hamiltonian and achieve an order of magnitude speedup over the conventional DQMC algorithm. Our approach is broadly applicable to many classical and quantum lattice MC algorithms.

Using synchrotron x-ray diffraction, we show that the long-accepted monoclinic structure of the “collapsed” high-pressure phases reported in seven lanthanide elements [Nd, Tb, Gd, Dy, Ho, Er, and (probably) Tm] is incorrect. In Tb, Gd, Dy, Ho, Er, and Tm we show that the collapsed phases have a 16-atom orthorhombic structure ($oF16$) not previously seen in the elements, whereas in Nd we show that it has an eight-atom orthorhombic structure ($oF8$) previously reported in several actinide elements. $oF16$ and $oF8$ are members of a new family of layered elemental structures, the discovery of which reveals that the high-pressure structural systematics of the lanthanides, actinides, and group-III elements (Sc and Y) are much more related than previously imagined. Electronic structure calculations of Tb, combined with quantum many-body corrections, confirm the experimental observation, and calculate that the collapsed orthorhombic phase is a ferromagnet, nearly degenerate with an antiferromagnetic state between 60 and 80 GPa. We find that the magnetic properties of Tb survive to the highest pressures obtained in our experiments (110 GPa). Further calculations of the collapsed phases of Gd and Dy, again using the correct crystal structure, show the former to be a type-A antiferromagnet, whereas the latter is ferromagnetic.

We study a class of spin-$1/2$ quantum ladder models with generalized plaquette interactions in the presence of a transverse field. We show that in certain parameter regimes, these models have strong zero modes responsible for the long relaxation times of edge spins. By exploiting an infinite set of symmetries in these systems, we show how their Hamiltonians can be represented, in each symmetry sector, by a transverse field Ising chain. Due to the presence of an extensive number of conserved quantities, even if the original system has no disorder, most of these symmetry sectors feature a quasirandom transverse field profile. This representation of the ladder system in terms of a disordered Ising chain allows us to explain the features of the edge autocorrelation function of the original system. Furthermore, we find what appears to be an interesting mechanism for slow decorrelation: Even in parameter regimes where the full ladder model does not possess an obvious strong zero mode, some of the initial information stored in the edge spins can be preserved for long times as a consequence of the existence of strong zero modes within individual symmetry sectors.