We study the electronic properties of monolayer transition metal dichalcogenide materials subjected to aliovalent doping, using Nb-doped ${\mathrm{WS}}_2$ as an exemplar. Scanning transmission electron microscopy imaging of the as-grown samples reveals an anisotropic Nb dopant distribution, prompting an investigation of anisotropy in electronic properties. Through electronic structure calculations on supercells representative of observed structures, we confirm that local Nb atom distributions are consistent with energetic considerations, although kinetic processes occurring during sample growth must be invoked to explain the overall symmetry-breaking. We perform effective band-structure and conductivity calculations on realistic models of the material that demonstrate that a high level of anisotropy can be expected in electronic properties including conductivity. In-plane anisotropy of the conductivity is predicted to be as high as 5:1, which is higher than previously observed in any TMDC system in the [Mo,W]${[\mathrm{S},\mathrm{Se}]}_2$ class.

We report an enhanced magnetoelastic contribution to the Gilbert damping in highly magnetostrictive ${\mathrm{Fe}}_{0.7}{\mathrm{Ga}}_{0.3}$ thin films. This effect is mitigated for perpendicular-to-plane fields, leading to a large anisotropy of the Gilbert damping in all of the films (up to a factor of 10 at room temperature). These claims are supported by broadband measurements of the ferromagnetic resonance linewidths over a range of temperatures (5 to 400 K), which serve to elucidate the effect of both the magnetostriction and phonon relaxation on the magnetoelastic Gilbert damping.

Geometry of the wave function is a central pillar of modern solid state physics. In this work, we unveil the wave-function geometry of two-dimensional semimetals with band crossing points (BCPs). We show that the Berry phase of BCPs are governed by the quantum metric describing the infinitesimal distance between quantum states. For generic linear BCPs, we show that the corresponding Berry phase is determined either by an angular integral of the quantum metric, or equivalently, by the maximum quantum distance of Bloch states. This naturally explains the origin of the $\pi$-Berry phase of a linear BCP. In the case of quadratic BCPs, the Berry phase can take an arbitrary value between 0 and $2\pi$. We find simple relations between the Berry phase, maximum quantum distance, and the quantum metric in two cases: (i) when one of the two crossing bands is flat; (ii) when the system has rotation and/or time-reversal symmetries. To demonstrate the implication of the continuum model analysis in lattice systems, we study tight-binding Hamiltonians describing quadratic BCPs. We show that, when the Berry curvature is absent, a quadratic BCP with an arbitrary Berry phase always accompanies another quadratic BCP so that the total Berry phase of the periodic system becomes zero. This work demonstrates that the quantum metric plays a critical role in understanding the geometric properties of topological semimetals.

We study the current-induced torques in asymmetric magnetic tunnel junctions containing a conventional ferromagnet and a magnetic Weyl semimetal contact. The Weyl semimetal hosts chiral bulk states and topologically protected Fermi arc surface states which were found to govern the voltage behavior and efficiency of current-induced torques. We report how bulk chirality dictates the sign of the nonequilibrium torques acting on the ferromagnet and discuss the existence of large fieldlike torques acting on the magnetic Weyl semimetal which exceed the theoretical maximum of conventional magnetic tunnel junctions. The latter are derived from the Fermi arc spin texture and display a counterintuitive dependence on the separation of the Weyl nodes. Our results shed light on the new physics of multilayered spintronic devices composed of magnetic Weyl semimetals, which might open doors for new energy-efficient spintronic devices.

$\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br} \left(\kappa \text{−}\mathrm{Br}\right)$ is an organic superconductor that changes to an Anderson-type insulator due to the random defects introduced by x-ray irradiation. In this study, we directly investigated the effects of irradiation on the local structures in the anion layer of $\kappa \text{−}\mathrm{Br}$ using x-ray fluorescence holography. The local structures around the Cu atoms in the anion layer were directly reconstructed from holograms. Prior to radiation damage, the reconstruction clearly shows the atomic images of the Br and N atoms that are directly coordinated to Cu. In the irradiated samples, the N images in the reconstructions were strongly suppressed as a result of the positional fluctuations of N introduced by the defects induced by irradiation. Using the previously proposed “bond-shifted” model and the thermal vibration amplitudes calculated from molecular dynamics simulations, we simulated the holograms and the atomic reconstructions of the anion layer. The simulations show that the “bond-shifted” model, along with thermal vibrations at 100 K, suppresses the N atomic images, as was observed in the experimental reconstructions.

Cubic $f$-electron compounds commonly exhibit highly anisotropic magnetic phase diagrams consisting of multiple long-range ordered phases. Field-driven metamagnetic transitions between them may depend not only on the magnitude, but also on the direction of the applied magnetic field. Examples of such behavior are plentiful among rare-earth borides, such as $R{\mathrm{B}}_6$ or $R{\mathrm{B}}_{12}$ ($R$ = rare earth). In this work, for example, we use torque magnetometry to measure anisotropic field-angular phase diagrams of La-doped cerium hexaborides, ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{B}}_6$ ($x=0,0.18,0.28,0.5$). One expects that field-directional anisotropy of phase transitions must be impossible to understand without knowing the magnetic structures of the corresponding competing phases and being able to evaluate their precise thermodynamic energy balance. However, this task is usually beyond the reach of available theoretical approaches, because the ordered phases can be noncollinear, possess large magnetic unit cells, involve higher-order multipoles of $4f$ ions rather than simple dipoles, or just lack sufficient microscopic characterization. Here we demonstrate that the anisotropy under field rotation can be qualitatively understood on a much more basic level of theory, just by considering the crystal-electric-field scheme of a pair of rare-earth ions in the lattice, coupled by a single nearest-neighbor exchange interaction. Transitions between different crystal-field ground states, calculated using this minimal model for the parent compound ${\mathrm{CeB}}_6$, possess field-directional anisotropy that strikingly resembles the experimental phase diagrams. This implies that the anisotropy of phase transitions is of local origin and is easier to describe than the ordered phases themselves.

The specific heat $C$ of the cuprate superconductors ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and ${\mathrm{Bi}}_{2+y}{\mathrm{Sr}}_{2-x-y}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ was measured at low temperatures (down to 0.5 K) for dopings $p$ close to $p^{★}$, the critical doping for the onset of the pseudogap phase. A magnetic field up to 35 T was applied to suppress superconductivity, giving direct access to the normal state at low temperatures, and enabling a determination of $C_{\mathrm{e}}$, the electronic contribution to the normal-state specific heat at $T\to 0$. In ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ at $x=p=0.22$, 0.24 and 0.25, $C_{\mathrm{e}}/T=15\mathrm{to}16\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ at $T=2\mathrm{K}$, values that are twice as large as those measured at higher doping ($p>0.3$) and lower doping ($p<0.15$). This confirms the presence of a broad peak in the doping dependence of $C_{\mathrm{e}}$ at $p^{★}\simeq 0.19$ as previously reported for samples in which superconductivity was destroyed by Zn impurities. Moreover, at those three dopings, we find a logarithmic growth as $T\to 0$ such that $C_{\mathrm{e}}/T\sim Bln(T_0/T)$. The peak versus $p$ and the logarithmic dependence versus $T$ are the two typical thermodynamic signatures of quantum criticality. In the very different cuprate ${\mathrm{Bi}}_{2+y}{\mathrm{Sr}}_{2-x-y}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$, we again find that $C_{\mathrm{e}}/T\sim Bln(T_0/T)$ at $p\simeq p^{★}$, strong evidence that this $ln(1/T)$ dependence of the electronic specific heat—first discovered in the cuprates ${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$—is a universal property of the pseudogap critical point.

We study the effect of the triplet proximity effect on Josephson transport in superconductor/half-metal/superconductor structures of atomic thickness beyond the quasiclassical approximation. Using the combination of microscopic ${\mathrm{Gor}}^{\prime }\mathrm{kov}$ formalism and tight-binding model we show that the full spin polarization inside the half-metal give rise to nonmonotonic temperature dependence of the critical current in S/HM/S Josephson junction with two spin active S/HM interfaces. We also calculate the magnetic moment inside the ${\mathrm{S}}_2$ layer in ${\mathrm{S}}_1/\mathrm{HM}/{\mathrm{S}}_2$ structure with spin-active ${\mathrm{S}}_1/\mathrm{HM}$ interface, which is induced by the spin-triplet superconducting correlations. Finally, we analyze the second-harmonic generation in ${\mathrm{S}}_1/\mathrm{HM}/{\mathrm{S}}_2$ structure with one spin-active interface.

NMR experiments on superfluid ${}^3\mathrm{He}$ in a nematic aerogel have shown that the stability of the half-quantum vortex (HQV) realized in the polar phase depends on the relative direction between the magnetic field and the anisotropy axis brought by the aerogel. The vortex energy in the polar phase is examined in terms of the Ginzburg-Landau free energy incorporating the dipole energy, and the reason why the HQV in the polar phase in nematic aerogels became unstable in a transverse magnetic field upon a field cooling is explained.

The Weyl semimetal in a constant magnetic field is investigated with the ground state for the lowest ($n=0$) Landau levels corresponding to spin-down (${\sigma }_3=-1$). In the presence of an electron-phonon coupling in the ground state, an attractive interaction for the polarized spin and opposite chirality is generated. A two-body attractive interaction projected to the two lowest ($n=0$) chiral Landau levels with the same spin polarization is obtained. The higher Landau level states ($n\ne 0$) are treated at the one-loop approximation with the two chiralities containing spins polarized as ${\sigma }_3=\pm 1$. Using the chiral anomaly in one dimension we show that the sliding phase is controlled by the electric field. This theory, for a Weyl semimetal with two nodes in the direction of the applied magnetic field, gives rise to a sliding charge density wave observed by Gooth et al. [Nature (London) 575, 315 (2019)].

The non-Hermitian skin effect can be realized through asymmetric hopping between forward and backward directions, where all the modes of the system are localized at one edge of a finite 1D lattice. Here we show theoretically that in a finite chain of 1D exciton-polariton micropillars with symmetric hopping, the inherent nonlinearity of the system can exhibit a double-sided skin (bi-skin) effect based of the fluctuations of the system, where the modes of the system are localized at the two edges of the system. To show the topological origin of such modes, we calculate the winding number. The bi-skin effect can be detected experimentally as an intensity drop at one edge of the chain and an increase at the opposite edge upon an increase of the polariton density under continuous wave excitation.

Polariton-based devices require materials where light-matter coupling under ambient conditions exceeds losses, but our current selection of such materials is limited. Here we measured the dispersion of polaritons formed by the $A$ and $B$ excitons in thin ${\mathrm{MoS}}_2$ slabs by imaging their optical near fields. We combined fully tunable laser excitation in the visible with a scattering near-field optical microscope to excite polaritons and image their optical near fields. We obtained the properties of bulk ${\mathrm{MoS}}_2$ from fits to the slab dispersion. The in-plane excitons are in the strong regime of light-matter coupling with a coupling strength ($40-100\mathrm{meV}$) that exceeds their losses by at least a factor of two. The coupling becomes comparable to the exciton binding energy, which is known as very strong coupling. ${\mathrm{MoS}}_2$ and other transition metal dichalcogenides are excellent materials for future polariton devices.

We consider a semiconducting nanowire with Rashba spin-orbit interaction subjected to a magnetic field and in the presence of strong electron-electron interactions. When the ratio between Fermi and Rashba momenta is tuned to $1/2$, two competing resonant multiparticle scattering processes are present simultaneously and the interplay between them brings the system into a gapless critical parafermion phase. This critical phase is described by a self-dual sine-Gordon model, which we are able to map explicitly onto the low-energy sector of the ${\mathbb{Z}}_4$ parafermion clock chain model. Finally, we show that by alternating regions in which only one of these two processes is present one can generate localized zero-energy parafermion bound states.

Mermin-Wagner-Coleman theorem predicts no long-range magnetic order at finite temperature in the two-dimensional (2D) isotropic systems, but it does predict a quasi-long-range order with a divergent correlation length at the Kosterlitz-Thouless (KT) transition for planar magnets. As a representative of two-dimensional planar antiferromagnets, single-layer ${\mathrm{CoPS}}_3$ carries the promise of monolayer antiferromagnetic platforms for the ultimately thin spintronics. Here, with the aid of magnetostriction which is sensitive to the local magnetic order, we observe the signature phonon mode splitting of below $T_{\mathrm{KT}}$ in monolayer ${\mathrm{CoPS}}_3$. The two-magnon signal in the monolayer one manifests the associated magnetic transition. It indicates the quasi-long-range order in an exact 2D planar spin model below KT phase transition. The ratio ($J$′/$J$) between the interlayer and intralayer interactions, which characterizes the 2D behaviors, is evaluated to be around 0.03. Our results provide an efficient method to detect the quasi-long-range antiferromagnetic ordering in the two-dimensional magnets down to the monolayer limit.

We present a first principles implementation of the dynamic transverse magnetic susceptibility in the framework of linear response time-dependent density functional theory. The dynamic susceptibility allows one to obtain the magnon dispersion as well as magnon lifetimes for a particular material, which strongly facilitates the interpretation of inelastic neutron scattering experiments as well as other spectroscopic techniques. We apply the method to Fe, Ni, and Co and perform a thorough convergence analysis with respect to the basis set size, $k$-point sampling, spectral smearing, and unoccupied bands. In particular, it is shown that while the gap error (acoustic magnon energy at $\mathbf{q}=\mathbf{0}$) is highly challenging to converge, the spin-wave stiffness and the dispersion relation itself are much less sensitive to convergence parameters. Our final results agree well with experimentally extracted magnon dispersion relations except for Ni, where it is well known that the exchange splitting energy is poorly represented in the local density approximation. We also find good agreement with previous first principles calculations and explain how differences in the calculated dispersion relations can arise from subtle differences in computational approaches.

We introduce a machine-learning density-functional-theory formalism for the spinless Hubbard model in one dimension at both zero and finite temperature. In the zero-temperature case this establishes a one-to-one relation between the site occupation and the total energy, which is then minimized at the ground-state occupation. In contrast, at finite temperature the same relation is defined between the Helmholtz free energy and the equilibrium site occupation. Most importantly, both functionals are semilocal, so that they are independent from the size of the system under investigation and can be constructed over exact data for small systems. These “exact” functionals are numerically defined by neural networks. We also define additional neural networks for finite-temperature thermodynamical quantities, such as the entropy and heat capacity. These can be either a functional of the ground-state site occupation or of the finite-temperature equilibrium site occupation. In the first case their equilibrium value does not correspond to an extremal point of the functional, while it does in the second case. Our work gives us access to finite-temperature properties of many-body systems in the thermodynamic limit.

We address a particular set of extended Su-Schrieffer-Heeger models with $2n$ sites in the unit cell $\left[\mathrm{SSH}\left(2n\right)\right]$, that we designate by sine-cosine models $\left[\mathrm{SC}\right(n\left)\right]$, with hopping terms defined as a sequence of $n$ sine-cosine pairs of the form $\{sin\left({\theta }_j\right),cos\left({\theta }_j\right)\}$, $j=1,\cdots ,n$. These models, when squared, generate a block-diagonal matrix representation with one of the blocks corresponding to a chain with uniform local potentials. We further focus our study on the subset of $\mathrm{SC}\left(2^{n-1}\right)$ chains that, when squared an arbitrary number of times (up to $n$), always generate a block which is again a sine-cosine model, if an energy shift is applied and if the energy unit is renormalized. We show that these $n$-times squarable models $\left[\mathrm{SSC}\right(n\left)\right]$ and their band structure are uniquely determined by the sequence of energy unit renormalizations and by the energy shifts associated with each step of the squaring process. Chiral symmetry is present in all sine-cosine chains and edge-state levels at the respective central gaps are protected by it. Zero-energy edge states in a $\mathrm{SSC}\left(j\right)$ chain (with $j<n$) of the Matryoshka sequence obtained squaring the $\mathrm{SSC}\left(n\right)$ chain with open boundary conditions become finite-energy edge states in noncentral band gaps of the $\mathrm{SSC}\left(n\right)$ chain. The extension to higher dimensions is also discussed.

The heavy fermion superconductor $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ is a candidate for chiral, time-reversal symmetry-breaking superconductivity with a nodal gap structure. Here, we microscopically visualized superconductivity and spatially inhomogeneous ferromagnetism in $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$. We observed linear-$T$ superfluid density, consistent with $d$-wave pairing symmetries including chiral $d$ wave, but did not observe the spontaneous magnetization expected for chiral $d$ wave. Local vortex pinning potentials had either four- or twofold rotational symmetries with various orientations at different locations. Taken together, these data support a nodal gap structure in $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ and suggest that chirality either is not present or does not lead to detectable spontaneous magnetization.

We show the formation of a nonequilibrium excitonic condensate in a bulk ${\mathrm{WSe}}_2$ coherently pumped in resonance with the lowest-energy exciton. The lifetime of the superfluid is addressed by studying the screened dynamics during and after the pump pulse. Intervalley scattering causes electron migration from the optically populated K valley to the conduction band minimum at $\mathrm{\Sigma }$. Due to the electron-hole imbalance at the K point a plasma of quasifree holes develops, which efficiently screens the interaction of the remaining excitons. We show that this plasma screening causes an ultrafast melting of the nonequilibrium condensate and that during melting coherent excitons and quasifree electron-hole pairs coexist. The time-resolved spectral function does exhibit a conduction and excitonic sidebands of opposite convexity and relative spectral weight that changes in time. Both the dependence of the time-dependent conduction density on the laser intensity and the time-resolved spectral function agree with recent ARPES experiments.