We analyze the dynamical response of a range of three-dimensional Kitaev quantum spin liquids, using lattice models chosen to explore the different possible low-energy spectra for gapless Majorana fermions, with either Fermi surfaces, nodal lines, or Weyl points. We find that the behavior of the dynamical structure factor is distinct in all three cases, reflecting the quasiparticle density of states in two fundamentally different ways. First, the low-energy response is either straightforwardly related to the power with which the low-energy density of states vanishes; or for a nonvanishing density of states, to the phase shifts encountered in the corresponding x-ray edge problem, whose phenomenology we extend to the case of Majorana fermions. Second, at higher energies, there is a rich fine structure, determined by microscopic features of the Majorana spectrum. Our theoretical results test the usefulness of inelastic neutron scattering as a probe of these quantum spin liquids: we find that although spin flips fractionalize, the main features of the dynamical spin response nevertheless admit straightforward interpretations in terms of Majorana and flux loop excitations.

Three-dimensional topological semimetals come in different variants, either containing Weyl points or Dirac lines. Here we describe a more complicated momentum-space topological defect where several separate Dirac lines connect with each other, forming a momentum-space equivalent of the real-space nexus considered before for helium-3. Close to the nexus the Dirac lines exhibit a transition from type I to type II lines. We consider a general model of stacked honeycomb lattices with the symmetry of Bernal (AB) stacked graphite and show that the structural mirror symmetries in such systems protect the presence of the Dirac lines, and also naturally lead to the formation of the nexus. By the bulk-boundary correspondence of topological media, the presence of Dirac lines lead to the formation of drumhead surface states at the side surfaces of the system. We calculate the surface state spectrum, and especially illustrate the effect of the nexus on these states.

We report on experimental study of the exciton-polariton emission (PE) polarization noise below and above the polariton lasing threshold under continuous-wave nonresonant excitation. The experiments were performed with a high-$Q$ graded $5\lambda /2$ GaAs/AlGaAs microcavity with four sets of three quantum wells in the strong-coupling regime. The PE polarization noise substantially exceeded in magnitude the shot-noise level and, in the studied frequency range (up to 650 MHz), had a flat spectrum. We have found that the polarization and intensity noise dependences on the pump power are strongly different. This difference is ascribed to the bosonic stimulation effect in spin-dependent scattering of the polaritons to the condensate. A theoretical model describing the observed peculiarity of the PE polarization noise is proposed.

We show that tunneling involving bosonic wires and/or boson integer quantum Hall (bIQH) edges is characterized by features that are far more universal than those in their fermionic counterpart. Considering a pair of minimal geometries, we examine the tunneling conductance as a function of energy (e.g., chemical potential bias) at high and low energy limits, finding a low energy enhancement and a universal high versus zero energy relation that hold for all wire/bIQH edge combinations. Beyond this universality present in all the different topological (bIQH-edge) and nontopological (wire) setups, we also discover a number of features distinguishing the topological bIQH edges, which include a current imbalance to chemical potential bias ratio that is quantized despite the lack of conductance quantization in the bIQH edges themselves. The predicted phenomena require only initial states to be thermal and thus are well suited for tests with ultracold bosons forming wires and bIQH states. For the latter, we highlight a potential realization based on single component bosons in the recently observed Harper-Hofstadter band structure.

We reinvestigate the putative giant spin splitting at the surface of ${\mathrm{SrTiO}}_3$ reported by Santander–Syro et al. [Nat. Mater. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on fractured (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a giant spin splitting while it is consistent with the unconventional Rashba-like splitting seen in band structure calculations that reproduce the experimentally observed ladder of quantum confined subbands.

A highlighting feature of Majorana bound states in two-dimensional topological superconductors is that they gain a phase factor of $\pi$ upon being orbited by a vortex. This work focuses on the vortex degree of freedom itself and demonstrates that the change in the Majorana state is accompanied by a fermion parity change within the vortex. Such a parity flip is interpreted as a higher dimensional analog of the fermion parity pump mechanism in superconducting wires as well as through general topological arguments. It is demonstrated in terms of level crossings in three different situations: in (i) spin-triplet paired superconductors and in proximity-induced superconducting systems involving (ii) quantum spin Hall-ferromagnet hybrids and (iii) Chern insulators.

We study the heat conductivity in Anderson insulators in the presence of a power-law interaction. Particle-hole excitations built on localized electron states are viewed as two-level systems randomly distributed in space and energy and coupled due to electron-electron interaction. A small fraction of these states form resonant pairs that in turn build a complex network allowing for energy propagation. We identify the character of energy transport through this network and evaluate the thermal conductivity. For physically relevant cases of two-dimensional and three-dimensional spin systems with $1/r^3$ dipole-dipole interaction (originating from the conventional $1/r$ Coulomb interaction between electrons), the found thermal conductivity $\kappa$ scales with temperature as $\kappa \propto T^3$ and $\kappa \propto T^{4/3}$, respectively. Our results may be of relevance also to other realizations of random spin Hamiltonians with long-range interactions.

We explore the reactivity of xenon with sulfur under high pressure, using unbiased structure searching techniques combined with first-principles calculations, which identify a stable ${\mathrm{XeS}}_2$ compound crystallized in a Laves phase with hypercoordinated (16-fold) Xe at 191 GPa and 0 K. Taking the thermal effects into account, we find that increasing the temperature could further stabilize it. The formation of ${\mathrm{XeS}}_2$ is a consequence of pressure-induced charge transfer from Xe to S atoms and the delocalization of Xe $5p$ and S $3p$ electrons. Meanwhile, the stabilization into a Laves phase of ${\mathrm{XeS}}_2$ is the result of delocalized chemical bonding and the need for optimum structure packing. The present discussion of the formation mechanism in ${\mathrm{XeS}}_2$ is general, and conclusions can be used to understand the formation of other Laves-phase compounds and the Xe chemistry that allows closed-shell Xe to participate in chemical reactions.

We report on a drastic change of the Ni collective magnetization dynamics when incorporated into a $\mathrm{Ni}\text{/}{\mathrm{V}}_2{\mathrm{O}}_3$ heterostructure. Two, unexpected, well-defined Ni ferromagnetic resonance (FMR) modes are observed in the coexistence region of the first-order ${\mathrm{V}}_2{\mathrm{O}}_3$ structural phase transition (SPT). The phase coexistence across the ${\mathrm{V}}_2{\mathrm{O}}_3$ SPT can explain the presence of the two resonance fields but not their anticrossing and large linewidth broadenings. Our results imply a strong coupling between the lattice dynamics of the strongly correlated oxide (${\mathrm{V}}_2{\mathrm{O}}_3$) and the magnon modes of the ferromagnet (Ni) in this hybrid. This and additional experiments on Ni grown on $\mathrm{SrTi}{\mathrm{O}}_3$, a prototypical second-order phase transition oxide, imply that these effects require the presence of first-order transitions in the oxides.

Among the interfacial transport modulations to the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (LAO/STO) heterostructure, mechanical strain has been proven to be an effective approach by growing the LAO/STO films on different substrates with varying lattice mismatches to STO. However, this lattice-mismatch-induced strain effect is static and biaxial, hindering the study of the strain effect in a dynamic way. In this work we realize dynamic and uniaxial strain to the LAO/STO oxide heterostructure at low temperature, through mechanical coupling from a magnetostrictive template. This anisotropic strain results in symmetry breaking at the interface and induces further splitting of the electronic band structure and therefore produces different conductivities along the $x$ and $y$ in-plane directions. In particular, we observe that along the strained direction the interface conductivity decreases by up to $70\%$ under a tensile strain, while it increases by $6.8\%$ under a compressive strain at 2 K. Also, it is revealed that the modulation on the interfacial transport property can be anisotropic, i.e., the resistance changes differently when an excitation current is parallel or perpendicular to the strain direction. This approach of strain engineering provides another degree of freedom for control of transport properties of oxide heterostructures and opens an additional way to investigate strain effects in materials science.

High-resolution inelastic neutron scattering reveals that the elementary magnetic excitations in multiferroic ${\mathrm{MnWO}}_4$ consist of low-energy dispersive electromagnons in addition to the well-known spin-wave excitations. The latter can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1) meV. Two electromagnon branches appear at lower energies of 0.07(1) and 0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric coupling and persist in both the collinear magnetic and paraelectric AF1 phase and the spin spiral ferroelectric AF2 phase. These excitations are associated with the Dzyaloshinskii-Moriya exchange interaction, which is significant due to the rather large spin-orbit coupling.

A phenomenology of magnetic chiral damping is proposed in the context of magnetic materials lacking inversion symmetry. We show that the magnetic damping tensor acquires a component linear in magnetization gradient in the form of Lifshitz invariants. We propose different microscopic mechanisms that can produce such a damping in ferromagnetic metals, among which local spin pumping in the presence of an anomalous Hall effect and an effective “s-d” Dzyaloshinskii-Moriya antisymmetric exchange. The implication of this chiral damping in terms of domain-wall motion is investigated in the flow and creep regimes.

Resonant x-ray scattering experiments on high-temperature superconductors and related cuprates have revealed the presence of intense paramagnon scattering at high excitation energies, of the order of several hundred meV. The excitation energies appear to show very similar behavior across all compounds, ranging from magnetically ordered, via superconductors, to heavy fermion systems. However, we argue that this apparent behavior has been inferred from the data through model fitting which implicitly imposes such similarities. Using model fitting that is free from such restrictions, we show that the paramagnons are not nearly as well defined as has been asserted previously, and that some paramagnons might not represent propagating excitations at all. Our work indicates that the data published previously in the literature will need to be reanalyzed with proper models.

We perform hole burning with a low-drift stabilized laser within the zero phonon line of the $4f-5d$ transition in ${\mathrm{Ce}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ at 2 K. The narrowest spectral holes appear for small applied magnetic fields and are $6\pm 4$ MHz wide (FWHM). This puts an upper bound on the homogeneous linewidth of the transition to $3\pm 2$ MHz, which is close to lifetime limited. The spin level relaxation time is measured to $72\pm 21$ ms with a magnetic field of 10 mT. A slow permanent hole burning mechanism is observed. If the excitation frequency is not changed the fluorescence intensity is reduced by more than $50\%$ after a couple of minutes of continuous excitation. The spectral hole created by the permanent hole burning has a width in the tens of MHz range, which indicates that a trapping mechanism occurs via the $5d$ state.

Using the Boltzmann formalism based on the first principles electronic structure and scattering rates, we investigate the current-induced spin accumulation and spin-orbit torques in FePt/Pt and Co/Cu bilayers in the presence of substitutional impurities. In FePt/Pt bilayers we consider the effect of intermixing of Fe and Pt atoms in the FePt layer and find a crucial dependence of spin accumulation and spin-orbit torques on the details of the distribution of these defects. In Co/Cu bilayers we predict that the magnitude and sign of the spin-orbit torque and spin accumulation depend very sensitively on the type of the impurities used to dope the Cu substrate. Moreover, simultaneously with impurity-driven scattering, we consider the effect of an additional constant quasiparticle broadening of the states at the Fermi surface to simulate phonon scattering at room temperature and discover that even a small broadening of the order of 25 meV can drastically influence the magnitude of the considered effects. We explain our findings based on the analysis of the complex interplay of several competing Fermi surface contributions to the spin accumulation and spin-orbit torques in these structurally and chemically nonuniform systems.

Resistive, magnetization, torque, specific heat, and scanning tunneling microscopy measurements are carried out on heavily-hole-doped ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ single crystals. A characteristic temperature $T^{*}\sim 13$ K, which is several times higher than the superconducting transition temperature $T_c=2.11$ K, is observed and possibly related to the superconducting fluctuation or the pseudogap state. A diamagnetic signal detected by torque measurements starts from the superconducting state, remains finite, and vanishes gradually until a temperature near $T^{*}$. Temperature-dependent resistivity and specific heat also show kinks near $T^{*}$. An asymmetric gap-like feature with the energy of 8.4 meV and a symmetric superconductivity-related gap of 2.4 meV on the scanning tunneling spectra are detected, and the pseudogap-related features disappear at temperatures up to at least 9 K. These observations by different experimental tools suggest the possible existence of superconducting fluctuations or a pseudogap state in the temperature range up to four to six times $T_c$ in ${\mathrm{CsFe}}_2{\mathrm{As}}_2$.

We report a strong difference in the sliding properties of the bidirectional charge-density wave (CDW) in the two-dimensional rare-earth tritelluride ${\mathrm{ErTe}}_3$ which occurs below $T_{\text{CDW1}}=265$ K with a wave vector along the $c$ axis and below $T_{\text{CDW2}}=165$ K with a wave vector along the $a$ axis; the excess current carried by the motion of the CDW is 10 times less for the lower CDW compared with the value of the upper one. We tentatively explain this result by a stronger pinning of the lower temperature CDW intricated with the upper one, which inhibits its motion and may generate a phase slippage lattice.

By the first-principles electronic structure calculations, we have systematically studied the electronic structures of recently discovered extremely large magnetoresistance (XMR) materials LaSb and LaBi. We find that both LaSb and LaBi are semimetals with the electron and hole carriers in balance. The calculated carrier densities on the order of ${10}^{20}{\mathrm{cm}}^{-3}$ are in good agreement with the experimental values, implying long mean-free time of carriers at low temperatures and thus high carrier mobilities. With a semiclassical two-band model, the charge compensation and high carrier mobilities naturally explain: (i) the XMR observed in LaSb and LaBi, (ii) the nonsaturating quadratic dependence of XMR on an external magnetic field, and (iii) the resistivity plateau in the turn-on temperature behavior at very low temperatures. The explanation of these features without resorting to the topological effect indicates that they should be the common characteristics of all electron-hole compensated semimetals.

The periodic Anderson model is widely studied to understand strong correlation physics and especially the competition of antiferromagnetism and singlet formation. In this paper we extend quantum Monte Carlo work on lattices with uniform numbers of neighbors to geometries in which the conduction electron sites can have variable coordination $z$. This situation is relevant both to recently discovered magnetic quasicrystals and also to magnetism in doped heavy fermion systems. Our key results are the presence of antiferromagnetic order at weak interorbital hybridization $V_{fd}$, and a delay in singlet formation to larger values of $V_{fd}$ on sites with larger $z$. The staggered magnetization tends to be larger on sites with higher $z$, providing insight into the behavior to be expected in crown, dice, and CaVO lattices.