Spin supersolids and spin superfluids reveal complex canted spin structures with independent order of longitudinal and transverse spin components. This work addresses the question whether these exotic phases can lead to spin-driven ferroelectricity. Here we report the results of dielectric and pyrocurrent measurements of $\mathrm{MnC}{\mathrm{r}}_2{\mathrm{S}}_4$ as function of temperature and magnetic field up to 60 T. This sulfide chromium spinel exhibits a Yafet-Kittel type spin structure at low temperatures. As function of external magnetic field, the manganese spins undergo a sequence of ordering patterns of the transverse and longitudinal spin components, which can be mapped onto phases as predicted by lattice-gas models including solid, liquid, super-fluid, and supersolid phases. By detailed dielectric and pyrocurrent measurements, we document a zoo of multiferroic phases with sizable ferroelectric polarization strongly varying from phase to phase. Using lattice-gas terminology, the title compound reveals multiferroic spin-superfluid and spin-supersolid phases, while the antiferromagnetic solid is paraelectric.

We report neutron-scattering and ac magnetic susceptibility measurements of the two-dimensional spin-1/2 frustrated magnet $\mathrm{BaCdVO}{\left({\mathrm{PO}}_4\right)}_2$. At temperatures well below $T_{\mathsf{N}}\approx 1\text{K}$, we show that only 34% of the spin moment orders in an up-up-down-down stripe structure. Dominant magnetic diffuse scattering and comparison to published $\mathrm{muon}-\mathrm{spin}-\mathrm{rotation}$ measurements indicates that the remaining 66% is fluctuating. This demonstrates the presence of strong frustration, associated with competing ferromagnetic and antiferromagnetic interactions, and points to a subtle ordering mechanism driven by magnon interactions. On applying magnetic field, we find that at $T=0.1$ K the magnetic order vanishes at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue that the putative high-field phase is a realization of the long-sought bond-spin-nematic state.

Extending a Boltzmann-Langevin theory to magnons, we show a universality of current-noise suppression in diffusive systems against the difference in the quantum-statistical properties of bosons and fermions. To this end, starting from a quantum kinetic equation for magnons subjected to a thermal gradient in dilute impurities, we derive a bosonic counterpart of the semiclassical Boltzmann-Langevin equation for electrons and evaluate a magnonic current noise in the diffusive insulating magnet. We theoretically discover that compared with a Poissonian shot noise of magnons in an insulating ferromagnetic junction, the magnonic shot noise is suppressed in the diffusive insulating bulk magnet and the noise-to-current ratio (Fano factor) at low temperatures exhibits a universal behavior, i.e., the same $\frac{1}{3}$-suppression as the one for electron transport in diffusive conductors, despite the difference in quantum-statistical properties. Finally, we show that our predictions are within experimental reach with current device and measurement technologies.

We have developed the multiperspective methods for discussing phase transition and evolution of molecular vibration using unpolarized and polarized Raman scatterings. There are three scattering characteristics serving as the criteria to judge the vibrational modes, molecular symmetry, and first-order phase transition. Except for the frequency collection of one or more phonon modes, remarkably, we first demonstrate in theory that polarized Raman scattering exhibits high sensitivity on dissecting first-order phase transition, such as the symmetry breaking and discontinuity during phase transition. Experimental results confirm that temperature dependencies of peak intensity and the depolarization ratio extracted from polarized spectra present the abrupt changes associated with the discontinuity of first-order phase transition. Thermal discontinuous transformation of the ${(\mathrm{K},\mathrm{Na})\mathrm{NbO}}_3$ lattice brings into physical correspondence with the molecule polarizability and order parameter of the system, i.e., spontaneous polarization, which is the first derivative of Gibbs free energy. Hence, these methods based on the scattering criteria could be broadly adapted for more extensive studies on the phase transition in condensed matter.

The band-filling effect, which is due to tuning the electron occupation number near the Fermi level, on the intrinsic magnetic damping parameter is demonstrated by employing high-quality epitaxial ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x$ alloy films, in which the Fermi level and the density of states can be continuously tuned by varying the Ni concentration $x$. The intrinsic magnetic damping parameter, measured by the time-resolved magneto-optical Kerr effect, changes weakly at small $x$ and increases sharply for $x>0.80$. The experimental results are well reproduced by the density functional theory calculation. More interestingly, the magnetic damping parameter and the density of states near the Fermi level share similar variation trends, demonstrating their correlation. The band-filling effect in $3d$ magnetic transition metal alloys provides a way to tune the magnetic damping parameter as a key element in the controlg of energy loss and speed of spintronic devices.

The Kondo lattice compound CePtPb crystallizes in the ZrNiAl-type structure with the space group $P\overline{6}2m$, where the ${\mathrm{Ce}}^{3+}$ ions form a quasikagome lattice in the $ab$ plane. We have constructed a temperature versus magnetic field ($T\text{−}H$) phase diagram for single-crystal CePtPb from electrical resistivity and specific heat measurements. Our results indicate the occurrence of long-range antiferromagnetic order below $T_{\mathrm{N}}=0.95\mathrm{K}$ and multiple magnetically ordered phases as $T_{\mathrm{N}}$ is suppressed by applied magnetic field. Although $T_{\mathrm{N}}$ can be continuously suppressed to zero by the field, the resistivity and specific heat close to the critical field $H_c\approx 6$ kOe do not follow simple models of quantum criticality. Above $H_c$, the electrical resistivity shows an unconventional $T^n$ dependence with $n>2$. We also present muon spin relaxation measurements, which reveal residual spin dynamics below $T_{\mathrm{N}}$ that are consistent with only two thirds of the Ce-$4f$ spins participating in the antiferromagnetic order.

We demonstrate that there exists a continuum Hamiltonian $H(\mathbf{r},\mathbf{p})$ that is formally the operator equivalent of the general tight-binding method, inheriting the associativity and Hermiticity of the latter operator. This provides a powerful and controlled method of obtaining effective Hamiltonians via Taylor expansion with respect to momentum and, optionally, deformation fields. In particular, for fundamentally nonperturbative defects, such as twist faults and partial dislocations, the method allows the deformation field to be retained to all orders, providing an efficient scheme for the generation of transparent and compact Hamiltonians for such defects. We apply the method to a survey of incommensurate physics in twist bilayers of graphene, graphdiyne, ${\mathrm{MoS}}_2$, and phosphorene. For graphene we are able to reproduce the “reflected Dirac cones” of the ${30}^{\circ }$ quasicrystalline bilayer found in a recent angle-resolved photoemission spectroscopy experiment, and we show that it is an example of a more general phenomenon of coupling by the moiré momentum. We show that incommensurate physics is governed by the decay of the interlayer interaction on the scale of the single-layer reciprocal lattices, and demonstrate that incommensurate scattering effects lead to a very rapid broadening of band manifolds as the twist angle is tuned through commensurate values.

We study non-Bloch bulk-boundary correspondence in a non-Hermitian Su-Schieffer-Heeger model in a domain wall configuration where the left and right bulks have different parameters. Focusing on the case where chiral symmetry is still conserved, we show that non-Hermitian skin effects of bulk states persist in the system, while the definition of the non-Bloch winding number of either bulk depends on parameters on both sides of the boundary. Under these redefined non-Bloch topological invariants, we confirm non-Bloch bulk-boundary correspondence under the domain wall configuration, which exemplifies the impact of boundary conditions in non-Hermitian topological systems.

Predicting the mechanical and thermal properties of quasi-two-dimensional (2D) transition metal dichalcogenides (TMDs) is an essential task necessary for their implementation in device applications. Although rigorous density-functional-theory–based calculations are able to predict mechanical and electronic properties, mostly they are limited to zero temperature. Classical molecular dynamics facilitates the investigation of temperature-dependent properties, but its performance highly depends on the potential used for defining interactions between the atoms. In this study, we calculated temperature-dependent phonon properties of single-layer TMDs, namely, ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$, by utilizing Stillinger-Weber–type potentials with optimized sets of parameters with respect to the first-principles results. The phonon lifetimes and contribution of each phonon mode in thermal conductivities in these monolayer crystals are systematically investigated by means of the spectral-energy-density method based on molecular dynamics simulations. The obtained results from this approach are in good agreement with previously available results from the Green-Kubo method. Moreover, detailed analysis of lattice thermal conductivity, including temperature-dependent mode decomposition through the entire Brillouin zone, shed more light on the thermal properties of these 2D crystals. The LA and TA acoustic branches contribute most to the lattice thermal conductivity, while ZA mode contribution is less because of the quadratic dispersion around the Brillouin zone center, particularly in ${\mathrm{MoSe}}_2$ due to the phonon anharmonicity, evident from the redshift, especially in optical modes, by increasing temperature. For all the considered 2D crystals, the phonon lifetime values are compelled by transition metal atoms, whereas the group velocity spectrum is dictated by chalcogen atoms. Overall, the lattice thermal conductivity is linearly proportional with inverse temperature.

The $4\pi$-periodic Josephson effect is an indicator of Majorana zero modes and a ground-state degeneracy which are central to topological quantum computation. However, the observability of a $4\pi$-periodic Josephson current-phase relation (CPR) is hindered by the necessity to fix the fermionic parity. As an alternative to a $4\pi$-periodic CPR, this paper proposes a chiral CPR for the $4\pi$-periodic Josephson effect. This is a CPR of the form $J\left(\varphi \right)\propto C|sin(\varphi /2\left)\right|$, describing a unidirectional supercurrent with the chirality $C=\pm 1$. Its nonanalytic dependence on the Josephson phase difference $\varphi$ translates into the $4\pi$-periodic CPR $J\left(\varphi \right)\propto sin(\varphi /2)$. The proposal requires a spin-polarized topological Josephson junction which is modeled here as a short link between spin-split superconducting channels at the edge of a two-dimensional topological insulator. In this case, $C$ coincides with the Chern number of the occupied spin band of the topological insulator. This paper details three scenarios of achieving a chiral CPR: By only Zeeman-like splitting, by Zeeman splitting combined with bias currents, and by an external out-of-plane magnetic field.

We study the coherent nonequilibrium dynamics of interacting two-dimensional systems after a quench from a trivial to a topological Chern insulator phase. While the many-body wave function is constrained to remain topologically trivial under local unitary evolution, we find that the Hall response of the system can dynamically approach a thermal value of the postquench Hamiltonian, even though the efficiency of this thermalization process is shown to strongly depend on the microscopic form of the interactions. Quite remarkably, the effective temperature of the steady-state Hall response can be arbitrarily tuned with the quench parameters. Our findings suggest a way of inducing and observing low-temperature topological phenomena in interacting ultracold atomic gases, where the considered quench scenario can be realized in current experimental setups.

The orthorhombic perovskite iridate ${\mathrm{SrIr}}_{0.8}{\mathrm{Sn}}_{0.2}{\mathrm{O}}_3$ is reported to be a new candidate of Slater insulator at low temperatures. Here, we present optical spectroscopy measurements on ${\mathrm{SrIr}}_{0.8}{\mathrm{Sn}}_{0.2}{\mathrm{O}}_3$ at different temperatures ($T$) across the paramagnetic-antiferromagnetic phase transition. The low energy reflectivity decreases with reducing $T$. Six phonon peaks in the spectrum at low $T$ are visible at room temperature, indicating the absence of crystal structure phase transition from 300 K to 10 K. The real part of conductivity spectra ${\sigma }_1\left(\omega \right)$ are obtained by employing the standard Kramers-Kronig transformation. In the paramagnetic state at room temperature, ${\sigma }_1\left(\omega \right)$ reaches a limiting value as $\omega \to 0$, implying an electrical conducting state. Upon decreasing $T$, the low frequency ${\sigma }_1\left(\omega \right)$ decreases and approaches zero with a concomitant opening of a direct gap. Detailed analysis of the low energy data shows that the gap opening is continuous and ${\sigma }_1\left(\omega \right)$ at the low energy gap edge follows an unusual ${\omega }^{1/2}$ dependence at low temperatures which is remarkably similar to other Slater insulators, such as ${\mathrm{NaOsO}}_3$.

We present magnetic and electrical transport properties such as resistivity, magnetoresistance, dielectric, and polarization of polycrystalline ${\mathrm{YbFe}}_2{\mathrm{O}}_4$. The ferrimagnetic transition temperature is measured at 243 K, followed by the two low-temperature transitions at $\sim 190$ and $\sim 65$ K, respectively. The magnetic properties including the $M-H$ hysteresis loops exhibit a strong temperature dependence and possibly indicate a spin-glass state below 65 K for ${\mathrm{YbFe}}_2{\mathrm{O}}_4$. The iron Mössbauer measurement at 295 K confirms the presence of two Fe sites. The measured resistivity can be modeled with the Mott's variable-range hopping model, $\rho \propto exp{(T_0/T)}^{1/4}$, indicating the electron hopping between ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ sites. The magnetoresistance effects up to 6% at 8 T were observed and the effects could be caused by the field-induced changes in the electron-hopping processes. The frequency-dependent complex dielectric constant has been found to be strongly influenced by the contact effects, and the polarization of polycrystalline ${\mathrm{YbFe}}_2{\mathrm{O}}_4$ does not show ferroelectricity.

Finding ways of creating, measuring, and manipulating Majorana bound states (MBSs) in superconducting-semiconducting nanowires is a highly pursued goal in condensed matter physics. It was recently proposed that a periodic covering of the semiconducting nanowire with superconductor fingers would allow both gating and tuning the system into a topological phase while leaving room for a local detection of the MBS wave function. We perform a detailed, self-consistent numerical study of a three-dimensional (3D) model for a finite-length nanowire with a superconductor superlattice including the effect of the surrounding electrostatic environment, and taking into account the surface charge created at the semiconductor surface. We consider different experimental scenarios where the superlattice is on top or at the bottom of the nanowire with respect to a back gate. The analysis of the 3D electrostatic profile, the charge density, the low-energy spectrum, and the formation of MBSs reveals a rich phenomenology that depends on the nanowire parameters as well as on the superlattice dimensions and the external back-gate potential. The 3D environment turns out to be essential to correctly capture and understand the phase diagram of the system and the parameter regions where topological superconductivity is established.

The electrical resistance, Hall resistance, and thermoelectric power of the Ising-like antiferromagnet $\mathrm{UIrS}{\mathrm{i}}_3$ were measured as functions of temperature and magnetic field. We have observed that the unequivocally different characters of first-order and second-order magnetic phase transitions lead to distinctly different magnetotransport properties in the neighborhood of corresponding critical temperatures and magnetic fields, respectively. The magnetic contributions to the electrical and Hall resistivity in the antiferromagnetic state, and the polarized and normal regimes of paramagnetic state are driven by different underlying mechanisms. Results of detailed measurements of magnetotransport in the vicinity of the tricritical point reveal that the Hall-resistivity steps at phase transitions change polarity just at this point. The jumps in field dependences of specific heat, electrical resistivity, Hall resistivity, and Seebeck coefficient at the first-order metamagnetic transitions indicate a Fermi surface reconstruction, which is characteristic of a magnetic-field-induced Lifshitz transition. The presented results emphasize the usefulness of measurements of electrical- and thermal-transport properties as sensitive probes of magnetic phase transformations in antiferromagnets sometimes hardly detectable by other methods.

The magnetic droplet soliton is a large-amplitude, coherently precessing wave state that exists in ferromagnetic thin films with perpendicular magnetic anisotropy. To effectively sustain a droplet, magnetic damping can be locally compensated in a nanocontact region that imparts spin-transfer torque; this has been successfully deployed in experiment to directly image the droplet and probe its dynamics electrically. However, theory predicts and experiments indicate the existence of a drift instability whereby the droplet is ejected from the spin-transfer-torque-active region and subsequently decays, an effect that may be enhanced or possibly induced by thermal fluctuations. Using soliton perturbation theory and large-deviation theory, this work determines the soliton ejection rate and the most likely path an ejected soliton tracks in the presence of thermal fields. These results lead to an effective lower bound on the stability of magnetic droplet solitons in spin-transfer-torque nanocontact devices operating at finite temperature and point to ways in which droplets can be made more robust.

We study thermal effects on pinning and creep in type-II superconductors where vortices interact with a low density $n_p$ of strong pointlike defects with pinning energy $e_p$ and extension $\xi$, the vortex core size. Defects are classified as strong if the interaction between a single pin and an individual vortex leads to the appearance of bistable solutions describing pinned and free vortex configurations. Extending the strong pinning theory to account for thermal fluctuations, we provide a quantitative analysis of vortex depinning and creep. We determine the thermally activated transitions between bistable states using Kramers rate theory and find the nonequilibrium steady-state occupation of vortex states. The latter depends on the temperature $T$ and vortex velocity $v$ and determines the current-voltage (or force-velocity) characteristic of the superconductor at finite temperatures. We find that the $T=0$ linear excess-current characteristic $v\propto (j-j_c)\mathrm{\Theta }(j-j_c)$, with its sharp transition at the critical current density $j_c$, keeps its overall shape but is modified in three ways due to thermal creep: a downward renormalization of $j_c$ to the thermal depinning current density $j_{\mathrm{dp}}\left(T\right)<j_c$, a smooth rounding of the characteristic around $j_{\mathrm{dp}}\left(T\right)$, and the appearance of thermally assisted flux flow $v\propto jexp(-U_0/k_{\mathrm{B}}T)$ at small drive $j\ll j_c$, with the activation barrier $U_0$ defined through the energy landscape at the intersection of free and pinned branches. This characteristic emphasizes the persistence of pinning of creep at current densities beyond critical.

Robust quantum systems rely on having a protective environment with minimized relaxation channels. Superconducting gaps play an important role in the design of such environments. The interaction of localized single spins with a conventional superconductor generally leads to intrinsically extremely narrow Yu-Shiba-Rusinov (YSR) resonances protected inside the superconducting gap. However, this may not apply to superconductors with more complex, energy-dependent order parameters. Exploiting the Fe-doped two-band superconductor ${\mathrm{NbSe}}_2$, we show that due to the nontrivial relation between its complex-valued and energy-dependent order parameters, YSR states are no longer restricted to be inside the gap. They can appear outside the gap (i.e., inside the coherence peaks), where they can also acquire a substantial intrinsic lifetime broadening. T-matrix scattering calculations show excellent agreement with the experimental data and relate the intrinsic YSR state broadening to the imaginary part of the host's order parameters. Our results suggest that nonthermal relaxation mechanisms contribute to the finite lifetime of the YSR states, even within the superconducting gap, making them less protected against residual interactions than previously assumed. YSR states may serve as valuable probes for nontrivial order parameters promoting a judicious selection of protective superconductors.

We demonstrate theoretically and confirm experimentally that nonlinear spin waves excited in thin yttrium iron garnet films are good candidates for squeezing vacuum quantum noise. The experimental demonstration is in the form of a measurement of spin-wave induced modulation instability (IMI) conducted in the classical regime. The experiment evidences strong phase locking of an idler wave parametrically generated in the film with a deterministic small-signal wave launched into the film from an external source. The theory predicts that the same behavior will be observed for vacuum quantum noise, resulting in squeezing of the noise.

Domain boundaries in lead phosphate, $\mathrm{P}{\mathrm{b}}_3{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$, are investigated by using second harmonic generation (SHG) microscopy. It is revealed that they are polar while the bulk is a purely ferroelastic, centrosymmetric material. $\mathrm{P}{\mathrm{b}}_3{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ contains W and $W{}^{\prime }$ walls with different orientations. The SHG microscopy shows that both $W$ and $W{}^{\prime }$ walls are polar, in striking contrast to the bulk, where the macroscopic polarity is forbidden by symmetry. The strongest polarity is observed in the W walls with a local symmetry of monoclinic $m$. The $W{}^{\prime }$ walls are also monoclinic but with symmetry 2. This material is hence a new candidate for domain-boundary-induced ferroelectricity while the bulk remains fully inactive. An application of small stress to a W wall easily changes the boundary configuration without changing the type of domain boundary; i.e., the W-wall nature is conserved. While $\mathrm{P}{\mathrm{b}}_3{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ was so far suggested as a device material for acousto-optic switchers, the present study adds the possibility that light scattering can be modified also by electric fields.