We study effects in noncentrosymmetric superconductors arising from their unique coupling of Cooper-pair condensate and elasticity. We show that although the much discussed Lifshitz coupling is not observable in a uniform bulk state, it strikingly endows dislocations with a fractional magnetic flux. We also predict a generation of voltage-free strain by a DC current in a $P$- and $T$-breaking Josephson junction. Viewing superconductors through the lens of higher-form symmetries we identify the Lifshitz coupling as a chemical potential for the approximately conserved winding number, drawing an analogy with pyroelectric insulators.

We study theoretically the large variations of the supercurrent through a normal dot that are induced by a small quasiparticle injection current from normal leads connected to the dot. We find that the supercurrent decomposes into a subgap contribution, which depends on the voltages applied to the normal leads, as well as a contribution with opposite sign from energies outside the gap, which is insensitive to the voltages. As the voltages gradually suppress the subgap contribution, a critical voltage exists above which the contribution from energies outside the gap dominates, leading to a sign reversal of the current-phase relation, namely a transition to a so-called $\pi$-junction behavior. We determine the critical voltage and analyze the robustness of the effect with respect to temperature and inelastic relaxation in the dot.

A general framework is proposed to solve the two-dimensional (2D) fully frustrated XY model for the Josephson junction arrays in a perpendicular magnetic field. The essential idea is to encode the ground-state local rules induced by frustrations in the local tensors of the partition function. The partition function is then expressed in terms of a product of a 1D transfer matrix operator, whose eigenequation can be solved by an algorithm of matrix product states rigorously. The singularity of the entanglement entropy for the 1D quantum analog provides a stringent criterion to distinguish various phase transitions without identifying a priori order parameter. Two very close phase transitions are determined at $T_{c1}\approx 0.4459$ and $T_{c2}\approx 0.4532$, respectively. The former corresponds to a Berezinskii-Kosterlitz-Thouless phase transition describing the phase coherence of XY spins, and the latter is an Ising-like continuous phase transition below which a chirality order with spontaneously broken $Z_2$ symmetry is established.

We perform a computational structure search and study the phase diagram of elemental sulfur up to 100 TPa from first-principles using ab initio random structure searching (airss), discovering six new phases with the transition sequence $\text{bcc}\to I{4}_1/acd\to cI16\to \alpha \text{-U}\to \text{hcp}\to A15\to \text{bcc}$. The electronic properties and charge density distributions of the new phases are extensively studied. Remarkably, upon transition to the $I{4}_1/acd$ phase at 1.24 TPa, the superconducting critical temperature of sulfur abruptly drops from 12 K to just 0.4 K, and above 5 TPa, we find the critical temperature to be zero up to at least 100 TPa. We show that this sudden suppression of superconductivity is coincident with the formation of a sequence of electride phases, which feature significantly reduced electron-phonon coupling and substantial charge localization. Despite ceasing to be a superconductor, sulfur remains metallic up to at least 100 TPa.

Exchange bias is a phenomenon critical to solid-state technologies that require spin valves or nonvolatile magnetic memory. The phenomenon is usually studied in the context of magnetic interfaces between antiferromagnets and ferromagnets, where the exchange field of the former acts as a means to pin the polarization of the latter. In the present study, we report an unusual instance of this phenomenon in the topological Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, where the magnetic interfaces associated with domain walls suffice to bias the entire ferromagnetic bulk. Remarkably, our data suggest the presence of a hidden order parameter whose behavior can be independently tuned by applied magnetic fields. For micron-size samples, the domain walls are absent, and the exchange bias vanishes, suggesting the boundaries are a source of pinned uncompensated moment arising from the hidden order. This mechanism suggests that exciting opportunities lie ahead for the application of topological materials in spintronic technologies.

The research field of magnonics proposes a low-energy wave-logic computation technology based on spin waves to complement the established complementary metal-oxide-semiconductor technology and provide a basis for emerging unconventional computation architectures. However, magnetic damping is a limiting factor for all-magnonic logic circuits and multidevice networks, ultimately rendering mechanisms to efficiently manipulate and amplify spin waves a necessity. In this regard, parallel pumping is a versatile tool since it allows one to selectively generate and amplify spin waves. While extensively studied in microscopic systems, nanoscaled systems are lacking investigation to assess the feasibility and potential future use of parallel pumping in magnonics. Here, we investigate a longitudinally magnetized 100-nm-wide magnonic nanoconduit using space- and time-resolved microfocused Brillouin-light-scattering spectroscopy. Employing parallel pumping to generate spin waves, we observe that the nonresonant excitation of dipolar spin waves is favored over the resonant excitation of short wavelength exchange spin waves. In addition, we utilize this technique to access the effective spin-wave relaxation time of an individual nanoconduit, observing a large relaxation time up to $115.0\pm \left(76\right)\mathrm{ns}$. Despite the significant decrease of the pumping efficiency in the investigated nanoconduit, a reasonably small threshold is found rendering parallel pumping feasible on the nanoscale.

Structural, electronic, and magnetic properties of bulk ilmenite ${\mathrm{CoTiO}}_3$ are analyzed in the framework of density functional theory, using the generalized gradient approximation and Hubbard-corrected approaches. We find that the G-type antiferromagnetic structure, which consists of antiferromagnetically coupled ferromagnetic $ab$ planes, is the ground state of the system, in agreement with experiments. Furthermore, cobalt titanates present two critical temperatures related to the breaking of the inter- and intralayer magnetic ordering. This would result in the individual planes remaining ferromagnetic even at temperatures above the Néel temperature. When spin-orbit coupling is included in our calculations, we find an out-of-plane magnetic anisotropy, which can be converted to an in-plane anisotropy with a small doping of electrons corresponding to about 2.5% Ti substitution for Co, consistent with experimental expectations. We thus present a disorder-dependent study of the magnetic anisotropy in bulk ${\text{CoTiO}}_3$, which will determine its magnon properties, including topological aspects.

We report a comprehensive study of the noncentrosymmetric NbReSi superconductor by means of muon-spin rotation and relaxation ($\mu \mathrm{SR}$) and nuclear magnetic resonance (NMR) techniques. NbReSi is a bulk superconductor with $T_c=6.5$ K, characterized by a large upper critical field, which exceeds the Pauli limit. Both the superfluid density ${\rho }_{\mathrm{sc}}\left(T\right)$ (determined via transverse-field $\mu \mathrm{SR}$) and the spin-lattice relaxation rate $T_1^{-1}\left(T\right)$ (determined via NMR) suggest a nodeless superconductivity (SC) in NbReSi. We also find signatures of multigap SC, here evidenced by the field-dependent muon-spin relaxation rate and the electronic specific-heat coefficient. The absence of spontaneous magnetic fields below $T_c$, as evinced from zero-field $\mu \mathrm{SR}$ measurements, indicates a preserved time-reversal symmetry in the superconducting state of NbReSi. Finally, we discuss possible reasons for the unusually large upper critical field of NbReSi, most likely arising from its anisotropic crystal structure.

We study the modification of the spatial coupling parameter between interacting ballistic exciton-polariton condensates in the presence of photonic spin-orbit coupling appearing from transverse electric–transverse magnetic (TE-TM) splitting in planar semiconductor microcavities. We propose a strategy to make the coupling strength between next-nearest neighbors stronger than between nearest neighbors, which inverts the conventional idea of the spatial coupling hierarchy between sites. Our strategy relies on the dominantly populated high-momentum components in the ballistic condensates which, in the presence of TE-TM splitting, lead to rapid radial precession of the polariton pseudospin. As a consequence, condensate pairs experience distance-periodic screening of their interaction strength, severely modifying their synchronization and condensation threshold solutions.

We investigate the multicritical behavior of the three-dimensional ${\mathbb{Z}}_2$ gauge Higgs model at the multicritical point (MCP) of its phase diagram, where one first-order transition line and two continuous Ising-like transition lines meet. The duality properties of the model determine some features of the multicritical behavior at the MCP located along the self-dual line. Moreover, we argue that the system develops a multicritical $XY$ behavior at the MCP, which is controlled by the stable $XY$ fixed point of the three-dimensional multicritical Landau-Ginzburg-Wilson field theory with two competing scalar fields associated with the continuous ${\mathbb{Z}}_2$ transition lines meeting at the MCP. This implies an effective enlargement of the symmetry of the multicritical modes at the MCP to the continuous group O(2). We also provide numerical results to support the multicritical $XY$ scenario.

The discovery of topological insulators has opened a new chapter of condensed matter physics. Recently, several experiments have confirmed that the magnetoelectric susceptibility of a topological insulator with time-reversal symmetry is quantized in terms of the fine-structure constant. To realize such measurements, a number of challenges have been overcome; particularly, high-precision time-domain terahertz polarimetry is devised. Here an experiment is proposed to observe the topological magnetoelectric effect. Based on the generalized multisphere Mie theory, it is shown that the optical activity of a cluster of topological insulator spheres elevates as metal spheres are deliberately brought near to them: The destructive interference of fields of the induced electric dipoles allows fields of the induced magnetic dipoles, which originate from the topological magnetoelectric effect, to play a prominent role. For example, for clusters of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and $\mathrm{InSb}$ spheres, the elevated azimuth and ellipticity angles are about $\pm {20}^{\circ }$. Such a giant optical activity serves as an optical bridge to the determination of the fine-structure constant.

The breaking of time-reversal and point-group spatial symmetries can have a profound impact on superconductivity. One of the most extraordinary effects, due to the application of a magnetic field, is represented by the Abrikosov vortices with charged supercurrents circulating around their cores. Whether a similar phenomenon can be obtained by exploiting spatial symmetry breaking, e.g., through electric fields or mechanical strain, is a fundamentally relevant but not yet fully settled problem. Here, we show that in two-dimensional spin-singlet superconductors with an unusually low degree of spatial symmetry content, vortices with supercurrents carrying angular momentum around the core can form and be energetically stable. The vortex has zero net magnetic flux since it is made up of counterpropagating Cooper pairs with opposite orbital moments. By solving self-consistently the Bogoliubov–de Gennes equations in real space, we demonstrate that the orbital vortex is stable and we unveil the spatial distribution of the superconducting order parameter around its core. The resulting amplitude has a characteristic pattern with a pronounced angular anisotropy that deviates from the profile of conventional magnetic vortices. These hallmarks guide predictions for the experimental detection.

We study a chain of spinless fermions with a multimer hopping term which kinematically favors the formation of multimers and competes with single-particle hopping. We argue that this model generically stabilizes two different multimer phases, as well as intermediate phases where the free-fermion and multimer fluids coexist and do not spatially separate. Using density-matrix renormalization group techniques, we establish the phase diagram of the model in the case of trimers. For one of the intermediate phases, hybridization between the fermions and trimers liquids does occur and the onset of their correlations is well captured by a generalized BCS-like ansatz. The case of tetramers is finally addressed.

Recent measurements of charge instabilities in overdoped compounds rekindled the proposal that cuprates become superconductors by long-range order through Josephson coupling between nanoscopic charge domains. We use the theory of phase-ordering dynamics to show that incommensurate charge density waves (CDWs) are formed in the CuO planes by a series of free-energy wells separated by steep barriers. Charge oscillations in these domains give rise to a net hole-hole attraction proportional to the height of these barriers. Concomitantly, the self-consistent calculations yield localized superconducting amplitudes in the CDW domains characterizing a granular superconductor. We show that a transition by long-range phase order promoted by Josephson coupling elucidates many well-known features of cuprates like the high magnetic penetration depth anisotropy and the origin of the pseudogap, among others. Furthermore, the average Josephson energy reproduces closely the planar superfluid density temperature dependence of La-based films and the superconducting giant proximity effects of cuprates, a 20-year-old open problem.

Sufficient disorder is believed to localize static and periodically driven interacting chains. With quasiperiodic driving by $D$ incommensurate tones, the fate of this many-body localization (MBL) is unknown. We argue that randomly disordered MBL exists for $D=2$, but not for $D\ge 3$. Specifically, a putative two-tone driven MBL chain is neither destabilized by thermal avalanches seeded by rare thermal regions, nor by the proliferation of long-range many-body resonances. For $D\ge 3$, however, sufficiently large thermal regions have continuous local spectra and slowly thermalize the entire chain. En route, we generalize the eigenstate thermalization hypothesis to the quasiperiodically driven setting, and we verify its predictions numerically. Two-tone driving enables new topological orders with edge signatures; our results suggest that localization protects these orders indefinitely.

We theoretically predict the emergence of 120-degree spin order as a nonequilibrium steady state in the photodriven Kondo-lattice model on a triangular lattice. In the system away from the half filling with ferromagnetic ground state, the photoexcitation of conduction electrons and the photoinduced renormalization of bandwidth cause reconstruction of the band structure and subsequent redistribution of the electrons through relaxations, which result in an electronic structure similar to that in the half-filled system at equilibrium, where the 120-degree spin order is stabilized. In this photoinduced 120-degree spin ordered phase, domains of different spin-ordered planes are formed, and vortices of spin chirality vectors called $Z_2$ vortices appear at points where multiple domains meet. We also discuss favorable conditions and experimental feasibility to observe the predicted photoinduced magnetic phase transition by investigating dependencies on the light parameters (amplitude, frequency, and polarization), the electron filling, the strength of Kondo coupling, and the effects of antiferromagnetic coupling among the localized spins. Photoinduced magnetic structures proposed so far have been limited to simple collinear (anti)ferromagnetic orders or local magnetic defects in magnets. The present work paves a way to the optical creation of complex noncollinear magnetisms as a global nonequilibrium steady phase in photodriven systems.

In materials with strong electron-phonon ($e$-ph) interactions, charge carriers can distort the surrounding lattice and become trapped, forming self-localized (small) polarons. We recently developed an ab initio approach based on canonical transformations to efficiently compute the formation and energetics of small polarons [N.-E. Lee et al., Phys. Rev. Mater. 5, 063805 (2021)]. A different approach based on a Landau-Pekar energy functional has been proposed in the recent literature [W. H. Sio et al., Phys. Rev. Lett. 122, 246403 (2019); Phys. Rev. B 99, 235139 (2019)]. In this work, we analyze and compare these two methods in detail. We show that the small polaron energy is identical in the two formalisms when using the same polaron wave function. We also show that our canonical transformation formalism can predict polaron band structures and can properly treat zero- and finite-temperature lattice vibration effects, although at present using a fixed polaron wave function. Conversely, the energy functional approach can compute the polaron wave function, but as we show here, it neglects lattice vibrations and cannot address polaron self-localization and thermal band narrowing. Taken together, this work relates two different methods developed recently to study polarons from first-principles, highlighting their merits and shortcomings and discussing them both in a unified formalism.