Photoexcited carriers in three-dimensional topological insulators (3D TIs) decay rapidly through the electron-electron and electron-phonon scattering. While most studies focus on such fast dynamics, recent experiments find the slow photovoltage (PV) dynamics arising from the band-bending potentials, in which the optical transitions in two-dimensional electron gas (2DEG) are effective. Although early investigations speculated the existence of multiple band-bending structures from the TI surface to the TI bulk, how PV and 2DEG are correlated in the presence of such multiple band bendings has been less explored. Here, we employ the combination of time- and angle-resolved photoemission spectroscopy (tr-ARPES) and ultrafast time-resolved terahertz (tr-THz) spectroscopy to investigate the PV and 2DEG dynamics in the prototypical topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our tr-ARPES analysis identifies two spatially separated PV dynamics associated with two types of band bending: one is the well-known surface PV, and another PV is formed deep within the bulk, which we call “internal bulk PV.” For the surface PV, our tr-THz spectra substantiate that the $\mu \mathrm{s}$-long transient signal arises from the surface-PV-induced increase of the TSS and 2DEG carrier density, which appears as a transient blueshift of a Fermi cutoff and an increased ARPES intensity in the tr-ARPES measurements. In contrast, the effect of the internal bulk PV shows only marginal changes in the 2DEG and TSS carrier densities but shifts the entire binding energy of the near-surface bands.

Measuring temperature at a high spatial resolution is a fundamental technique for investigating local static and dynamic thermal properties in various materials and devices. Here, we report temperature measurement by electron energy loss spectroscopy using monochromated electron probes of 1–2 nm diameter and large scattering vectors of electrons and based on the principle of detailed balancing between the annihilation and creation of phonons. Temperatures defined by optical and acoustic phonons are evaluated separately. We demonstrate that the use of acoustic phonons is advantageous for low-temperature measurements with higher accuracy. The scheme is applied to evaluating local temperatures in a diamond nanowire during Joule heating. We also discuss the Joule heating mechanism and key factors for improving temperature measurement accuracy.

The sine-Gordon model appears as the low-energy effective field theory of various one-dimensional gapped quantum systems. Here we investigate the dynamics of generic, nonintegrable systems belonging to the sine-Gordon family at finite temperature within the semiclassical approach. Focusing on timescales where the effect of nontrivial quasiparticle scatterings becomes relevant, we obtain universal results for the long-time behavior of dynamical correlation functions. We find that correlation functions of vertex operators behave neither ballistically nor diffusively but follow a stretched exponential decay in time. We also study the full counting statistics of the topological current and find that distribution of the transferred charge is non-Gaussian with its cumulants scaling nonuniformly in time.

We present a SU(2) gauge theory of fluctuating magnetic order in the two-dimensional Hubbard model. The theory is based on a fractionalization of electrons in fermionic chargons and bosonic spinons. The chargons undergo Néel or spiral magnetic order below a density-dependent transition temperature $T^{*}$. Fluctuations of the spin orientation are described by a nonlinear sigma model obtained from a gradient expansion of the spinon action. The spin stiffnesses are computed from a renormalization group improved random phase approximation. Our approximations are designed for moderate, not for strong, Hubbard interactions. The stiffnesses are strongly doping dependent with discontinuities at half-filling and a pronounced electron-hole asymmetry. The spinon fluctuations prevent magnetic long-range order of the electrons at any finite temperature. The phase with magnetic chargon order shares characteristic features with the pseudogap regime in high-$T_c$ cuprates: a strong reduction of charge carrier density, a spin gap, and Fermi arcs. A substantial fraction of the pseudogap regime exhibits electronic nematicity.

Lowered symmetry enables access to a wide set of responses not typically accessible in high symmetry materials. Prime examples are time-reversal forbidden quantum geometric photocurrent responses (e.g., linear injection and circular shift photocurrents) that are thought to vanish in nonmagnetic materials. Here we argue that polariton drag processes enable to unblock such quantum geometric photocurrents even in nonmagnetic and centrosymmetric materials. Strikingly, we uncover how a cooperative effect between finite $\mathbf{q}$ irradiation and the Fermi surface position leads to a polariton selective photoexcitation (PSP). PSP enables to directly address carriers within tight momentum resolved windows of the Fermi surface to yield giant enhancements of quantum geometric photocurrents. This selectivity enables to directly track momentum resolved quantum geometric quantities along the Fermi surface providing a new tool to interrogate the quantum geometry of high symmetry materials.

Dimerized quantum magnets provide a unique possibility to investigate the Bose-Einstein condensation of magnetic excitations in crystalline systems at low temperature. Here, we model the low-temperature magnetic properties of the recently synthesized spin $S=1$ dimer system ${\mathrm{K}}_2\mathrm{Ni}{\left({\mathrm{MoO}}_4\right)}_2$ and propose it as a candidate material for triplon and quintuplon condensation. Based on a first-principles analysis of its electronic structure, we derive an effective spin dimer model that we first solve within a mean-field approximation to refine its parameters in comparison to experiment. Finally, the model is solved by employing a numerically exact quantum Monte Carlo technique which leads to magnetic properties in good agreement with experimental magnetization and thermodynamic results. We discuss the emergent spin model of ${\mathrm{K}}_2\mathrm{Ni}{\left({\mathrm{MoO}}_4\right)}_2$ in view of the condensation of magnetic excitations in a broad parameter regime. Finally, we comment on a geometrical peculiarity of the proposed model and discuss how it could host a supersolid phase upon structural distortions.

It is now possible to use quasicrystals to search for novel topological phenomena enhanced by their peculiar structure characterized by an irrational number and high-dimensional primitive vectors. Here, we extend the concept of a topological insulator with an emerging staggered local magnetic flux (i.e., without external fields), similar to Haldane's honeycomb model, to the Penrose lattice as a quasicrystal. The Penrose lattice consists of two different tiles, where the ratio of the numbers of tiles corresponds to an irrational number. Contrary to periodic lattices, the periodicity of the energy spectrum with respect to the magnetic flux no longer exists, reflecting the irrational number in the Penrose lattice. Calculating the Bott index as a topological invariant, we find topological phases appearing in a fractal energy spectrum similar to the Hofstadter butterfly. More intriguingly, by folding the one-dimensional aperiodic magnetic flux into a two-dimensional periodic flux space, the fractal structure of the energy spectrum is extended to a higher dimension, whose section corresponds to the Hofstadter butterfly.

We present a density functional theory analysis of the negatively charged nitrogen-vacancy (NV) defect complex located at or close to the core of ${30}^{\circ }$ and ${90}^{\circ }$ partial glide dislocations in diamond. Formation energies, electronic densities of states, structural deformations, hyperfine structure, and zero-field splitting parameters of NV centers in such structurally distorted environments are analyzed. The formation energies of the NV centers are up to 3 eV lower at the dislocation cores compared to the bulk values of crystalline diamond. We found that the lowest energy configuration of the NV center at the core of a ${30}^{\circ }$ partial glide dislocation is realized when the axis of the NV center is oriented parallel to the dislocation line. This special configuration has a stable triplet ground state. Its hyperfine constants and zero-field splitting parameters deviate by only $3\%$ from values of the bulk NV center. Hence, this is an interesting candidate for a self-assembly of a linear array of NV centers along the dislocation line.

Surface acoustic waves (SAWs) are widely involved in today's communication and sensing networks. Recent exploration of topological SAWs has provided promising prospects for developing future hybrid SAW devices. In this paper, by designing tight-binding phononic crystals on a semi-infinite substrate, we revealed topological disclination states, i.e., a kind of bound state in topologically condensed matters, for the SAWs. Numerical simulations show that the bound states induced by disclination in the SAW system are more diverse than in previous electromagnetic and mechanical systems. SAW resonances in the disclinations can be efficiently excited by appropriate mechanical stimulation with decent qualities. These states are robust and flexible for design, providing options for future multifunctional and interdisciplinary SAW manipulators, processors, sensors, and other on-chip integrated components.

Using the corner-transfer matrix renormalization group to contract the tensor network that describes its partition function, we investigate the nature of the phase transitions of the hard-square model, one of the exactly solved models of statistical physics for which Baxter has found an integrable manifold. The motivation is twofold: assess the power of tensor networks for such models, and probe the 2D classical analog of a 1D quantum model of hard-core bosons that has recently attracted significant attention in the context of experiments on chains of Rydberg atoms. Accordingly, we concentrate on two planes in the 3D parameter space spanned by the activity and the coupling constants in the two diagonal directions. We first investigate the only case studied so far with Monte Carlo simulations, the case of opposite coupling constants. We confirm that, away and not too far from the integrable 3-state Potts point, the transition out of the period-3 phase appears to be unique in the Huse-Fisher chiral universality class, albeit with significantly different exponents as compared to Monte Carlo. We also identify two additional phase transitions not reported so far for that model, a Lifshitz disorder line, and an Ising transition for large enough activity. To make contact with 1D quantum models of Rydberg atoms, we then turn to a plane where the ferromagnetic coupling is kept fixed, and we show that the resulting phase diagram is very similar, the only difference being that the Ising transition becomes first-order through a tricritical Ising point, in agreement with Baxter's prediction that this plane should contain a tricritical Ising point, and in remarkable, almost quantitative agreement with the phase diagram of the 1D quantum version of the model.

Recent demonstrations of the electrical switching of antiferromagnets (AFs) have given an enormous impulse to the field of AF spintronics. Many of these observations are plagued by nonmagnetic effects that are very difficult to distinguish from the actual magnetic ones. Here, we study the electrical switching of thin $\left(5\mathrm{n}\mathrm{m}\right)$ $\mathrm{NiO}$ films in $\mathrm{Pt}/\mathrm{NiO}$ devices using magnetic fields up to $15\mathrm{T}$ to quantitatively disentangle these magnetic and nonmagnetic effects. We demonstrate that these fields suppress the magnetic components of the electrical switching of $\mathrm{NiO}$, but leave the nonmagnetic components intact. Using a monodomainization model the contributions are separated, showing how they behave as a function of the current density. These results show that combining electrical methods and strong magnetic fields can be an invaluable tool for AF spintronics, allowing for implementing and studying electrical switching of AFs in more complex systems.

Recently it was discovered that, under elevated pressures, ${\mathrm{MoB}}_2$ exhibits superconductivity at a critical temperature $T_c$ as high as 32 K. The superconductivity appears to develop following a pressure-induced structural transition from the ambient pressure $\mathrm{R}\overline{3}\mathrm{m}$ structure to an ${\mathrm{MgB}}_2$-like P6/mmm structure. This suggests that remarkably high $T_c$ values among diborides are not restricted to ${\mathrm{MgB}}_2$ as previously appeared to be the case, and that similarly high $T_c$ values may occur in other diborides if they can be coerced into the ${\mathrm{MgB}}_2$ structure. In this paper, we show that density functional theory calculations indicate that phonon free energy stabilizes the P6/mmm structure over the $\mathrm{R}\overline{3}\mathrm{m}$ at high temperatures across the ${\mathrm{Nb}}_{1-x}{\mathrm{Mo}}_x{\mathrm{B}}_2$ series. X-ray diffraction confirms that the synthesized Nb-substituted ${\mathrm{MoB}}_2$ adopts the ${\mathrm{MgB}}_2$ crystal structure. High magnetic field electrical resistivity measurements and specific heat measurements demonstrate that ${\mathrm{Nb}}_x{\mathrm{Mo}}_{1-x}{\mathrm{B}}_2$ exhibits superconductivity with $T_c$ as high as 8 K and critical fields approaching 6 T.

There has been much recent progress on exotic surface critical behavior, yet the classical-quantum correspondence of special and extraordinary-log criticality remains largely unclear. Employing worm Monte Carlo simulations, we explore the surface criticality at an emergent superfluid-Mott insulator critical point in the Villain representation, which is believed to connect classical and quantum O(2) critical systems. We observe a special transition with the thermal and magnetic renormalization exponents $y_t=0.58\left(1\right)$ and $y_h=1.690\left(1\right)$, respectively, which are close to recent estimates from models with discrete spin variables. The existence of extraordinary-log universality is evidenced by the critical exponent $\widehat{q}=0.58\left(2\right)$ from two-point correlation and the renormalization-group parameter $\alpha =0.28\left(1\right)$ from superfluid stiffness, which obey the scaling relation of extraordinary-log critical theory and recover the logarithmic finite-size scaling of critical superfluid stiffness in open-edge quantum Bose-Hubbard model. Our results bridge recent observations of surface critical behavior in the classical statistical mechanical models [Parisen Toldin, Phys. Rev. Lett. 126, 135701 (2021); Hu et al., Phys. Rev. Lett. 127, 120603 (2021); Parisen Toldin et al., Phys. Rev. Lett. 128, 215701 (2022)] and the open-edge quantum Bose-Hubbard model [Sun $et$ $al.$, arXiv:2205.00878 (2022)].

We propose the platform of a Yang-Lee anyon system which is constructed from Majorana bound states in topological superconductors. Yang-Lee anyons, described by the nonunitary conformal field theory with the central charge $c=-22/5$, are nonunitary counterparts of Fibonacci anyons, obeying the same fusion rule but exhibiting nonunitary non-Abelian braiding statistics. We consider a topological superconductor junction system coupled with dissipative electron baths, which realizes a non-Hermitian interacting Majorana system. Numerically estimating the central charge, we examine the condition that the non-Hermitian Majorana system can simulate the Ising spin model of the Yang-Lee edge singularity and confirm that by controlling model parameters in a feasible way the Yang-Lee edge criticality is realized. On the basis of this scenario, we present a scheme for the fusion, measurement, and braiding of Yang-Lee anyons in our proposed setup.

Flip-flop processes due to magnetic dipole-dipole interaction between neighboring ions in crystals doped by rare-earth ions are one of the mechanisms of relaxation between hyperfine levels. Modeling of this mechanism has so far been macroscopic, characterized by an average rate describing the relaxation of all ions. Here, however, we present a microscopic model of flip-flop interactions between individual nuclear spins of dopant ions. Every ion is situated in a unique local environment in the crystal, where each ion has different distances and a unique orientation relative to its nearest neighbors, as determined by the lattice structure. Thus each ion has a unique flip-flop rate and the collective relaxation dynamics of all ions in a bulk crystal is a sum of many exponential decays, giving rise to a distribution of rates rather than a single average decay rate. We employ this model to calculate flip-flop rates in ${\mathrm{Pr}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ and show experimental measurements of population decay of the ground state hyperfine levels at $\sim 2$ K. We also present a method to measure rates of individual transitions from hole burning spectra that requires significantly fewer fitting parameters in theoretical rate equations compared to earlier work. Furthermore, we measure the effect of external magnetic field on the flip-flop rates and observe that the rates slow down by two orders of magnitude in a field of 5–10 mT.

The thermodynamic and electronic-transport properties of trigonal ${\mathrm{EuMg}}_2{\mathrm{Bi}}_2$ in $ab$-plane magnetic fields ${\mathbf{H}}_x$ and the A-type antiferromagnetic structure have recently been reported. The Eu magnetic moments with spin $S=7/2$ remain locked in the $ab$ plane up to and above the $ab$-plane critical field ${\mathbf{H}}_x^{\mathrm{c}}=27.5$ kOe at which the Eu moments become parallel to ${\mathbf{H}}_x$. Here additional measurements at low fields are reported that reveal a new spin-reorientation transition at a field $H_{c1}\approx 465$ Oe where the Eu moments remain in the $ab$ plane but become perpendicular to $H_x$. At higher fields, the moments cant toward the field resulting in $M\propto H_x$ up to ${\mathbf{H}}_x^{\mathrm{c}}$. Similar results are reported from measurements of the magnetic properties of ${\mathrm{EuMg}}_2{\mathrm{Sb}}_2$ single crystals, where $H_{\mathrm{c}1}\approx 220$ Oe is found. Theory is formulated that models the low-field magnetic behavior of both materials, and the associated anisotropies are calculated. The $ab$-plane trigonal anisotropy in ${\mathrm{EuMg}}_2{\mathrm{Sb}}_2$ is found to be significantly smaller than in ${\mathrm{EuMg}}_2{\mathrm{Bi}}_2$.

The combination of spin-orbit coupling and superconductivity induces unconventional spin-triplet correlation in Ising superconductors. We theoretically investigate the spin transport through a normal metal–Ising superconductor junction, showing that Ising superconductors also have the characteristic of spin superconductivity. Due to the existence of spin-triplet Cooper pairs, not only charge supercurrent but also spin supercurrent can transport in Ising superconductors. We analyze the transport process in the junction, which is mainly contributed by the equal-spin Andreev reflection and spin-flip reflection, and calculate the spin conductance and the spin injection efficiency under different conditions. Our findings broaden the boundary of spin superconductivity and reveal the potential applications of Ising superconductors in spintronics, especially in controlled long-distance dissipationless spin transport.

A recent demonstration of the periodic oscillation of resistance in the thin film of the ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ superconductor with a hole in the film suggests that charge-$4e$ and charge-$6e$ Cooper pairs may have condensed in this compound. While exciting, such interpretation calls for a precise determination of the effective area for the passage of Cooper pairs from one end of the lead to the other. Unlike the traditional Little-Parks effect where the rim around the hole is thin, the effective hole area is not obviously defined for the “thick-rim geometry” adopted in the experiment. Here, we note that the experiment was conducted in a regime where the superconductivity is strongly fluctuating, which motivates an analysis based on the spacetime formulation of the time-dependent Ginzburg-Landau theory. We argue that under appropriate conditions, the optimal semiclassical path is not the geometrically shortest one, but the one that moves along the edge of the hole and takes advantage of the reduced fluctuations at the boundary of the hole. The condition for the crossover from the geometrically shortest path to the path that sticks to the wall is clarified. In such a scenario, the geometric area of the hole indeed emerges as the effective area for the flux, providing a theoretical justification to the interpretation given by J. Ge et al. (arXiv:2201.10352). The conclusion of our analysis may have implications for similar experiments in other superconductors where the geometry of the device is not obviously that of the Little-Parks experiment employing the thin wall, but of the thick-rim type such as used in the experiment.

Topology, symmetry, electron correlations, and the interplay between them have formed the cornerstone of our understanding of quantum materials in recent years and are used to identify new emerging phases. While the first two give a fair understanding of noninteracting and, in many cases, weakly interacting wave function of electron systems, the inclusion of strong correlations could change the picture substantially. The Kondo lattice model is a paradigmatic example of the interplay of electron correlations and conduction electrons of a metallic system, describing heavy fermion materials and also fractionalized Fermi liquid pertaining to an underlying gauge symmetry and topological orders. In this work we study a superconducting Kondo lattice model, a network of one-dimensional Kitaev superconductors Kondo coupled to a lattice of magnetic moments. Using slave-particle representation of spins and exact numerical calculations, we obtain the phase diagram of the model in terms of Kondo coupling $J_K$ and identify a topological order phase for $J_K<J_K^c$ and a Kondo compensated phase for $J_K>J_K^c$, where $J_K^c$ is the critical point. Setting the energy scales of electron hopping and pairing to unity, the mean-field theory calculations achieve $J_K^c=2$ and in exact numerics we found $J_K^c\simeq 1.76$, both of which show that the topological order is a robust phase. We argue that in terms of slave particles, the compensated phase corresponds to an invertible phase, and a Mott insulating transition leads to a topological order phase. Furthermore, we show that in the regime $J_K<J_K^c$ in addition to the low-energy topological states, a branch of subgap states appears inside the superconducting gap.