We demonstrate the ability of low energy electron microscopy (LEEM) to extract fundamental information in surface phase transitions during in situ observations of complex semiconductor surfaces. We utilize established LEEM techniques and develop a methodology that enables us to calculate the surface entropy difference using only LEEM measurements without the need for external characterization. We demonstrate the effectiveness of the unified method by monitoring the phase coexistence during the first-order transition between the $c(8\times 2)$ and $(6\times 6)$ phases on the surface of GaAs(001) at a range of temperatures relevant for epitaxy. The coexistence behavior with temperature and the fluctuations of phase boundaries are measured and analyzed to obtain the entropy difference and stress difference between the phases. The calculated values show that the entropy difference is not large enough to stabilize the $(6\times 6)$ phase with respect to the $c(8\times 2)$ by itself, suggesting that the elastic relaxation during the coexistence between the two phases is necessary to stabilize the $(6\times 6)$ phase.

We use first-principles calculations to investigate how deviations from perfect chemical order affect the magnetocrystalline anisotropy energy (MAE) in $L{1}_0$ FeNi. We first analyze the local chemical environment of the Fe atoms in various partially ordered configurations, using the orbital magnetic moment anisotropy (OMA) as proxy for a local contribution to the MAE. We are able to identify a specific nearest neighbor configuration and use this “favorable environment” to successfully design various structures with MAE higher than the perfectly ordered system. However, a systematic analysis of the correlation between local environment and OMA using smooth overlap of atomic positions indicates only a partial correlation, which exists only if the deviation from full chemical order is not too large, whereas in general no such correlation can be identified even using up to third nearest neighbors. Guided by the observation that the identified favorable environment implies an Fe-rich composition, we investigate the effect of randomly inserting additional Fe into the nominal Ni planes of the perfectly ordered structure. We find that the MAE increases with Fe content, at least up to 62.5% Fe. Thus, our paper shows that the perfectly ordered case is not the one with highest MAE and that an increased MAE can be obtained for slightly Fe-rich compositions.

The origin and nature of glassy dynamics presents one of the central enigmas of condensed-matter physics across a broad range of systems ranging from window glass to spin glasses. The spin-ice compound ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, which is perhaps best known as hosting a three-dimensional Coulomb spin liquid with magnetically charged monopole excitations, also falls out of equilibrium at low temperature. How and why it does so remains an open question. Based on an analysis of low-temperature diffuse neutron-scattering experiments employing different cooling protocols alongside recent magnetic noise studies, combined with extensive numerical modeling, we argue that upon cooling, the spins freeze into what may be termed a “structural magnetic glass,” without an $a$ priori need for chemical or structural disorder. Specifically, our model indicates the presence of frustration on two levels, first producing a near-degenerate constrained manifold inside which phase ordering kinetics is in turn frustrated. A remarkable feature is that monopoles act as sole annealers of the spin network and their pathways and history encode the development of glass dynamics, allowing the glass formation to be visualized. Our results suggest that spin ice ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ provides one prototype of magnetic glass formation specifically and a setting for the study of kinetically constrained systems more generally.

We develop a theory for a two-component miscible dipolar condensate in a planar trap. Using numerical solutions and a variational theory we solve for the excitation spectrum and identify regimes where density- and spin-roton excitations are favored. We characterize the various instabilities that can emerge in this system over a wide parameter regime and present results for the stability phase diagram. Importantly this allows us to identify the parameter regimes where a novel roton-immiscibility transition can occur, driven by the softening of the spin-roton excitation.

Frustrated magnets with a strong spin-lattice coupling can show rich magnetic phases and the associated fascinating phenomena. A promising platform is the breathing pyrochlore magnet ${\mathrm{CuInCr}}_4{\mathrm{S}}_8$ with localized $S=3/2{\mathrm{Cr}}^{3+}$ ions, which is proposed to be effectively viewed as an $S=6$ Heisenberg antiferromagnet on the face-centered-cubic lattice. Here we unveil that ${\mathrm{CuInCr}}_4{\mathrm{S}}_8$ exhibits a complex magnetic phase diagram with a small phase pocket ($A$ phase) by means of magnetization, magnetostriction, magnetocapacitance, and magnetocaloric-effect measurements in pulsed high magnetic fields of up to 60 T. Remarkably, the appearance of $A$ phase is accompanied by anomalous magnetostrictive and magnetocapacitive responses, suggesting the emergence of helimagnetism in contrast to the neighboring commensurate magnetic phases. Besides, the high-entropy nature is confirmed in the high-temperature side of $A$ phase. These features are potentially related to a thermal fluctuation-driven multiple-$q$ state caused by the magnetic frustration, which has been theoretically predicted but yet experimentally undiscovered in insulating magnets.

We study the role of electron-electron interactions near integer and abelian fractional quantum Hall (QH) transitions using composite fermion (CF) representations. Interaction effects are encapsulated in CF theories as gauge fluctuations. Without gauge fluctuations, the CF system realizes a ‘dual’ representation of the noninteracting QH transition. With gauge fluctuations, the system is governed by a gauged nonlinear sigma model (NLSM) with a $\theta \mathrm{term}$. While the transition is described by a strong-coupling fixed point of the NLSM, we are nevertheless able to deduce two of its properties. With $1/r$ interactions, (1) the transition has a dynamical exponent $z=1$, and (2) all transitions are “superuniversal”: fractional and integer QH transitions are in the same universality class. With short-range interactions, $z=2$ and the fate of superuniversality remains unclear.

The ${\mathbb{Z}}_2$ topological metals $R{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($R$ = K, Rb, Cs) with a layered kagome structure provide a unique opportunity to investigate the interplay between charge order, superconductivity, and topology. Here, we report muon-spin relaxation/rotation ($\mu \mathrm{SR}$) measurements performed on $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ across a broad temperature range, in order to uncover the nature of the charge density wave order and superconductivity in this material. From zero-field $\mu \mathrm{SR}$, we find that spontaneous magnetic fields appear below 50 K, which is well below the charge density wave transition ($T^{*}\sim 93$ K). We show that these spontaneous fields are dynamic in nature making it difficult to associate them with a hidden static order. The superconducting state of $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ is found to preserve time-reversal symmetry, and the transverse-field $\mu \mathrm{SR}$ results are consistent with a superconducting state that has two fully open gaps.

We investigate symmetry-preserving gapped boundary of $(2+1)$-dimensional $\left[\right(2+1\left)\mathrm{D}\right]$ topological phases with global symmetry, which can be either bosonic or fermionic. We develop a general algebraic description for gapped boundary condition for symmetry-enriched or fermionic topological phases, extending the framework of Lagrangian algebra anyon for bosonic phases without symmetry. We then focus on application to the case with U(1) symmetry. We derive new obstructions to symmetry-preserving gapped boundary for U(1)${}^f$-symmetric $(2+1)\mathrm{D}$ fermionic topological phases, which are beyond chiral central charge $c_{-}$ and electric Hall conductivity ${\sigma }_H$. These obstructions are given by a simple Gauss-Milgram-type formula valid for supermodular category and regarded as a higher version of $c_{-}$ and ${\sigma }_H$.

With the ever-increasing energy need to process big data, the realization of low-power computing technologies, such as superconducting logic and memories, has become a pressing issue. Developing fast and nonvolatile superconducting memory elements, however, remains a challenge. Superconductor-ferromagnet hybrid devices offer a promising solution, as they combine ultrafast manipulation of spins with dissipationless readout. Here, we present a type of nonvolatile Josephson junction memory that utilizes the bistable magnetic texture of a single mesoscopic ferromagnet. We use micromagnetic simulations to design an ellipse-shaped planar junction structured from a Co/Nb bilayer. The ellipse can be prepared as uniformly magnetized or as a pair of vortices at zero applied field. The two states yield considerably different critical currents, enabling reliable electrical readout of the element. We describe the mechanism that controls the critical current by applying numerical calculations to quantify the local stray field from the ferromagnet, which shifts the superconducting interference pattern. Our approach presents a route towards realizing superconducting memory applications by combining micromagnetic modeling with bistable spin-textured junctions.

Weyl fermions (WFs) in the type-II Weyl semimetal (WSM) ${\mathrm{WTe}}_2$ are difficult to resolve experimentally because the Weyl bands disperse in an extremely narrow region of the ($E\text{−}k$) space. Here, by using DFT-assisted high-resolution ${}^{125}\mathrm{Te}$ solid-state nuclear magnetic resonance (ssNMR) in the temperature range 50–700 K, we succeeded in detecting low energy WF excitations and monitoring their evolution with temperature. Remarkably, WFs are observed to emerge at $T\sim 120$ K, while at lower temperatures ${\mathrm{WTe}}_2$ behaves as a trivial metal. This intriguing phenomenon is induced by the rapid raise of the Fermi level upon heating, which crosses the Weyl bands only for $T>120$ K. The abrupt change of the NMR parameters at this temperature is signature of a topological Lifshitz transition instead of a cursive energy-bands crossing.

We develop a method for bosonizing the Fermi surface based on the formalism of the coadjoint orbits. This allows one to parametrize the Fermi surface by a bosonic field that depends on the spacetime coordinates and on the position on the Fermi surface. The Wess-Zumino-Witten term in the effective action, governing the adiabatic phase acquired when the Fermi surface changes its shape, is completely fixed. As an effective field theory the action also involves a Hamiltonian, which contains, beside the kinetic energy and the Landau interaction, terms with arbitrary number of derivatives and fields. We show that the resulting local effective field theory captures both linear and nonlinear effects in Landau's Fermi liquid theory. The approach can be extended to incorporate spin degrees of freedom and the charge-2 fields corresponding to the BCS order parameter.

The equilibrium properties of a weakly interacting atomic Bose gas across the Berezinskii-Kosterlitz-Thouless (BKT) and Bose-Einstein condensation (BEC) phase transitions are numerically investigated through a dimensionality crossover from two to three dimensions. The crossover is realized by confining the gas in an experimentally feasible hybridized trap which provides homogeneity along the planar $xy$ directions through a box potential in tandem with a harmonic transverse potential along the transverse $z$ direction. The dimensionality is modified by varying the frequency of the harmonic trap from tight to loose transverse trapping. Our findings, based on a stochastic (projected) Gross-Pitaevskii equation, showcase a continuous shift in the character of the phase transition from BKT to BEC, and a monotonic increase of the identified critical temperature as a function of dimensionality, with the strongest variation exhibited for small chemical potential values up to approximately twice the transverse confining potential.

For materials where spin-orbit coupling is competitive with electronic correlations, the spatially anisotropic spin-orbital wave functions can stabilize degenerate states that lead to many and diverse quantum phases of matter. Here we find evidence for a dynamical spin-orbital state preceding a $T^{*}$ = 50 K order-disorder spin-orbital ordering transition in the $j=3/2$ lacunar spinel ${\mathrm{GaTa}}_4{\mathrm{Se}}_8$. Above $T^{*}, {\mathrm{GaTa}}_4{\mathrm{Se}}_8$ has an average cubic crystal structure, but total scattering measurements indicate local noncubic distortions of ${\mathrm{Ta}}_4$ tetrahedral clusters for all measured temperatures $2<T<300$ K. Inelastic neutron-scattering measurements reveal the dynamic nature of these local distortions through symmetry forbidden optical phonon modes that modulate $j=3/2$ molecular orbital occupation as well as intercluster Ta-Se bonds. Spin-orbital ordering at $T^{*}$ cannot be attributed to a classic Jahn-Teller mechanism and, based on our findings, we propose that intercluster interactions acting on the scale of $T^{*}$ act to break global symmetry. The resulting staggered intercluster dimerization pattern doubles the unit cell, reflecting a spin-orbital valence bond ground state.

We revisit the fate of the skin modes in many-body non-Hermitian fermionic systems. Contrary to the single-particle case, the many-body ground state cannot exhibit an exponential localization of all eigenstates due to the Pauli exclusion principle. However, asymmetry can still exist in the density profile, which can be quantified using the imbalance between the two halves of the system. Using the non-Hermitian Su-Schrieffer-Heeger (SSH) chain as an illustration, we show the existence of two distinct scaling regimes for the imbalance. In the first one, the imbalance grows linearly with the system size, as generically expected. In the second one, the imbalance saturates to a finite value. By combining high-precision exact diagonalization calculations and analytical arguments, we observe that the imbalance does not scale when the occupied bands can be deformed to their Hermitian limit. This suggests a direct connection between the corresponding bulk topological invariants and the skin effect in many-body systems. Importantly, this relation also holds for interacting systems.