The evolution of an attractive polarized two-component Fermi gas at zero temperature is analyzed as its polarization is progressively decreased, from full polarization (corresponding to the polaronic limit) down to a critical polarization when superfluidity sets in. This critical polarization and the nature of the associated superfluid instability are determined within a fully self-consistent $t$-matrix approach implemented exactly at zero temperature. In this way, the polarization-vs-coupling phase diagram at zero temperature is constructed throughout the whole BCS-BEC crossover. Depending on the coupling strength of the interparticle interaction between the two components, the superfluid instability can be either toward a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase or toward a standard polarized BCS phase. The evolution with polarization of the quasiparticle parameters in the normal Fermi gas turns out to be notably different in the two cases. When the instability is toward a polarized BCS superfluid, quasiparticles in the proximity of the two Fermi surfaces remain well defined for all polarizations. When the instability is instead toward an FFLO superfluid, precursor effects become apparent upon approaching the FFLO quantum critical point (QCP), where the quasiparticle residues vanish and the effective masses diverge. This behavior leads to a complete breakdown of the quasiparticle picture characteristic of a Fermi liquid, similarly to what occurs in heavy-fermion materials at an antiferromagnetic QCP. At unitarity, the system is further investigated at finite temperature, making it possible to identify a non-Fermi liquid region in the temperature-vs-polarization phase diagram associated with the underlying FFLO QCP.

We present a general approach to the derivation of the effective anisotropy field which determines the dynamical behavior of magnetic spins according to the Landau-Lifshitz-Gilbert equation. The approach is based on the gradient in spherical polar coordinates with the final results being expressed in Cartesian coordinates as usually applied in atomistic and micromagnetic model calculations. The approach is generally valid for all orders of anisotropies including higher-order combinations of azimuthal and rotational anisotropies often found in functional magnetic materials such as permanent magnets and an emerging class of antiferromagnetic materials with applications in spintronics. Anisotropies are represented in terms of spherical harmonics which have the important property of rational temperature scaling. Effective field vectors are given for anisotropies up to sixth order, presenting a unified framework for implementing higher-order magnetic anisotropies in numerical simulations.

Excitation of spin waves of a required frequency and directional spectrum is among the crucial tasks in optomagnonics. Here we investigate the generation of spin waves in an iron garnet thin film by a train of femtosecond laser pulses with ultimately high repetition rate of up to 10 GHz and compare it with the case of 1-GHz repetition rate. The periodic optical excitation with repetition rate close to the frequency of the ferromagnetic resonance amplifies spin waves with particular phase velocity and wavelength, which are tunable across a wide range by small variations of the frequency detuning, and can be adjusted by the magnitude of the applied external magnetic field. For pulses of the same fluence, the 10-GHz pulse rate provides a significant resonant increase of the spin-wave amplitude by 11.5 times with respect to single-pulse excitation while the 1-GHz pulse rate provides only a 1.5 times advancement. Moreover, variation of the detuning frequency provides different regimes of the spin-wave propagation: short- and long-distance propagation along the magnetic field direction and appearance of an “X” line shape in the directionality pattern, making the considered optical approach of spin-wave generation promising for designing magnonic devices.

Linear response theory for the case of a uniaxially anisotropic superparamagnetic nanoparticle suspended in a fluid is developed for the situations where, along with the probing field, a stationary bias field is present. The built up description allows for both mechanisms of magnetic relaxation available to the particle: the internal (relaxation of the magnetic moment inside the particle) and external (relaxation together with the particle body due to its Brownian orientation diffusion in a fluid). In this framework, longitudinal dynamic magnetic susceptibility of such a particle is considered. It is confirmed that at zero bias field, frequency dependence of the out-of-phase component of the dynamic susceptibility (absorption spectrum) has two maxima if anisotropy energy is only several times greater than thermal energy. The presence of these peaks is a direct consequence of the bistability of uniaxial magnetic nanoparticles. The magnetizing field changes position and height of these maxima. Moreover, it is shown that in a presence of the bias the spectrum can acquire a third maximum if the Brownian rotation of the suspended particle is retarded with respect to establishment of the intrinsic magnetic equilibrium in it. Necessary conditions for such a situation are analyzed, and criteria for the possible appearance of the additional third absorption peak are indicated.

We investigate the robustness of topological superconductors under the perturbing influence of a finite charge current. To this aim, we introduce a modified Kitaev Hamiltonian parametrically dependent on the quasiparticle momentum induced by the current. Using different quantifiers of the topological phase, such as the Majorana polarization and the edge state quantum conditional mutual information, we prove the existence of a finite critical value of the quasiparticle momentum below which edge modes and topological superconductivity survive. We also discuss how a finite current breaks time reversal symmetry and changes the topological class in the Altland-Zirnbauer classification scheme compared to the case of isolated systems. Our findings provide a nontrivial example of the interplay between topology and the nonequilibrium physics of open quantum systems, a relation of crucial importance in the quest to a viable topological quantum electronics.

Motivated by recent experimental evidence signaling chiral charge order in vanadium-based kagome superconductors, we investigate theoretically the impact of chiral flux charge order on the experimental outcomes for normal and superconducting (SC) properties. It is revealed that the spectral weight on the Fermi surface (FS) is partially gapped by chiral flux charge order, with the reservation of the spectral weight on the $M$ points and the midpoint between the two adjacent $M$ points, resulting in the momentum-dependent energy gap being consistent with recent experimental observations. More importantly, by considering the influence of chiral flux charge order, we find that a conventional fully gapped SC pairing state evolves into a nodal gap feature for the spectral weight due to the spectral gap modulations on the FS. As a result, the U-shaped density of states (DOS) deforms to the V-shaped DOS along with the residual DOS near the Fermi energy. These results bear some resemblance to the experimental observations, and they may serve as a promising proposal to mediate the divergent or seemingly contradictory experimental outcomes regarding SC pairing symmetry.

Weyl and Dirac relativistic fermions are ubiquitous in topological matter. Their relativistic character enables high-energy physics phenomena like the chiral anomaly to occur in the solid state, which allows us to experimentally probe and explore fundamental relativistic theories. Here, we show that, on smooth interfaces between a trivial and a topological material, massless Weyl and massive Dirac fermions intrinsically coexist. The emergence of the latter, known as Volkov-Pankratov states, is directly revealed by magneto-optical spectroscopy, evidencing that their energy spectra are perfectly controlled by the smoothness of the topological interface. Simultaneously, we reveal the optical absorption of the zero-energy chiral Weyl state, whose wave function is drastically transformed when the topological interface is smooth. Artificial engineering of the topology profile thus provides a textbook system to explore the rich relativistic energy spectra in condensed matter heterostructures.

We study numerically a bosonic analog of the Kitaev honeycomb model, which is a minimal model for $4d^4/5d^4$ quantum magnets with honeycomb lattice geometry. We construct its phase diagram by a combination of Landau theory analysis and quantum Monte Carlo simulations. In particular, we show that the phase boundaries between the paramagnetic state and magnetically ordered states are generically fluctuation-induced first-order phase transitions. Our results are potentially applicable to ${\mathrm{Ru}}^{4+}$- and ${\mathrm{Ir}}^{5+}$-based honeycomb magnets.

The planar Hall effect in Weyl semimetals has been proposed as a key feature of chiral anomaly and nontrivial Berry curvature, while the effect of fundamental spatial symmetry remains to be explored. Here, we show with general symmetry analysis and lattice model calculations that the planar Hall effect can be significantly modulated by a certain class of spatial symmetry operations. The odd terms of conductivity with respect to magnetic field, including the linear terms, vanish in the plane perpendicular to the axis of twofold rotation ($C_2$) or the $C_2$ combined symmetry operations (mirror, twofold screw rotation, glide plane). When applying the magnetic field perpendicular to the glide plane, we find that the planar Hall conductivity in bulk forbidden by glide symmetry becomes nonzero in its quasi-two-dimensional counterpart with broken glide symmetry. It is further clarified that the nonzero planar Hall conductivity comes from surface states. This signature can be used to distinguish bulk and surface transport in Weyl semimetals.

We investigate the charged $q/2$-body interacting Sachdev-Ye-Kitaev (SYK) model in the grand-canonical ensemble. By treating $q$ as a large parameter, we are able to analytically study its phase diagram. By varying the chemical potential or temperature, we find that the system undergoes a phase transition between low and high entropies in the maximally chaotic regime. A similar transition in entropy is seen in charged anti–de Sitter (AdS) black holes transitioning between a large and small event horizon. Approaching zero temperature, we find a first-order chaotic-to-nonchaotic quantum phase transition, where the finite extensive entropy drops to zero. This again has a gravitational analog—the Hawking-Page transition between a large black hole and thermal radiation. An analytical study of the critical phenomena associated with the continuous phase transition provides us with mean-field van der Waals critical and effective exponents. We find that all analogous power laws are shared with several charged AdS black hole phase transitions. Together, these findings indicate a connection between the charged $q\to \infty$ SYK model and black holes.

There has been much interest over many years in studying charged systems after the artificial imposition of periodic boundary conditions, and correcting for the resulting divergence of the electrostatic energy density. A correction for cubic cells was derived by Makov and Payne in 1995, and its leading error term is of the form $L^{-5}$ for a cube of side $L$. Most modern density functional theory codes use a different treatment of the $\mathrm{Z}\alpha$ energy term to that used by Makov and Payne, resulting in an error term of the form $L^{-3}$ if their correction is used unmodified. This paper shows how the Makov and Payne result can be made consistent with modern practice.

Interfaces in MgO/FeCoB/MgO trilayer have been studied with grazing incident nuclear resonance scattering using the x-ray standing wave technique. High depth selectivity of the present method allows one to measure magnetism and structure at the two interfaces of FeCoB, namely, FeCoB on MgO and MgO on FeCoB, independently, yielding an intriguing result that both interfaces are not symmetric. A high-density layer with an increased magnetic hyperfine field at the FeCoB-on-MgO interface suggests different growth mechanisms at the two interfaces. The azimuthal angle-dependent magneto-optic Kerr effect measurements reveal the presence of unusual uniaxial magnetic anisotropy (UMA) in the trilayer. An in situ temperature-dependent study discovered that this UMA systematically reduces with temperature. After annealing at $250{}^{\circ }\mathrm{C}$, the trilayer starts following the standard Stoner-Wohlfarth model for in-plane UMA. The trilayer becomes isotropic at $450{}^{\circ }\mathrm{C}$ with an order-of-magnitude increase in coercivity. The asymmetry at the interfaces is in turn explained by boron diffusion from the FeCoB interface layer into the nearby MgO layer. Stress-induced UMA is observed in the boron-deficient FeCoB layer, superimposed with the bulk FeCoB layer, and found to be responsible for unusual UMA. The temperature-dependent variation in the UMA and coercivity can be understood in terms of variations in the internal stresses and coupling between FeCoB bulk and the interface layer.

A one-dimensional boundary of a two-dimensional topological superconductor can host a number of topologically protected chiral modes. Combining two topological superconductors with different topological indices, it is possible to achieve a situation when only a given number of channels ($m$) are topologically protected, while others are not and therefore are subject to Anderson localization in the presence of disorder. We study transport properties of such quasi-one-dimensional quantum wires with broken time-reversal and spin-rotational symmetries (class D) and calculate the average conductance, its variance and the third cumulant, as well as the average shot noise power. The results are obtained for arbitrary wire length, tracing a crossover from the diffusive Drude regime to the regime of strong localization where only $m$ protected channels conduct. Our approach is based on the nonperturbative treatment of the nonlinear supersymmetric sigma model of symmetry class D with two replicas developed in our recent publication [D. S. Antonenko et al., Phys. Rev. B 102, 195152 (2020)]. The presence of topologically protected modes results in the appearance of a topological Wess-Zumino-Witten term in the sigma-model action, which leads to an additional subsidiary series of eigenstates of the transfer-matrix Hamiltonian. The developed formalism can be applied to study the interplay of Anderson localization and topological protection in quantum wires of other symmetry classes.

In this article we develop an effective medium model to characterize the electron wave propagation in graphene based nanostructures with electrostatic and magnetic vector potentials imposed on their surface. We use a numerical algorithm to determine the effective medium parameters of the heterostructure and calculate the electronic band structure of the system. We apply our formalism to analyze superlattices with solely a magnetic potential and reveal that the response of the structure remains reciprocal and is characterized by a decrease in the velocity of the charge carriers. We also study the response of superlattices with both potentials superimposed on graphene and show that the response of the system becomes nonreciprocal with a dispersion characterized by a tilted Dirac cone. We demonstrate that it is possible to alternate between type-I, type-II, or type-III Dirac cones by properly tuning the amplitude of the potentials.

We report the anomalous magnetotransport properties in high-quality single-crystal ${\mathrm{VSb}}_2$ without V atom vacancies. ${\mathrm{VSb}}_2$ shows typical metallic behavior with convex-upward characteristics, which can be explained using a parallel-resistor model. The three-dimensional weak antilocalization effect leads to a large saturation magnetoresistance (MR) up to $595\%$, which disappears when the magnetic field is along the $ab$ plane. Instead, a clear Shubnikov–de Haas oscillation at low temperatures is observed, based on which the fast Fourier transformation spectrum analyses reveal three fundamental frequencies. The two-carrier fitting of Hall resistivity suggests that the saturation MR originates from the giant magnitude difference of the two carrier concentrations. Moreover, the angle-resolved magnetoresistance is measured by rotating the sample around its $c$ axis and $ab$ plane. A transition from twofold symmetry to isotropy is observed.

We perform thermodynamic and transport measurements on a magnetic charge-glass former, $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{CsCo}{\left(\mathrm{SCN}\right)}_4$, to discuss the $\pi -d$ interaction occurring in the interlayer direction. An anomalous temperature dependence and sharp peak structure of the heat capacity are observed in magnetic fields, indicating a field-induced transition in this compound. Furthermore, a sharp peak of the thermoelectric power is also observed in magnetic fields, supporting the presence of the field-induced transition. Comparison with a nonmagnetic analog compound indicates that the introduction of magnetic ${\mathrm{Co}}^{2+}$ ions $(S=3/2)$ triggers this transition. This strongly suggests that the $\pi -d$ interaction effectively works in this compound and affects the physical properties. To explain the field-induced phenomenon in this compound, we propose a speculative scenario in which the $\pi -d$ interaction between the $\pi$ spins in the insulating domains and the $d$ spin of ${\mathrm{Co}}^{2+}$ induces the reconstruction of the domain structure in the CG state.

In many classical and quantum systems described by an effective non-Hermitian Hamiltonian, spectral phase transitions, from an entirely real-energy spectrum to a complex spectrum, can be observed as a non-Hermitian parameter in the system is increased above a critical value. A paradigmatic example is provided by systems possessing parity-time ($\mathcal{PT}$) symmetry, where the energy spectrum remains entirely real in the unbroken $\mathcal{PT}$ phase while a transition to complex energies is observed in the broken $\mathcal{PT}$ phase. Such spectral phase transitions are universally sharp. However, when the system is slowly and periodically cycled, the phase transition can become smooth, i.e., imperfect, owing to the complex Berry phase associated to the cyclic adiabatic evolution of the system. This remarkable phenomenon is illustrated by considering the spectral phase transition of the Wannier-Stark ladders in a $\mathcal{PT}$-symmetric class of two-band non-Hermitian lattices subjected to an external dc field, revealing that a nonvanishing imaginary part of the Zak phase—the Berry phase picked up by a Bloch eigenstate evolving across the entire Brillouin zone—is responsible for imperfect spectral phase transitions.

The Ising cage-net model, first proposed by Prem et al. [Phys. Rev. X 9, 021010 (2019)], is a representative type I fracton model with nontrivial non-Abelian features. In this paper, we calculate the ground state degeneracy of this model and find that, even though it follows a similar coupled layer structure as the X-cube model, the Ising cage-net model cannot be “foliated” in the same sense as X-cube as defined in Shirley et al. [Phys. Rev. X 8, 031051 (2018)]. A more generalized notion of “foliation” is hence needed to understand the renormalization group transformation of the Ising cage-net model. The calculation is done using an operator algebra approach that we develop in this paper, and we demonstrate its validity through a series of examples.