Since the discovery of pressure-induced superconductivity in the two-leg ladder system ${\mathrm{BaFe}}_2{X}_3$ ($X=\mathrm{S}$, Se), with the $3d$ iron electronic density $n=6$, quasi-one-dimensional iron-based ladders have attracted considerable attention. Here, we use density-functional theory to predict that the novel $n=6$ iron ladder ${\mathrm{BaFe}}_2{\mathrm{Te}}_3$ could be stable with a similar crystal structure as ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$. Our results also indicate that ${\mathrm{BaFe}}_2{\mathrm{Te}}_3$ will display a complex $2\times 2$ block-type magnetic order. Due to the magnetic striction effects of this block order, ${\mathrm{BaFe}}_2{\mathrm{Te}}_3$ should be a magnetic noncollinear ferrielectric system with a net polarization $0.31\mu \mathrm{C}/{\mathrm{cm}}^2$. In general, the similar electronic density and magnetic ground state of Te- and Se-based ladders indicates both should display similar properties. In particular, the physical and structural similarity with ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ suggests that ${\mathrm{BaFe}}_2{\mathrm{Te}}_3$ could become superconducting under high pressure.

We present a comprehensive experimental study of the magnetic structure, magnetic and dielectric properties of the rare-earth orthochromite-orthoferrite solid-solution series $\mathrm{YbC}{\mathrm{r}}_{1-x}\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_3$ ($0.1\le x\le 0.6$). Room-temperature synchrotron x-ray diffraction analysis reveals the absence of any superlattice reflections, which excludes the formation of a $B$-site-ordered double-perovskite-like phase and establishes the complete solid solubility of Fe at the Cr site within the framework of orthorhombic Pbnm structure. We demonstrate that canted antiferromagnetic ground state of $\mathrm{YbCr}{\mathrm{O}}_3$ is converted to a ferrimagnetic with Fe doping, in addition to an increase in the magnetic ordering temperature. An unusual, second magnetic transition (first-order in nature) appears for $x\ge 0.3$ samples below the ferrimagnetic transition temperature (e.g., at 70 K for $x=0.4$), which is identified as the spin reorientation of transition metal ions from the neutron powder diffraction measurements, and primarily, driven by the $f\text{−}d$ exchange interaction. A clear evidence of the anomalous behavior of coercivity and exchange bias field is found around the spin reorientation temperature, which is characterized by a significant change in the magnetocrystalline anisotropy due to spin reorientation of transition metal ions. Temperature-dependent dielectric data exhibit the magnetoelectric coupling as well as a ferroelectric relaxor-like state at the onset of ferrimagnetic ordering. Here, we reveal the anomalous behavior of the exchange bias field and significant magnetoelectric coupling around the spin reorientation and ferrimagnetic transitions, respectively, in $\mathrm{YbC}{\mathrm{r}}_{1-x}\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_3$.

Our general interest is in self-consistent-field (scf) theories of disordered fermions. They generate physically relevant subensembles (“scf ensembles”) within a given Altland-Zirnbauer class. We are motivated to investigate such ensembles (i) by the possibility to discover new fixed points due to (long-range) interactions; (ii) by analytical scf theories that rely on partial self-consistency approximations awaiting a numerical validation; and (iii) by the overall importance of scf theories for the understanding of complex interaction-mediated phenomena in terms of effective single-particle pictures. In this paper we present an efficient, parallelized implementation solving scf problems with spatially local fields by applying a kernel-polynomial approach. Our first application is the Boguliubov-deGennes theory of the attractive-$U$ Hubbard model in the presence of on-site disorder; the sc fields are the particle density $n\left(\mathbf{r}\right)$ and the gap function $\mathrm{\Delta }\left(\mathbf{r}\right)$. For this case, we reach system sizes unprecedented in earlier work. They allow us to study phenomena emerging at scales substantially larger than the lattice constant, such as the interplay of multifractality and interactions or the formation of superconducting islands. For example, we observe that the coherence length exhibits a nonmonotonic behavior with increasing disorder strength already at moderate $U$. With respect to methodology our results are important because we establish that partial self-consistency (“energy-only”) schemes as typically employed in analytical approaches tend to miss qualitative physics such as island formation.

We investigate 't Hooft anomalies in the ${\mathbb{C}\mathbb{P}}^{N-1}$ model in space-time dimensions higher than two and identify two types of anomalies: one is a mixed anomaly between the $\mathrm{PSU}\left(N\right)$ flavor-rotation and magnetic symmetries and the other is between the reflection and magnetic symmetries. The latter indicates that even in the absence of the flavor symmetry, the model cannot have a unique gapped ground state as long as the reflection and magnetic symmetries are respected. We also clarified the condition for the 't Hooft anomalies to survive under monopole deformations, which explicitly break the magnetic symmetry down to its discrete subgroup. Besides, we explicitly show how the identified 't Hooft anomalies match in the low-energy effective description of symmetry broken phases—the Néel, $\mathrm{U}\left(1\right)$ spin liquid, and the valence bond solid phases. An application to the finite-temperature phase diagram of the four-dimensional ${\mathbb{C}\mathbb{P}}^{N-1}$ model is also discussed.

We consider the spin-echo dynamics of a nitrogen-vacancy (NV)-center qubit based on the $S=1$ ground-state spin manifold, caused by a dynamically polarized nuclear environment. We show that the echo signal acquires then a nontrivially time-dependent phase shift. This effect should be observable for polarization $\approx 0.5$ of nuclei within $\sim 1$ nm from the qubit, and for the NV center initialized in a superposition of $m=0$ and either $m=1$ or $m=-1$ states. This phase shift is much smaller when the NV center is prepared in a superposition of $m=1$ and $m=-1$ states, i.e., when the qubit couples to the spin environment in a way analogous to that of spin-$1/2$. For a nuclear environment devoid of spins strongly coupled to the qubit, the phase shift is well described within Gaussian approximation, which provides an explanation for the dependence of the shift magnitude on the choice of states on which the qubit is based, and makes it clear that its presence is related to the linear response of the environment perturbed by an evolving qubit. Consequently, its observation signifies the presence of an environment-mediated self-interaction of the qubit and, hence, it invalidates the notion that the nuclear environment acts as a source of external noise driving the qubit. We also show how a careful comparison of the echo signal from qubits based on $m=0,1$ and $m=\pm 1$ manifolds can distinguish between effectively Gaussian and non-Gaussian environments.

A van der Waals heterostructure combined with intrinsic magnetism and topological orders have recently paved attractive avenues to realize quantum anomalous Hall effects. In this work, using first-principles calculations and effective model analysis, we propose that the robust quantum anomalous Hall states with sizable band gaps emerge in the van der Waals heterostructure of germanene/${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. This heterostructure possesses high thermodynamic stability, thus facilitating its experimental fabrication. Furthermore, we uncover that the proximity effect enhances the coupling between the germanene and ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ layers, inducing the nontrivial band gaps in a wide range from 29 to 72 meV. The chiral edge states inside the band gap, leading to Hall conductance quantized to $-e^2/h$, are clearly visible. These findings provide an ideal candidate to detect the quantum anomalous Hall states and realize further applications to nontrivial quantum transport at a high temperature.

We study the conductance of a time-reversal-symmetric helical electronic edge coupled antiferromagnetically to a magnetic impurity, employing analytical and numerical approaches. The impurity can reduce the perfect conductance $G_0$ of a noninteracting helical edge by generating a backscattered current. The backscattered steady-state current tends to vanish below the Kondo temperature $T_K$ for time-reversal-symmetric setups. We show that the central role in maintaining the perfect conductance is played by a global $U\left(1\right)$ symmetry. This symmetry can be broken by an anisotropic exchange coupling of the helical modes to the local impurity. Such anisotropy, in general, dynamically vanishes during the renormalization group (RG) flow to the strong-coupling limit at low temperatures. The role of the anisotropic exchange coupling is further studied using the time-dependent numerical renormalization group method, uniquely suitable for calculating out-of-equilibrium observables of strongly correlated setups. We investigate the role of finite-bias voltage and temperature in cutting the RG flow before the isotropic strong-coupling fixed point is reached, and extract the relevant energy scales and the manner in which the crossover from the weakly interacting regime to the strong-coupling backscattering-free screened regime is manifested. Most notably, we find that at low temperatures the conductance of the backscattering current follows a power-law behavior $G\sim {(T/T_K)}^2$, which we understand as a strong nonlinear effect due to time-reversal-symmetry breaking by the finite bias.

Using density functional theory, we investigate the electronic and magnetic properties of $3d$ transition-metal adatoms adsorbed on a monolayer of Mn on W(110). Mn/W(110) has a noncollinear cycloidal spin-spiral ground state with an angle of about ${173}^{\circ }$ between magnetic moments of adjacent Mn rows. It allows one to rotate the spin orientation of an adsorbed magnetic adatom quasi-continuously. Therefore, this surface is ideally suited for manipulating the spin direction of individual atoms and exploring their magnetic properties using scanning tunneling microscopy (STM). The adsorbed V and Cr transition-metal adatoms couple antiferromagnetically to the nearest-neighbor Mn atom of Mn monolayer while Mn, Fe, Co, and Ni couple ferromagnetically. The magnetic moments of the $3d$ adatoms are large and show a Hund's rule type of trend with a peak in the middle of the series. We find large spin splitting of the $3d$ transition-metal adatoms, large spin polarization of the local vacuum density of states up to 73% at the Fermi energy, and significant tunneling anisotropic magnetoresistance enhancement up to 27%. We conclude that such large values stem from the strong hybridization between the adatoms and the Mn atoms of the monolayer. Furthermore, identification of spin orientations of the adatom using spin-polarized STM is possible at small bias voltages only for Co and V adatoms.

The chiral $p$-wave superconducting state is comprised of spin-triplet Cooper pairs carrying a finite orbital angular momentum. For the case of a periodic lattice, calculating the net magnetization arising from this orbital component presents a challenge as the circulation operator $\widehat{\mathbf{r}}\times \widehat{\mathbf{p}}$ is not well defined in the Bloch representation. This difficulty has been overcome in the normal state, for which a modern theory is firmly established. Here, we derive the extension of this normal-state approach, generating a theory which is valid for a general superconducting state, and go on to perform model calculations for a chiral $p$-wave state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. The results suggest that the magnitude of the elusive edge current in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is finite, but lies below experimental resolution. This provides a possible solution to the longstanding controversy concerning the gap symmetry of the superconducting state in this material.

Synthesis of monolayer FeSe on $\mathrm{SrTi}{\mathrm{O}}_3$, with greatly enhanced superconductivity compared to bulk FeSe, remains difficult. Lengthy annealing within a certain temperature window is always required to achieve superconducting samples as reported by different groups around the world, but the mechanism of annealing in inducing superconductivity has not been elucidated. We grow FeSe films on $\mathrm{SrTi}{\mathrm{O}}_3$ by molecular beam epitaxy and adjust the stoichiometry by depositing additional small amounts of Fe atoms. The monolayer films become superconducting after the Fe deposition without annealing, and show similar superconducting transition temperatures as those of the annealed films in transport measurements. We also demonstrate on the 5-unit-cell films that the FeSe multilayer films can be reversibly tuned between the nonsuperconducting $\sqrt{5}\times \sqrt{5}$ phase with Fe vacancies and the superconducting 1 × 1 phase. Our results reveal that the anneal process in essence removes Fe vacancies and the additional Fe deposition serves as a more efficient way to achieve superconductivity. This work highlights the significance of stoichiometry in the superconductivity of FeSe thin films and provides an easy path for superconducting samples.

Magnetometers with exceptional sensitivity are highly demanded in solving a variety of physical and engineering problems, such as measuring Earth's weak magnetic fields and prospecting mineral deposits and geological structures. It has been shown that the non-Hermitian degeneracy at exceptional points (EPs) can provide a new route for that purpose, because of the nonlinear response to external perturbations. One recent work [H. Yang et al. Phys. Rev. Lett. 121, 197201 (2018)] has made the first step to realize the second-order magnonic EP in ferromagnetic bilayers respecting the parity-time symmetry. In this paper, we generalize the idea to higher-order cases by considering ferromagnetic trilayers consisting of a gain, a neutral, and a (balanced-)loss layer. We observe both second- and third-order magnonic EPs by tuning the interlayer coupling strength, the external magnetic field, and the gain-loss parameter. We show that the magnetic sensitivity can be enhanced by three orders of magnitude comparing to the conventional magnetic tunneling junction-based sensors. Our results pave the way for studying high-order EPs in purely magnetic system and for designing magnetic sensors with ultrahigh sensitivity.

We show that the phase of a spin-torque oscillator generically acquires a geometric contribution on slow and cyclic variation of the parameters that govern its dynamics. As an example, we compute the geometric phase that results from a cyclic excursion of the magnitude of the external field and the current. We consider thermal phase fluctuations and conclude that the geometric phase should be experimentally observable at room temperature with current set-ups. We briefly comment on arrays of spin-torque oscillators and possible applications.

We study how shear moduli of a correlated metal change across superconducting phase transitions. Using a microscopic theory we explain why for most classes of superconductors this change is small. The Fe-based and the A15 systems are notable exceptions where the change is boosted by five orders of magnitude. We show that this boost is a consequence of enhanced nematic correlation. The theory explains the unusual temperature dependence of the orthorhombic shear and the back-bending of the nematic transition line in the superconducting phase of the Fe-based systems.

We have studied the electrical transport in the charge-density wave material $1T-\mathrm{TiS}{\mathrm{e}}_2$ microflakes. In the low temperatures, the logarithmic temperature-dependent resistivity corrections were observed. In particular, the negative magnetoresistances in low magnetic fields were further measured and well described by the Hikami-Larkin-Nagaoka theory. All the experimental results demonstrate the weak localization effect in the $1T-\mathrm{TiS}{\mathrm{e}}_2$ microflakes. Furthermore, the power-law dependence of the extracted phase coherence length on temperature is $\sim T^{-0.6}$, indicating the presence of the two-dimensional electron-electron interaction in the $1T-\mathrm{TiS}{\mathrm{e}}_2$ microflakes.

We investigate the spin and orbital correlations of a superexchange model with spin $S=1$ and orbital $L=1$ relevant for $5d^4$ transition-metal Mott insulators [O. N. Meetei et al., Phys. Rev. B 91, 054412 (2015)], using exact diagonalization and density matrix renormalization group (DMRG). For spin-orbit coupling $\lambda =0$, the orbitals are in an entangled state that is decoupled from the spins. We find two phases with increasing $\lambda$: (I) the $S2$ phase with two peaks in the structure factor for $\lambda \le {\lambda }_{c1}\approx 0.34J$ where $J$ is the ferromagnetic exchange; and, (II) the $S1$ phase for ${\lambda }_{c1}<\lambda \le {\lambda }_{c2}\approx 1.2J$ with emergent antiferromagnetic correlations. Both $S1$ and $S2$ phases are shown to exhibit power-law correlations, indicative of a gapless spectrum. Upon increasing $\lambda >{\lambda }_{c2}$ leads to a product state of local spin-orbital singlets that exhibit exponential decay of correlations, indicative of a gapped phase. We obtain insights into the phases from the well-known Uimin-Lai-Sutherland model in an external field that provides an approximate description of our model within mean-field theory.

The cuprous delafossite, ${\mathrm{CuFeO}}_2$, has received significant attention in recent years as a potential photocathode material in photoelectrochemical water-splitting cells. Presented herein is an investigation of the electronic structure of ${\mathrm{CuFeO}}_2$ in the framework of density functional theory. We have benchmarked three of the most popular formulations for the treatment of the electron exchange and correlation interactions, highlighting their strengths and weaknesses in predicting electronic structures compatible with the available spectroscopic measurements. Although some features are correctly reproduced by the simplest approach, which is based on the generalized gradient approximation, this fails in describing the fundamental semiconducting character of the material. The introduction of the fully self-consistent Hubbard $U$ correction in the exchange correlation functional accounts explicitly for the on-site Coulomb interaction among localized $d$ electrons, thereby opening a gap in the band structure. However, our results indicate that the $U$ correction disrupts the crystal-field splitting of the $t_{2g}$ and $e_g$ states, resulting in an inaccurate description of the conduction-band edge. We provide a qualitative and quantitative analysis to explain why the $t_{2g}$ and $e_g$ states behave differently when the Hubbard correction is switched on. We find that best practice for accurate, yet computationally viable, simulations of CFO makes use of hybrid functionals, where the fraction of exact exchange is not arbitrarily selected but tuned according to the static dielectric constant of the material. In this case, theoretical predictions are found to be in excellent agreement with experimental results.

SnTe is a narrow band-gap topological crystalline insulator (TCI), whereas SnSe is a normal semiconductor. We report Raman study of SnTe and SnSe as a function of pressure at room temperature along with first-principles density functional theory calculations. Under pressure, isostructural transition is observed in SnTe, as revealed by the anomalous softening of the strongest Raman mode up to 1.5 GPa, accompanied by an increase in the linewidth. Our first-principles calculations show that the mirror Chern number of SnTe does not change and the TCI phase remains unaffected by pressure. Raman signatures of its phase transition at 1.5 GPa are associated with phonon instability at the $\mathrm{\Gamma }$ point and inversion of the lowest-energy conduction bands. An anomaly in the electron-phonon coupling results in anomalous behavior of the Raman modes at this pressure. Further, SnTe undergoes structural transitions at $\sim 5.8, \sim 12$, and $\sim 18.3\mathrm{GPa}$. The 5.8-GPa transition is associated with a structural transition from the ambient cubic ( $Fm\overline{3}m$ ) to orthorhombic (Pnma) phase, which is no longer a topological insulator, resulting in a topological phase transition. Above the transition pressure of 12 GPa, another orthorhombic Pnma[GeS] phase is stabilized, coexisting with the Pnma phase. The reduction in the number of observed Raman modes above $\sim 18.3\mathrm{GPa}$ and enthalpy calculations show a transition from orthorhombic (Pnma) to a more symmetric cubic ($\mathrm{Pm}\overline{3}\mathrm{m}$) structure. Our high-pressure study of SnSe, on the other hand, reveals that it undergoes two phase transitions: one from the orthorhombic (Pnma) structure to the orthorhombic (Cmcm) structure at $\sim 6.2\mathrm{GPa}$ and the other at $\sim 12.9\mathrm{GPa}$, in which the Cmcm phase undergoes a semimetal to metal transition. Density functional theory calculations capture the contrast in the pressure-dependent behavior of the topological crystalline insulator SnTe and the normal semiconductor SnSe.

We develop a semianalytical model to describe bound states in the continuum (BICs) in photonic crystal slabs. We model leaky modes supported by photonic crystal slabs as a transverse Fabry-Perot resonance composed of a few propagative Bloch waves bouncing back and forth vertically inside the slab. This multimode Fabry-Perot model accurately predicts the existence of BICs and their positions in the parameter space. We show that, regardless of the slab thickness, BICs cannot exist below a cutoff frequency, which is related to the existence of the second-order Bloch wave in the photonic crystal. Thanks to the semianalyticity of the model, we investigate the dynamics of BICs with the slab thickness in symmetric and asymmetric photonic crystal slabs. We evidence that, as the horizontal mirror symmetry is broken, the symmetry-protected BICs that exist in symmetric structures at the $\mathrm{\Gamma }$ point of the dispersion diagram become resonance-trapped BICs, but only for specific values of the slab thickness.