We explore the formation and collective modes of Bose-Einstein condensate of Dirac magnons (Dirac BEC). While we focus on two-dimensional Dirac magnons, an employed approach is general and could be used to describe Bose-Einstein condensates with linear quasiparticle spectrum in various systems. By using a phenomenological multicomponent model of pumped boson population together with bosons residing at Dirac nodes, the formation and time evolution of condensates of Dirac bosons is investigated. The condensate coherence and its multicomponent nature are manifested in the Rabi oscillations whose period is determined by the gap in the spin-wave spectrum. A Dirac nature of the condensates could be also probed by the spectrum of collective modes. It is shown that the Haldane gap provides an efficient means to tune between the gapped and gapless collective modes as well as controls their stability.

Hund's coupling in multiorbital systems allows for the possibility of even-parity orbital-antisymmetric spin-triplet pairing, which can be stabilized by spin-orbit coupling (SOC). While this pairing expressed in the orbital basis is uniform and spin-triplet, it appears in the band basis as a pseudospin-singlet, with the momentum dependence determined by the SOC and the underlying triplet character remaining in the form of interband pairing active away from the Fermi energy. Here, we examine the role of momentum-dependent SOC in generating nontrivial pairing symmetries, as well as the hidden triplet nature associated with this interorbital pairing, which we dub a “shadowed triplet.” Applying this concept to ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we first derive several forms of SOC with $d$-wave form factors from a microscopic model, and subsequently we show that for a range of SOC parameters, a pairing state with $s+i{d}_{xy}$ symmetry can be stabilized. Such a pairing state is distinct from pure spin-singlet and -triplet pairings due to its unique character of pseudospin-energy locking. We discuss experimental probes to differentiate the shadowed triplet pairing from conventional pseudospin-triplet and -singlet pairings.

This paper presents an exact analytical theory for field emission from dielectric coated cathode surfaces, by solving the one-dimensional (1D) Schrödinger equation with a double-triangular potential barrier introduced by the coating. The effects of the cathode material (work function and Fermi energy), dielectric properties (dielectric constant, electron affinity, and thickness), applied dc field strength, and cathode temperature are analyzed in detail. For 1D flat cathode surfaces with coating, it is found that the emission current density can be larger than the uncoated case when the dielectric constant is smaller than a certain value ${\varepsilon }_{\mathrm{diel}}^{\mathrm{th}}$ and the dielectric thickness is larger than the threshold value $d_{\mathrm{th}}\left[\mathrm{nm}\right]\approx {\varepsilon }_{\mathrm{diel}}W/eF$ with the dielectric constant ${\varepsilon }_{\mathrm{diel}}<{\varepsilon }_{\mathrm{diel}}^{\mathrm{th}}$, where $W$ is the work function of the cathode material, $F$ the applied dc field, and $e$ the elementary charge. Our quantum model is also compared with a modified Fowler-Nordheim equation for a double barrier, showing qualitatively good agreement. Our study provides insights for designing field emitters with higher efficiency and better stability.

The fractionalization of microscopic degrees of freedom is a remarkable manifestation of strong interactions in quantum many-body systems. Analytical studies of this phenomenon are primarily based on two distinct frameworks: field theories of partons and emergent gauge fields, or coupled arrays of one-dimensional quantum wires. We unify these approaches for two-dimensional spin systems. Via exact manipulations, we demonstrate how parton gauge theories arise in microscopic wire arrays and explicitly relate spin operators to emergent quasiparticles and gauge-field monopoles. This correspondence allows us to compute physical correlation functions within both formulations and leads to a straightforward algorithm for constructing parent Hamiltonians for a wide range of exotic phases. We exemplify this technique for several chiral and nonchiral quantum spin liquids.

Yttrium sulfides are found to exhibit rich phase diagram and diverse properties under pressures. Here, we systematically investigate the stability of binary Y-S system up to 50 GPa using first-principles swarm-intelligence structure search. We identify seven hitherto unknown stoichiometries (${\mathrm{Y}}_7{\mathrm{S}}_6$, ${\mathrm{Y}}_6{\mathrm{S}}_5$, ${\mathrm{Y}}_7{\mathrm{S}}_8$, ${\mathrm{Y}}_6{\mathrm{S}}_7$, ${\mathrm{Y}}_5{\mathrm{S}}_6$, ${\mathrm{Y}}_3{\mathrm{S}}_2$, and ${\mathrm{YS}}_3$) that are energetically stable with respect to the decomposition into elemental components. All phases exhibit metallic nature, of which ${\mathrm{YS}}_3$ shows potential superconductivity with estimated $T_c$ of 18.5 K at 50 GPa. The $T_c$ in ${\mathrm{YS}}_3$ is comparable to the 17 K superconductivity in elements yttrium (89.3 GPa) and sulfur (160 GPa), but could be achieved at much lower pressure. The high superconductivity is attributed to the high electronic density of states from S atoms at Fermi level. Additionally, ${\mathrm{Y}}_3{\mathrm{S}}_2$ is predicted to be a layer-structured magnetic electride with magnetic moment of 0.5 $\mu B$ per formula unit at 6 GPa, which transforms to a three-dimensional phase with weak superconductivity around 17 GPa.

Ladder models of ultracold atoms offer a versatile platform for the experimental and theoretical study of different phenomena and phases of matter linked to the interplay between artificial gauge fields and interactions. Strongly correlated helical states are known to appear for specific ratios of the particle and magnetic flux densities, and they can often be interpreted as a one-dimensional limit of fractional quantum Hall states, thus being called pretopological. Their signatures, however, are typically hard to observe due to the small gaps characterizing these states. Here we investigate bosonic ladder models at filling factor $\nu =1$. Based on bosonization, renormalization group, and matrix product state simulations we pinpoint two strongly correlated helical phases appearing at this resonance. We show that one of them can be accessed in systems with two-species hardcore bosons and on-site repulsions only, thus amenable for optical lattice experiments. Its signatures are sizable and stable over a broad range of parameters for realistic system sizes.

For systems with spatial and nonspatial symmetries, the topological classification depends not only on these symmetries but also on the commutation/anticommutation relations between spatial and nonspatial symmetries. The coexistence of spatial and nonspatial symmetries together with appropriate commutation/anticommutation relations between them can give rise to crystalline and higher-order topological phases, which host gapless boundary modes. Alternatively, space-time symmetries in a Floquet system can take the role of spatial symmetries in deciding the topological classification. Promoting a spatial symmetry to a space-time symmetry can alter the commutation relations, which in turn can modify the topological properties of the system. We show how a coherently excited phonon mode can be used to promote a spatial symmetry with which the static system is always trivial to a space-time symmetry which supports a nontrivial Floquet topological phase. We demonstrate this effect by considering two systems: The first is a second-order topological superconductor, and the second is a first-order crystalline topological insulator. In both these cases, a coherently excited phonon mode is responsible for promoting the reflection symmetry to a time-glide symmetry. This newly introduced symmetry allows the previously trivial system to host gapless modes. In the first case, these are protected corner modes, while in the second case, these are gapless edge modes.

Majorana fermions act as their own antiparticle, and they have long been thought to be confined to the realm of pure theory. However, interest in them has recently resurfaced, as it was realized through the work of Kitaev that some experimentally accessible condensed matter systems can host these exotic excitations as bound states on the boundaries of one-dimensional chains, and that their topological and non-Abelian nature holds promise for quantum computation. Unambiguously detecting the experimental signatures of Majorana bound states has turned out to be challenging, as many other phenomena lead to similar experimental behavior. Here, we computationally study a ring comprised of two Kitaev model chains with tunnel coupling between them, where an applied magnetic field allows for Aharonov-Bohm interference in transport through the resulting ring structure. We use a nonequilibrium Green's function technique to analyze the transport properties of the ring in both the presence and absence of Majorana zero modes. Further, we show that these results are robust against weak disorder in the presence of an applied magnetic field. This computational model suggests another signature for the presence of these topologically protected bound states can be found in the magnetic field dependence of devices with loop geometries.

Although it is well documented that the vortex core in a micromagnet can switch polarization in response to magnetic field pulses, the temperature dependence of this process has not yet been addressed by experiments. Using scanning transmission x-ray microscopy, we investigate the magnetic vortex dynamics in a ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ microplatelet at temperatures ranging from $150\mathrm{K}$ up to $T_{\mathrm{C}}$ at $350\mathrm{K}$. The time-resolved images reveal qualitatively different dynamic regimes as a function of temperature and applied field, as the relative strengths of the micromagnetic energy terms strongly vary over this temperature range. By explicitly accounting for the temperature dependence of the magnetic parameters in our micromagnetic simulations, we found excellent agreement with the experiments over the full measurement range. In line with previous models, the simulations reveal that the vortex core switches polarization when it reaches a critical velocity that mainly depends on the strength of the exchange interaction. It is therefore strongly temperature dependent, thus explaining our observation of the different dynamical regimes.

Certain lattices with specific geometries have one or more spectral bands that are strictly flat, i.e., the electron energy is independent of the momentum. This can occur robustly irrespective of the specific couplings between the lattices sites due to the lattice symmetry, or it can result from fine-tuned couplings between the lattice sites. While the theoretical picture behind flat electronic bands is well developed, experimental realization of these lattices has proven challenging. Utilizing scanning tunneling microscopy (STM) and spectroscopy (STS), we manipulate individual vacancies in a chlorine monolayer on Cu(100) to construct various atomically precise one-dimensional (1D) lattices with engineered flat bands. We realize experimentally both gapped and gapless flat band systems with single or multiple flat bands. We also demonstrate tunability of the energy of the flat bands and how they can be switched on and off by breaking and restoring the symmetry of the lattice geometry. The experimental findings are corroborated by tight-binding calculations. Our results constitute the first experimental realizations of engineered flat bands in a 1D solid-state system and pave the way toward the construction of, e.g., topological flat band systems and experimental tests of flat-band-assisted superconductivity in a fully controlled system.

We numerically study the effects of nonmagnetic impurities (vacancies) in the spin-$S$ Heisenberg antiferromagnet on the kagome lattice. For a range of low but nonzero temperatures, and spin values that extend down to $S=2$, we find that the magnetization response to an external magnetic field is consistent with the response of emergent “half-orphan” degrees of freedom that are expected to dominate the response of the corresponding classical magnet in a similar temperature range whenever there are two vacancies on the same triangle. Specifically, for all spin values we have considered (from $S=1/2$ to 4), there is a large enhancement of the local susceptibility of the lone spin on such a triangle with two vacancies; in the presence of a uniform magnetic field $h$, this lone-spin behaves effectively as an almost free spin $S$ in an effective field $h/2$. Quite remarkably, in the zero-temperature limit, the ground state in the presence of a half-orphan has a nonzero total spin value $S_{\text{GS}}$ that shows a trend similar to $S/2$ when $S\ge 2$. These qualitative aspects of the response differ strikingly from the more conventional response of diluted samples without such half-orphan degrees of freedom. We discuss how these findings could be checked experimentally.

Motivated by recent experimental and numerical progress on various cluster Mott insulators, we study an extended Hubbard model on a breathing kagome lattice with a single electron orbital and $1/6$ electron filling. Two distinct types of cluster localization are found in the cluster Mott regime due to the presence of the electron repulsion between neighboring sites, rather than from the on-site Hubbard interaction in the conventional Mott insulators. We introduce a unified parton construction framework to accommodate both types of cluster Mott insulating phase as well as a trivial Ferm liquid metal and discuss the phase transitions in the phase diagram. It is shown that, in one of the cluster localization phases, the strong intersite repulsion results into locally metallic behavior within one of two triangular clusters on the breathing kagome lattice. We further comment on experimental relevance to existing Mo-based cluster magnets.

When light is incident on a medium with a spatially disordered index of refraction, interference effects lead to near-perfect reflection when the number of dielectric interfaces is large, so that the medium becomes a “transparent mirror.” We investigate the analog of this effect for electrons in twisted bilayer graphene (TBG), for which local fluctuations of the twist angle give rise to a spatially random Fermi velocity. In a description that includes only spatial variation of Fermi velocity, we derive the incident-angle-dependent localization length for the case of quasi-one-dimensional disorder by mapping this problem onto a one-dimensional Anderson localization. The localization length diverges at normal incidence as a consequence of Klein tunneling, leading to a power-law decay of the transmission when averaged over incidence angle. In a minimal model of TBG, the modulation of twist angle also shifts the location of the Dirac cones in momentum space in a way that can be described by a random gauge field, and thus Klein tunneling is inexact. However, when the Dirac electron's incident momentum is large compared to these shifts, the primary effect of twist disorder is only to shift the incident angle associated with perfect transmission away from zero. These results suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics.

Majorana fermions are often proposed to be realized by singling out one Fermi surface without spin degeneracy via spin-orbit coupling, and then imposing boundaries or defects. In this work, we take a different route starting with two degenerate Fermi surfaces without spin-orbit coupling, and show that by the method of “kink on boundary,” the dispersive chiral Majorana fermions can be realized in superconducting systems with $p\pm is$ pairings. The surfaces of these systems develop spontaneous magnetizations whose directions are determined by the boundary orientations and the phase difference between the $p$- and $s$-component gap functions. Along the magnetic domain walls on the surface, there exist chiral Majorana fermions propagating unidirectionally, which can be conveniently dragged and controlled by external magnetic fields. Furthermore, the surface magnetization is shown to be a magnetoelectric effect based on a Ginzburg-Landau free-energy analysis. We also discuss how to use the proximity effects to realize chiral Majorana fermions by performing the kink on boundary method.

We report giant bulk rectification in the bulk trigonal superconductor $\mathrm{PbTa}{\mathrm{Se}}_2$. Nonreciprocal magnetotransport originating from a vortex ratchet motion was successfully observed, showing a significant enhancement during the superconducting transition, which is considered a model behavior for three-dimensional noncentrosymmetric superconductors with strong vortex pinning. Moreover, an unprecedented sign change of nonreciprocal transport was observed with increasing current, which indicates that the rectification effect can be tuned by the current value as well as the magnetic field. The present results represent the rich vortex dynamics and offer a principle for current control of rectification in noncentrosymmetric bulk superconductors.

The geometric properties of a lattice can have profound consequences on its band spectrum. For example, symmetry constraints and geometric frustration can give rise to topologicially nontrivial and dispersionless bands, respectively. Line-graph lattices are a perfect example of both of these features: Their lowest energy bands are perfectly flat, and here we develop a formalism to connect some of their geometric properties with the presence or absence of fragile topology in their flat bands. This theoretical work will enable experimental studies of fragile topology in several types of line-graph lattices, most naturally suited to superconducting circuits.

An analytical formula is derived for particle and energy densities of fermions and bosons, and their ballistic momentum and energy currents for anisotropic energy dispersions in generalized dimensions. The formulation considerably simplifies the comparison of the statistical properties and ballistic particle and energy transport currents of electrons, acoustic phonons, and photons in various dimensions in a unified manner. Assorted examples of its utility are discussed, ranging from blackbody radiation to Schottky diodes and ballistic transistors, quantized electrical and thermal conductance, generalized ballistic Seebeck and Peltier coefficients, their Onsager relations, the generalized Wiedemann-Franz law and the robustness of the Lorenz number, and ballistic thermoelectric power factors, all of which are obtained from the single formula. The new formulation predicts a thermoelectric power factor behavior of three-dimensional Dirac bands which has not been observed yet.

We study a periodically driven qubit coupled to a quantized cavity mode. Despite its apparent simplicity, this system supports a rich variety of exotic phenomena, such as topological frequency conversion as recently discovered in Martin et al. [Phys. Rev. X 7, 041008 (2017)]. Here we report on a qualitatively different phenomenon that occurs in this platform, where the cavity mode's oscillations lock their frequency to a rational fraction $r/q$ of the driving frequency $\mathrm{\Omega }$. This phenomenon, which we term quantum frequency locking, is characterized by the emergence of $q$-tuplets of stationary (Floquet) states whose quasienergies are separated by $\mathrm{\Omega }/q$, up to exponentially small corrections. The Wigner functions of these states are nearly identical, and exhibit highly regular and symmetric structure in phase space. Similarly to Floquet time crystals, these states underlie discrete time-translation symmetry breaking in the model. We develop a semiclassical approach for analyzing and predicting quantum frequency locking in the model, and use it to identify the conditions under which it occurs.

For massive database-driven materials research, there are increasing demands for both fast and accurate quantum mechanical computational tools. Contemporary density functional theory (DFT) methods can be fast, sacrificing their accuracy, or be precise, consuming a significant amount of resources. Here, to overcome such a problem, we present a DFT method that exploits self-consistent determinations of the on-site and intersite Hubbard interactions ($U$ and $V$) simultaneously and obtain band gaps of diverse materials in the accuracy of the $GW$ method at a standard DFT computational cost. To achieve the self-consistent evaluation of $U$ and $V$, we adapt a recently proposed Agapito–Curtarolo–Buongiorno Nardelli pseudohybrid functional for $U$ to implement a density functional of $V$. This method is found to be appropriate for considering various interactions such as local Coulomb repulsion, covalent hybridization, and their coexistence. We also obtain good agreements between computed and measured band gaps of low-dimensional systems, thus meriting this approach for large-scale as well as high-throughput calculations for various bulk and nanoscale materials with a higher accuracy.

We demonstrate how the properties of light-induced electronic Floquet states in solids impact natural physical observables, such as transport properties, by capturing the environmental influence on the electrons. We include the environment as dissipative processes, such as interband decay and dephasing, often ignored in Floquet predictions. These dissipative processes determine the Floquet band occupations of the emergent steady state, by balancing out the optical driving force. In order to benchmark and illustrate our framework for Floquet physics in a realistic solid, we consider the light-induced Hall conductivity in graphene recently reported by McIver et al. [Nat. Phys. 16, 38 (2020)]. We show that the Hall conductivity is estimated by the Berry flux of the occupied states of the light-induced Floquet bands, in addition to the kinetic contribution given by the average band velocity. Hence, Floquet theory provides an interpretation of this Hall conductivity as a geometric-dissipative effect. We demonstrate this mechanism within a master equation formalism, and obtain good quantitative agreement with the experimentally measured Hall conductivity, underscoring the validity of this approach which establishes a broadly applicable framework for the understanding of ultrafast nonequilibrium dynamics in solids.