We theoretically explore the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction mediated by spin-3/2 quasiparticles in half-Heusler topological semimetals in quasi-two-dimensional geometries. We find that while the Kohn-Luttinger part of the effective Hamiltonian gives rise to generalized Heisenberg coupling of the form ${\mathcal{H}}_{\mathrm{RKKY}}\propto {\sigma }_{1,i}{\mathcal{I}}_{ij}{\sigma }_{2,j}$ with a symmetric matrix ${\mathcal{I}}_{ij}$, the addition of a small antisymmetric linear spin-orbit coupling term leads to a Dzyaloshinskii-Moriya type of coupling with an antisymmetric matrix ${\mathcal{I}}_{ij}^{\prime }$. As a general aspect, we demonstrate that besides revealing the usual oscillatory dependence on the distance, all coupling strengths depend strongly on the relative orientation of the two impurities with respect to the lattice. This yields a strongly anisotropic behavior for ${\mathcal{I}}_{ij}$ such that by only rotating one impurity around another at a constant distance, we can see further oscillations of the RKKY couplings. As we will explain, this unprecedented effect is very unique to electronic systems such as half-Heusler topological semimetals, which combine spin-orbit coupling with strongly anisotropic Fermi surfaces. We further find that all of the RKKY terms have two common features: (i) a tetragonal warping in their map of spatial variations in a similar way to the two-dimensional Fermi lines of the system, and (ii) a complex beating pattern consisting of different periodicities. Intriguingly, all these features survive in all dopings, and we see them in both electron- and hole-doped cases. In addition, due to the lower dimensionality combined with the effects of different spin-orbit couplings, we see that only one symmetric off-diagonal term, ${\mathcal{I}}_{xy}$, and two antisymmetric (Dzyaloshinskii-Moriya) components, ${\mathcal{I}}_{xz}^{\prime }$ and ${\mathcal{I}}_{yz}^{\prime }$, are nonvanishing, while the remaining three off-diagonal components are identically zero. This manifests in another drastic difference of RKKY interaction in half-Heusler topological semimetals compared to the electronic systems with a spin-1/2 effective description.

We present a hierarchical quantum master equation (HQME) approach, which allows the numerically exact evaluation of higher-order current cumulants in the framework of full counting statistics for nonequilibrium charge transport in nanosystems. The novel methodology is exemplarily applied to a model of vibrationally coupled electron transport in a molecular nanojunction. We investigate the influence of cotunneling on avalanchelike transport, in particular in the nonresonant transport regime, where we find that inelastic cotunneling acts as a trigger process for resonant avalanches, which manifests itself as finite temperature oscillations in the current fluctuations. We also demonstrate that the correction to the elastic noise upon opening of the inelastic transport channel is nonuniversal and strongly affected by the nonequilibrium excitation of the vibration.

An extensive theoretical and experimental investigation has been conducted on fermion-pair condensation and exciton condensation as distinct classes of Bose-Einstein-like condensation. In this Rapid Communication, the existence of a fermion-exciton condensate—a single quantum state in which the characters of both fermion-pair and exciton condensates coexist—is established computationally in the low-particle-number ($N$) limit and theoretically in the large-$N$ thermodynamic limit. The trade-off between the fermion-pair and excitonic character of the fermion-exciton condensate is shown to be elliptic in nature. The possibility that the properties of fermion-exciton condensates could be a hybrid of the properties of fermion-pair condensates and exciton condensates is discussed, and future experimental and computational exploration of this class of condensate, which may potentially be realizable in a bilayer of superconductors, is anticipated.

In this Rapid Communication we analyze the efficiency of operations based on transferring charge from a quantum dot (QD) to two coupled topological superconductors, which can be used for performing non-Abelian operations on Majorana bound states (MBSs). We develop a method which allows us to describe the full time evolution of the system as the QD energy is manipulated. Using a full counting statistics analysis, we set bounds to the operation timescales. The lower bound depends on the superconducting phase difference due to a partial decoupling of the different MBS parity sectors, while the upper bound is set by the tunneling of quasiparticles to the MBSs. Using realistic parameters, we find the existence of a regime where the operation can be carried out with a fidelity close to unity. Finally, we propose the use of a two-operation protocol to quantify the effect of the dephasing and accumulated dynamical phases, demonstrating their absence for certain superconducting phase differences.

The laws of thermodynamics allow work extraction from a single heat bath provided that the entropy decrease of the bath is compensated for by another part of the system. We propose a thermodynamic quantum engine that exploits this principle and consists of two electrons on a double quantum dot (QD). The engine is fueled by providing it with singlet spin states, where the electron spins on different QDs are maximally entangled, and its operation involves only changing the tunnel coupling between the QDs. Work can be extracted since the entropy of an entangled singlet is lower than that of a thermal (mixed) state, although they look identical when measuring on a single QD. We show that the engine is an optimal thermodynamic engine in the long-time limit. In addition, we include a microscopic description of the bath and analyze the engine's finite-time performance using experimentally relevant parameters.

Previous works on deformed graphene predict the existence of valley-polarized states, however, optimal conditions for their detection remain challenging. We show that in the quantum Hall regime, edgelike states in strained regions can be isolated in energy within Landau gaps. We identify precise conditions for conducting edgelike states to be valley polarized. By the appropriate design of strain profiles these states can be positioned at chosen locations in the sample. A map of the local density of states as a function of energy and position reveals a unique braid pattern that serves as a fingerprint to identify valley polarization.

Using synchrotron x-ray diffraction, we investigate how the tetragonal-to-orthorhombic phase transition in the ferrimagnetic $\left(c>a\right)$ spinel ${\mathrm{Mn}}_3{\mathrm{O}}_4$ is driven by the $t_2$ orbital degeneracy of ${\mathrm{Cu}}^{2+}\left(3d^9\right)$ ions doped in its tetrahderal sites. At high temperatures where the orthorhombic phase initially appears, we observe that the local elongations of Cu-doped tetrahedra cause the unit cells to contract along the same direction. The signs of the local and global lattice strains finally agree with each other when the orthorhombic phase transition is completed below $T_{\mathrm{N}}=42.5$ K. Using neutron diffraction, we report that ${\mathrm{Co}}^{2+}\left(3d^7\right)$ doping also stabilizes the orthorhombic phase but without enhancing the associated lattice strains. These results are consistent with the scenario that the orthorhombic instability of the undoped ${\mathrm{Mn}}_3{\mathrm{O}}_4$ is driven by the spin-lattice coupling.

We investigate the effect of nonlocal interactions on the photodoped Mott insulating state of the two-dimensional Hubbard model using a nonequilibrium generalization of the dynamical cluster approximation. In particular, we compare the situation where the excitonic states are lying within the continuum of doublon-holon excitations to a setup where the excitons appear within the Mott gap. In the first case, the creation of nearest-neighbor doublon-holon pairs by excitations across the Mott gap results in enhanced excitonic correlations, but these excitons quickly decay into uncorrelated doublons and holons. In the second case, photoexcitation results in long-lived excitonic states. While in a low-temperature equilibrium state, excitonic features are usually not evident in single-particle observables such as the photoemission spectrum, we show that the photoexcited nonequilibrium system can exhibit in-gap states associated with the excitons. The comparison with exact-diagonalization results for small clusters allows us to identify the signatures of the excitons in the photoemission spectrum.

A valence band in narrow HgTe quantum wells contains well-conductive Dirac-like light holes at the $\mathrm{\Gamma }$ point and a poorly conductive heavy hole subband located in the local valleys. Here we propose and employ two methods to measure the density of states for these heavy holes. The first method uses a gate-recharging technique to measure thermodynamical entropy per particle. As the Fermi level is tuned with gate voltage from a light to heavy subband, the entropy increases dramatically, and the value of this increase gives an estimate for the density of states. The second method determines the density of states for heavy holes indirectly from the gate voltage dependence of the period of the Shubnikov–de Haas oscillations for light holes. The results obtained by both methods are in reasonable agreement with each other. Our approaches can be applied to measure large effective carrier masses in other two-dimensional gated systems.

Self-sustained oscillations, limit cycles, are a fundamental phenomenon unique to nonlinear dynamic systems of high-dimensional phase space. They enable understanding of a wide range of cyclic processes in natural, social, and engineering systems. Here we show that limit cycles form in coupled polariton cavities following the breaking of Josephson coupling, leading to frequency-comb emission. The limit cycles and destruction of Josephson coupling both appear due to interplay between strong polariton-polariton interaction and a dissipative contribution to the cavity coupling. The resulting nonlinear dynamics of the condensates is characterized by asymmetric population distribution and nontrivial average phase difference between the two condensates, and by time-periodic modulation of their amplitudes and phases. The latter is manifested by coherent emission of new equidistant frequency components. The emission spectrum resembles that of a microfrequency comb, but originates from a fundamentally different mechanism than that of existing frequency combs. It allows nonresonant excitation with a power input much below the conventional semiconductor laser threshold. The comb line spacing is determined by the interaction and coupling strengths, and is adjustable up to multiterahertz frequency. The work establishes coupled polariton cavities as an experimental platform for rich nonlinear dynamic phenomena.

We develop a tight-binding model based on linear combination of atomic orbitals (LCAO) methods to describe the electronic structure of arrays of acceptors, where the underlying basis states are derived from an effective-mass-theory solution for a single acceptor in either the spherical approximation or the cubic model. Our model allows for arbitrarily strong spin-orbit coupling in the valence band of the semiconductor. We have studied pairs and dimerized linear chains of acceptors in silicon in the “independent-hole” approximation, and investigated the conditions for the existence of topological edge states in the chains. For the finite chain we find a complex interplay between electrostatic effects and the dimerization, with the long-range Coulomb attraction of the hole to the acceptors splitting off states localized at the end acceptors from the rest of the chain. A further pair of states then splits off from each band, to form a pair localized on the next-to-end acceptors, for one sense of the bond alternation and merges into the bulk bands for the other sense of the alternation. We confirm the topologically nontrivial nature of these next-to-end localized states by calculating the Zak phase. We argue that for the more physically accessible case of one hole per acceptor these long-range electrostatic effects will be screened out; we show this by treating a simple phenomenologically screened model in which electrostatic contributions from beyond the nearest neighbors of acceptor each pair are removed. Topological states are now found on the end acceptors of the chains. In some cases the termination of the chain required to produce topological states is not the one expected on the basis of simple geometry (short versus long bonds); we argue this is because of a nonmonotonic relationship between the bond length and the effective Hamiltonian matrix elements between the acceptors.

The time evolution of a quantum dot exciton in Coulomb interaction with wetting layer carriers is treated using an approach similar to the independent boson model. The role of the polaronic unitary transform is played by the scattering matrix, for which a diagrammatic, linked cluster expansion is available. Similarities and differences to the independent boson model are discussed. A numerical example is presented.

Multi-Weyl semimetals are new types of topological semimetals whose topological charge is equal to the value of the winding number $J$. Here, we investigate the single-particle ballistic scattering on a rectangular barrier in multi-Weyl semimetals. Because this system has a crystallographic anisotropy, the scattering properties depend on the mutual orientation of the crystalline axis and the barrier. For different $J$, the vertical component of the wave vector $k_{\perp }$ and the corresponding probability current density $j_{\perp }$ satisfies $j_{\perp }\propto k_{\perp }^{2J-1}$. In the case of a barrier perpendicular to the $z$-axis, it is found that the reflectionless incident angles are determined by geometrical resonances between the barrier width and the de Broglie length of the scattered electrons in the barrier region. In the $z$-axis direction, the local minimum conductance $G_{\min }$ occurs when the chemical potential equals the barrier height and $G_{\min }\propto 1/L^{2/J}$, where $L$ is the width of the barrier. Differently, in the case of a barrier perpendicular to the $x$-axis, the angular distribution of the transmission probability is no longer rotation invariant. For the double-Weyl semimetals ($J=2$), the transmission probability decreases rapidly to 0 as the barrier width $L$ increases for a normal incidence, which is similar to conventional nonrelativistic electrons. It is interesting that perfect transmission is again found for normally incident Weyl fermions for the triple-Weyl semimetals ($J=3$). In this case, the tunneling indicates a property similar to that in the case of $J=1$.

One of the main challenges in the field of thermal radiation is to actively control the near-field radiative heat transfer (NFRHT) between closely spaced bodies. In this context, the use of an external magnetic field has emerged as a very attractive possibility and a plethora of physical phenomena have been put forward in the last few years. Here, we predict some additional magnetic-field-induced phenomena that can take place in the context of NFRHT between planar layered structures containing magneto-optical (MO) materials (mainly doped semiconductors like InSb). In particular, we predict the possibility of increasing the NFRHT upon applying an external magnetic field in an asymmetric structure consisting of two infinite plates made of InSb and Au. We also study the impact of a magnetic field in the NFRHT between structures containing MO thin films and show that the effect is more drastic than in their bulk counterparts. Finally, we systematically investigate the anisotropic thermal magnetoresistance, i.e., the dependence of the radiative heat conductance on the orientation of an external magnetic field, in the case of two infinite plates made of InSb and show that one can strongly modulate the NFRHT by simply changing the orientation of the magnetic field. All the phenomena predicted in this work can be experimentally tested with existent technology and provide new insight into the topic of active control of NFRHT.

We study the dynamics and interactions of the critical fluctuations of the Verwey transition in magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) in the pretransition region by means of inelastic neutron scattering experiments on a natural single crystal. We find that a ${\mathrm{\Delta }}_5$ mode interacts strongly with a central peak of order parameter fluctuations, whose width is determined by the order parameter coherence time. This is compared with pump-probe experiments, which we formerly explained in terms of fluctuation-assisted stimulated Raman scattering. Our estimates of the order parameter correlation time from experiments in the energy domain (inelastic neutron scattering experiments) and in the time domain (pump-probe experiments) coincide, thus giving further credit to our previous interpretation of pump-probe experiments and confirming that the Verwey transition is of the order-disorder type, without phonon softening.

Using the first-principles quantum Monte Carlo calculations, we investigate the previously established universal scaling, as well as the corresponding classification schemes, for the Néel temperature $T_N$ and the staggered magnetization density $M_s$ of three-dimensional dimerized quantum antiferromagnets. Particularly, the calculations are done on both the stacked honeycomb and cubic lattices. In addition to simulating models with two types of antiferromagnetic couplings (bonds) like those examined in earlier studies, here a tunable parameter controlling the strength of the third type of bond is introduced. Interestingly, while minor effects due to microscopic details appear, the data for the studied models with two types of bonds do fall on top of the universal scaling curves determined previously. Moreover, the most striking result suggested in our study is that with the presence of three kinds of bonds in the investigated models, the considered scaling relations between $T_N$ and $M_s$ can be classified by the coordination number of the underlying lattice geometries. The findings presented here broaden the applicability of the associated classification schemes formerly discovered. In particular, these results not only are interesting from a theoretical point of view but also can serve as useful guidelines for the relevant experiments.

The high-field (H,T) phase diagram of the multiferroic lacunar spinel $\mathrm{Ge}{\mathrm{V}}_4{\mathrm{S}}_8$ has been studied by ultrasound, magnetization, and pyrocurrent experiments in magnetic fields up to 60 T. The title compound consists of molecular building blocks, with vanadium ${\mathrm{V}}_4$ clusters characterized by a unique electron density. These vanadium tetrahedra constitute a Jahn-Teller active entity, which drive an orbital-ordering transition at 30 K with the concomitant appearance of ferroelectricity. Ultrasound and magnetization experiments reveal sharp anomalies in magnetic fields of 46 T, which are associated with a first-order phase transition into an orbitally disordered state characterized by significant field and temperature hystereses. We report a sequence of complex magnetic, polar, and orbitally ordered states, i.e., the appearance of two orbitally ordered phases OO1 and OO2 for ${\mu }_{0}H<45\mathrm{T}$ and $T<30\mathrm{K}$. Beyond the paraelectric phase we further evidenced three ferroelectric phases, FE1, FE2, and FE3. Finally, antiferromagnetic (AFM) order $(T<15\text{\hspace{0.17em}K})$ and fully polarized ferromagnetic order $\left({\mu }_{0}H>60\mathrm{T}\right)$ have been observed in $\mathrm{Ge}{\mathrm{V}}_4{\mathrm{S}}_8$. At low temperatures and for fields below 40 T, AFM order coexists with the polar phase FE3 identifying a multiferroic state. Our results demonstrate a fascinating competition of the different orders, which the material manifests in high magnetic fields and at low temperatures.

We show that pinning of surface spins affects the hysteresis properties of the core-shell magnetic nanostuctures of different shapes, sizes, and different spin interactions, namely, Ising, XY, and Heisenberg models. The asymmetry in hysteresis loops occuring due to pinning turns out to be more prominent in an inverse core-shell structure where spin interaction in the core is antiferromagnetic and that in the shell is ferromagnetic. Monte Carlo simulations of the inverse core-shell nanostructures show that the exchange bias, even under zero-field-cooled conditions, increases with increase of both the pinning density and the fraction of up spins among the pinned ones. The exchange bias also exhibits a switch—from negative to positive—depending on the fraction of up spins pinned. These results are remarkably well reproduced by a simple model of the outermost surface layer. The surface spin pinning appears to affect the magnetic properties of heterostructures as well, besides nanostructures.

We study the heat current through two capacitively coupled quantum dots coupled in series with two conducting leads at different temperatures $T_L$ and $T_R$ in the spinless case (valid for a high applied magnetic field). Our results are also valid for the heat current through a single quantum dot with strongly ferromagnetic leads pointing in opposite directions (so that the electrons with given spin at the dot can jump only to one lead) or through a quantum dot with two degenerate levels with destructive quantum interference and high magnetic field. Although the charge current is always zero, the heat current is finite when the interdot Coulomb repulsion $U$ is taken into account due to many-body effects. We study the thermal conductance as a function of temperature and the dependence of the thermal current with the couplings to the leads, $T_L-T_R$, energy levels of the dots and $U$, including conditions for which an orbital Kondo regime takes place. When the energy levels of the dots are different, the device has rectifying properties for the thermal current. We find that the ratio between the thermal current resulting from a thermal bias $T_L>T_R$ and the one from $T_L<T_R$ is maximized for particular values of the energy levels, one above and the other below the Fermi level.