An accurate prediction of atomic diffusion in Fe alloys is challenging due to thermal magnetic excitations and magnetic transitions. We investigate the diffusion of Mn in bcc Fe using an effective interaction model and first-principles based spin-space averaged relaxations in magnetically disordered systems. The theoretical results are compared with the dedicated radiotracer measurements of ${}^{54}\mathrm{Mn}$ diffusion in a wide temperature range of 773 to 1173 K, performed by combining the precision grinding (higher temperatures) and ion-beam sputtering (low temperatures) sectioning techniques. The temperature evolution of Mn diffusion coefficients in bcc iron in theory and experiment agree very well and consistently reveal a reduced acceleration of Mn solute diffusion around the Curie point. By analyzing the temperature dependencies of the ratio of Mn diffusion coefficients to self-diffusion coefficients we observe a dominance of magnetic disorder over chemical effects on high-temperature diffusion. Therefore, the missing acceleration mainly reflects an anomalous behavior of the Mn solute in the magnetically ordered low-temperature state of the Fe host, as compared to other transition metals.

The nonreciprocal propagation, reflection, and absorption of light are most strongly pronounced in the magneto-optical Faraday and Kerr effects that are observed in any paramagnetic and diamagnetic material in the presence of a magnetic field, as well as spontaneous nonreciprocal effects in ferromagnets and ferrimagnets. But nonreciprocity in antiferromagnets is not at all an obvious effect. In this paper, we analyze reciprocal and nonreciprocal propagation of light in a magnetoelectric antiferromagnet ${\mathrm{CuB}}_2{\mathrm{O}}_4$ that demonstrates a large variety of intriguing optical effects. We present a description of the magnetic and antiferromagnetic nonreciprocal propagation and absorption of light on the basis of space and time symmetry considerations in the paramagnetic and commensurate antiferromagnetic state below the Néel temperature. We employ the expansion of the dielectric permittivity tensor as a function of the wave vector of propagating light, an applied magnetic field, and the antiferromagnetic and ferromagnetic order parameters. All these contributions to the dielectric permittivity serve as a basis for solving the relevant Maxwell's equations. Such an approach allowed us to establish the basic mechanisms affecting the propagation of light and sources of nonreciprocity, and it can be applied to other antiferromagnetic crystals. Results of calculations are compared with available experimental data for ${\mathrm{CuB}}_2{\mathrm{O}}_4$.

Thermoacoustic amplifiers are analyzed in the framework of nonreciprocal Willis coupling. The closed form expressions of the effective properties are derived, showing that an applied temperature gradient causes the appearance of a nonreciprocal Willis coupling. Even and nonreciprocal Willis couplings are exhibited already in the first-order Taylor expansion of the solution and are of equal modulus but opposite sign, thus suggesting that the even Willis coupling is a reaction to the nonreciprocity introduced by the temperature gradients. These Willis couplings cause a coalescence point in the $k$ space, which deviates from $\text{Re}\left(k\right)=0$ (with $k$ the wave number) and is thus a zero-group-velocity point, as well as the opening of an amplification gap at low frequency. Effective parameters and scattering properties are found in excellent agreement with experimental results. This article paves the way to further control the acoustic waves at very low frequencies with nonreciprocal systems.

Superconductivity emerges in $1T\text{−}{\mathrm{TiSe}}_2$ when its charge density wave (CDW) order is suppressed by Cu intercalation or pressure. Since the CDW state is thought to be an excitonic insulator, an interesting question is whether the superconductivity is also mediated by excitonic fluctuations. We investigated this question as to the nature of doping induced superconductivity in ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$ by asking about its consistency with the phonon-mediated pairing. We employed the ab initio density functional theory and density functional perturbation theory to compute the electron-phonon coupling Eliashberg function from which to calculate the superconducting (SC) critical temperature $T_c$. The calculated $T_c$ as a function of the doping concentration $x$ exhibits a dome shape with the maximum $T_c$ of $2-6$ K at $x\approx 0.05$ for the Coulomb pseudopotential $0\le {\mu }^{*}\le 0.1$. The maximal $T_c$ was found to be pinned to the quantum critical point where the CDW is completely suppressed and the corresponding phonon mode becomes soft. Underlying physics is that the reduced phonon frequency enhances the electron-phonon coupling constant $\lambda$, which overcompensates the frequency decrease to produce a net increase of $T_c$. The doping induced superconductivity in ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$ seems to be consistent with the phonon-mediated pairing. A comparative discussion was made with the pressure induced superconductivity in ${\mathrm{TiSe}}_2$.

Understanding the influence of vibrational degrees of freedom on transport through a heterostructure poses considerable theoretical and numerical challenges. In this work, we use the density-matrix renormalization group method together with local basis optimization to study the half-filled Holstein model in the presence of a linear potential, either isolated or coupled to tight-binding leads. In both cases, we observe a decay of charge density wave states at a sufficiently strong potential strength. Local basis optimization selects the most important linear combinations of local oscillator states to span the local phonon space. These states are referred to as optimal modes. We show that many of these local optimal modes are needed to capture the dynamics of the decay, that the most significant optimal mode on the initially occupied sites remains well described by a coherent state typical for small polarons, and that those on the initially empty sites deviate from the coherent-state form. Additionally, we compute the current through the structure in the metallic regime as a function of voltage. For small voltages, we reproduce results for the Luttinger parameters. As the voltage is increased, the effect of larger electron-phonon coupling strengths becomes prominent. Further, the most significant optimal mode remains almost unchanged when going from the ground state to the current-carrying state in the metallic regime.

We investigate the ground-state phase diagram of the spinless Haldane-Hubbard model in the presence of quenched disorder, contrasting results obtained from both exact diagonalization and the density matrix renormalization group, applied to a honeycomb cylinder. The interplay of disorder, interactions, and topology gives rise to a rich phase diagram and, in particular, highlights the possibility of a disorder-driven trivial-to-topological transition in the presence of finite interactions. That is, the topological Anderson insulator, demonstrated in noninteracting settings, is shown to be stable in the presence of sufficiently small interactions before a charge density wave Mott insulator sets in. We further promote a finite-size analysis of the transition to the ordered state in the presence of disorder, finding a mixed character of first- and second-order transitions in finite lattices, tied to the specific conditions of disorder realizations and boundary conditions used.

We present an experimental study of the electronic states of the nanolamellar compound ${\mathrm{Ti}}_2\mathrm{SnC}$ by means of angle-resolved photoemission spectroscopy (ARPES). Experiments were performed on macroscopic single crystals, and the observed Fermi surface and band structure were systematically compared with the output of density functional theory calculations. The fine details of the Fermi surface predicted by theory are duly evidenced by the ARPES measurements. The multiply connected Fermi surface (FS) is formed by a combination of quasi-two-dimensional corrugated tubes and quasi-one-dimensional thin plates, centered on high-symmetry axes and points, respectively. The quasi-one-dimensional FS region displays a clear Dirac-like dispersion. An evanescent surface state is observed in addition to the bulk bands.

We report an unconventional class of pressure-induced quantum phase transitions, possessing two bicritical points at 6.25 GPa in URhSn. This unique transformation accompanies a Fermi surface reconstruction, demarcating competing ordered phases suitably described with a localized and itinerant description of the magnetic $5f$ electrons. Ferromagnetic fluctuations over a wide range of temperatures and pressures in the pressure-induced low temperature phase are evidenced by a robust $T^{5/3}$ temperature dependence of resistivity up to 11 GPa, which is a characteristic of an elusive marginal Fermi-liquid state.

Recent experiments showed the transfer of orbital angular momentum (OAM) from a nonresonant laser onto an exciton-polariton condensate, despite earlier views that the phase information of such a laser should be lost during the process of polariton condensation. We study with a phenomenological theory the interplay of a usual angular momentum independent gain and an angular momentum preserving gain. We find that even when the latter is much smaller, it is enough to favor condensation into a given orbital angular momentum state. This further allows a breaking of symmetry in the system, which further manifests in nonreciprocal one-way propagation in a lattice of coupled rings. Even though we consider only Hermitian reciprocal coupling between rings, the local non-Hermiticity generates an effective nonreciprocal coupling and supports a non-Hermitian topological invariant (winding number) associated to a non-Hermitian skin effect.

We develop a microscopic and atomistic theory of electron-spin-based qubits in gated quantum dots in a single layer of transition metal dichalcogenides. The qubits are identified with two degenerate locked spin and valley states in a gated quantum dot. The two qubit states are accurately described using a multimillion atom tight-binding model solved in wave-vector space. The spin-valley locking and strong spin-orbit coupling result in two degenerate states, one of the qubit states being spin down located at the $+K$ valley of the Brillouin zone, and the other state located at the $-K$ valley with spin up. We describe the qubit operations necessary to rotate the spin-valley qubit as a combination of the applied vertical electric field, enabling spin-orbit coupling in a single valley, with a lateral strongly localized valley-mixing gate.

By using the tight-binding model and nonequilibrium Green's function method (NEGF), we study the band structures and transport properties of a silicene nanoribbon with a line defect where a bulk energy gap is opened due to the sublattice symmetry breaking. The flat subband bends downwards or upwards due to the effect of the line defect. The spin-orbit coupling induces the quantum spin Hall states. Especially, the energy band depends on the distance between the line defect and the edge of the nanoribbon. The effects of the on-site energies on the band spectra of the two defect configurations are different. There always exists one band gap for different on-site energies for the defect configuration of case 1. However, a gapless state and a band gap can be modulated by changing the on-site energy, the sublattice potential, and spin-orbit couplings for the defect configuration of case 2. Accordingly, the variation trends of the conductance including zero conductance can be well understood in terms of the combined effect of the sublattice potential, the on-site energy, and spin-orbit couplings on the band structures. Thus it is easy and effective to modulate the transport property of the silicene nanoribbon with the defect configuration of case 2 by utilizing the sublattice potential, the on-site energy, and spin-orbit couplings. This study is of great significance for the fabrication and the modulation of the transport property of silicene-based devices.

Dynamical magnetic impurities (MI) are considered as a possible origin for suppression of the ballistic helical transport on edges of two-dimensional (2D) topological insulators. The MIs provide a spin-flip backscattering of itinerant helical electrons. Such a backscattering reduces the ballistic conductance if the exchange interaction between the MI and the electrons is anisotropic and the Kondo screening is unimportant. It is well known that the isotropic MIs do not suppress the helical transport in systems with axial spin symmetry of the electrons. We show that, if this symmetry is broken, the isotropic MI acquires an effective anisotropy and suppresses the helical conductance. The peculiar underlying mechanism is a successive backscattering of the electrons which propagate in the same direction and have different energies. The respective correction to the linear conductance is determined by the allowed phase space of the electrons and scales with temperature as $T^4$. Hence, it disappears at small temperatures. This qualitatively distinguishes effects governed by the MIs with the induced and bare anisotropy; the latter is temperature independent. If $T$ is smaller than the applied bias, finite $eV$, the allowed phase space is provided by the bias and the differential conductance scales as ${\left(eV\right)}^4$. We point out regimes where the combined effect of the MI and the broken spin symmetry dominates over that governed by electron interactions.

Long-sought quantum spin liquids usually emerge in geometrically frustrated magnets. Less commonly, unfrustrated magnets can harbor a variety of quantum spin liquids if charge and/or orbital degrees of freedom are involved. Jahn-Teller distortion of ${\mathrm{Cu}}^{2+}{\mathrm{O}}_6$ octahedra is absent in hexagonal ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ suggesting a Cu $3d$ spin-orbital liquid state. Here, by means of time-resolved resonant x-ray scattering, we show that hexagonal ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ exhibits a charge-orbital dynamics which is absent in the orthorhombic phase of ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ with Jahn-Teller distortion and Cu $3d$ orbital order. The time scale of charge-orbital dynamics manifests in the coherent phonons at 6 GHz. The present work reveals a role of electron-lattice entanglement in the quantum spin-orbital liquid state.

Motivated by the potential for cupratelike superconductivity in monolayer rare-earth nickelate superlattices, we study the effects of crystal field splitting, lattice distortions, and strain on the charge, magnetic, and orbital order in undoped two-dimensional (2D) nickelate monolayers $R\mathrm{Ni}{\mathrm{O}}_3$. We use a two-band Hubbard model to describe the low-energy electron states, with correlations controlled by an effective Hubbard $U$ and Hund's $J$. The electrons are coupled to the octahedral breathing-mode lattice distortions. Treating the lattice semiclassically, we apply the Hartree-Fock approximation to obtain the phase diagram for the ground state as a function of the various parameters. We find that the 2D confinement leads to strong preference for the planar $d_{x^2-y^2}$ orbital even in the absence of a crystal-field splitting. The $d_{x^2-y^2}$ polarization is enhanced by adding a crystal field splitting, whereas coupling to breathing-mode lattice distortions weakens it. However, the former effect is stronger, leading to $d_{x^2-y^2}$ orbital and antiferromagnetic order at reasonable values of $U,J$ and thus to the possibility to realize cupratelike superconductivity in this 2D material upon doping. We also find that the application of tensile strain enhances the cupratelike phase and phases with orbital polarization in general, by reducing the $t_2/t_1$ ratio of next-nearest to nearest neighbor hopping. On the contrary, systems with compressive stress have an increased hopping ratio and consequently show a preference for ferromagnetic (FM) phases, including, unexpectedly, the out-of-plane $d_{3z^2-r^2}$ FM phase.

Topological pairing of composite fermions has led to remarkable ideas, such as excitations obeying non-Abelian braid statistics and topological quantum computation. We construct a $p$-wave paired Bardeen-Cooper-Schrieffer (BCS) wave function for composite fermions in the torus geometry, which is a convenient geometry for formulating momentum space pairing as well as for revealing the underlying composite-fermion Fermi sea. Following the standard BCS approach, we minimize the Coulomb interaction energy at half filling in the lowest and the second Landau levels, which correspond to filling factors $\nu =1/2$ and $\nu =5/2$ in GaAs quantum wells, by optimizing two variational parameters that are analogous to the gap and the Debye cutoff energy of the BCS theory. Our results show no evidence for pairing at $\nu =1/2$ but a clear evidence for pairing at $\nu =5/2$. To a good approximation, the highest overlap between the exact Coulomb ground state at $\nu =5/2$ and the BCS state is obtained for parameters that minimize the energy of the latter, thereby providing support for the physics of composite-fermion pairing as the mechanism for the $5/2$ fractional quantum Hall effect. We discuss the issue of modular covariance of the composite-fermion BCS wave function, and calculate its Hall viscosity and pair correlation function. By similar methods, we look for but do not find an instability to $s$-wave pairing for a spin-singlet composite-fermion Fermi sea at half-filled lowest Landau level in a system where the Zeeman splitting has been set to zero.

We propose a scheme to manipulate the Majorana qubit states other than braiding. By utilizing electron transport through Majorana zero modes and tuning the magnetic flux in the setup, the aim of Majorana qubit state manipulation can be fulfilled. The principal part in the setup is an Aharonov-Bohm (AB) interferometer consisting of two topological superconducting chains (TSCs) connected to two leads. We obtain the exact master equation as well as its solution and study the general properties of the real-time dynamics of the Majorana qubit states. The parity of the Majorana qubit state can be almost perfectly polarized by tuning the bias and can also be flipped by switching the sign of the cross couplings between the TSCs and the leads. Moreover, the qubit coherence shows a phase rigidity due to the intrinsic particle-hole symmetry of the Majorana AB interferometer, which is different from that in the double-quantum-dot AB interferometers without topological properties.

Helical edge states of two-dimensional topological insulators show a gap in the density of states (DOS) and suppressed conductance in the presence of ordered magnetic impurities. Here we will consider the dynamical effects on the DOS and transmission when the magnetic impurities are driven periodically. Using the Floquet formalism and Green's functions, the system properties are studied as a function of the driving frequency and the potential energy contribution of the impurities. We see that increasing the potential part closes the DOS gap for all driving regimes. The transmission gap is also closed, showing a pronounced asymmetry as a function of energy. These features indicate that the dynamical transport properties could yield valuable information about the magnetic impurities.

We identify time-reversal symmetry breaking mixed-parity superconducting states that feature eight Majorana corner modes in properly cleaved three-dimensional cubic crystals. Namely, when an odd-parity isotropic $p$-wave pairing coexists with cubic symmetry preserving even-parity octupolar $d_{x^2-y^2}+i{d}_{3z^2-r^2}$ pairing, the gapless surface Majorana modes of the former get localized at the eight corners, thus yielding an intrinsic third-order topological superconductor (TOTSC). A cousin $d_{xy}+i{d}_{3z^2-r^2}$ pairing also accommodating eight corner Majorana modes, by virtue of breaking the cubic symmetry, in contrast, yields an extrinsic TOTSC. We identify a doped octupolar (topological or trivial) Dirac insulator as a suitable platform to sustain such unconventional superconductors, realized from an intraunit cell pairing. Finally, we argue that the proposed TOTSC can be experimentally realizable in NaCl and other structurally similar compounds under high pressure.

Exceptional degeneracies, at which both eigenvalues and eigenvectors coalesce, and parity-time $\left(\mathcal{PT}\right)$ symmetry, reflecting balanced gain and loss in photonic systems, are paramount concepts in non-Hermitian systems. We here complete the topological classification of exceptional nodal degeneracies protected by $\mathcal{PT}$ symmetry in up to three dimensions and provide simple example models whose exceptional nodal topologies include previously overlooked possibilities such as second-order knotted surfaces of arbitrary genus, third-order knots, and fourth-order points.