Hybrid density functional calculations are performed for a variety of systems containing $d^9$ ions $(\mathrm{C}{\mathrm{u}}^{2+}$ and $\mathrm{A}{\mathrm{g}}^{2+})$ and exhibiting quasi-one-dimensional magnetic properties. In particular, we study fluorides containing these ions in a rarely encountered compressed octahedral coordination that forces the unpaired electron into the local $d\left(z^2\right)$ orbital. We predict that such systems should exhibit exchange anisotropies surpassing that of $\mathrm{S}{\mathrm{r}}_2\mathrm{Cu}{\mathrm{O}}_3$, one of the best realizations of a one-dimensional system known to date. In particular, we predict that the interchain coupling in the $\mathrm{A}{\mathrm{g}}^{2+}$-containing $\left[\mathrm{AgF}\right]\left[\mathrm{B}{\mathrm{F}}_4\right]$ system should be nearly four orders of magnitude smaller than the intrachain interaction. Our results indicate that quasi-one-dimensional spin-$\frac{1}{2}$ systems containing chains with spin sites in the $d{\left(z^2\right)}^1$ local ground state could constitute a versatile model for testing modern theories of quantum many-body physics in the solid state.

The interactions between acceptors in semiconductors are often treated in qualitatively the same manner as those between donors. Acceptor wave functions are taken to be approximately hydrogenic and the standard hydrogen molecule Heitler-London model is used to describe acceptor-acceptor interactions. But due to valence band degeneracy and spin-orbit coupling, acceptor states can be far more complex than those of hydrogen atoms, which brings into question the validity of this approximation. To address this issue, we develop an acceptor-acceptor Heitler-London model using single-acceptor wave functions of the form proposed by Baldereschi and Lipari, which more accurately capture the physics of the acceptor states. We calculate the resulting acceptor-pair energy levels and find, in contrast to the two-level singlet-triplet splitting of the hydrogen molecule, a rich ten-level energy spectrum. Our results, computed as a function of interacceptor distance and spin-orbit coupling strength, suggest that acceptor-acceptor interactions can be qualitatively different from donor-donor interactions, and should therefore be relevant to the control of two-qubit interactions in acceptor-based qubit implementations, as well as the magnetic properties of a variety of p-doped semiconductor systems. Further insight is drawn by fitting numerical results to closed-form energy-level expressions obtained via an acceptor-acceptor Hubbard model.

The coupling between a 2D semiconductor quantum well and an optical cavity gives rise to combined light-matter excitations, the exciton-polaritons. These were usually measured when the conduction band is empty, making the single polariton physics a simple single-body problem. The situation is dramatically different in the presence of a finite conduction-band population, where the creation or annihilation of a single exciton involves a many-body shakeup of the Fermi sea. Recent experiments in this regime revealed a strong modification of the exciton-polariton spectrum. Previous theoretical studies concerned with nonzero Fermi energy mostly relied on the approximation of an immobile valence-band hole with infinite mass, which is appropriate for low-mobility samples only; for high-mobility samples, one needs to consider a mobile hole with large but finite mass. To bridge this gap, we present an analytical diagrammatic approach and tackle a model with short-ranged (screened) electron-hole interaction, studying it in two complementary regimes. We find that the finite hole mass has opposite effects on the exciton-polariton spectra in the two regimes: in the first, where the Fermi energy is much smaller than the exciton binding energy, excitonic features are enhanced by the finite mass. In the second regime, where the Fermi energy is much larger than the exciton binding energy, finite mass effects cut off the excitonic features in the polariton spectra, in qualitative agreement with recent experiments.

We study the Haldane model under strain using a tight-binding approach, and compare the obtained results with the continuum-limit approximation. As in graphene, nonuniform strain leads to a time-reversal preserving pseudomagnetic field that induces (pseudo-)Landau levels. Unlike a real magnetic field, strain lifts the degeneracy of the zeroth pseudo-Landau levels at different valleys. Moreover, for the zigzag edge under uniaxial strain, strain removes the degeneracy within the pseudo-Landau levels by inducing a tilt in their energy dispersion. The latter arises from next-to-leading order corrections to the continuum-limit Hamiltonian, which are absent for a real magnetic field. We show that, for the lowest pseudo-Landau levels in the Haldane model, the dominant contribution to the tilt is different from graphene. In addition, although strain does not strongly modify the dispersion of the edge states, their interplay with the pseudo-Landau levels is different for the armchair and zigzag ribbons. Finally, we study the effect of strain in the band structure of the Haldane model at the critical point of the topological transition, thus shedding light on the interplay between nontrivial topology and strain in quantum anomalous Hall systems.

Edge states in narrow quasi-two-dimensional nanostructures determine, to a large extent, their electric, thermoelectric, and magnetic properties. Nonmagnetic edge states may quite often lead to topological-insulator-type behavior. However, another scenario develops when the zigzag edges are magnetic and the time reversal symmetry is broken. In this work we report on the electronic band structure modifications, electrical conductance, and thermoelectric properties of narrow zigzag nanoribbons with spontaneously magnetized edges. Theoretical studies based on the Kane-Mele-Hubbard tight-binding model show that for silicene, germanene, and stanene both the Seebeck coefficient and the thermoelectric power factor are strongly enhanced for energies close to the charge neutrality point. A perpendicular gate voltage lifts the spin degeneracy of energy bands in the ground state with antiparallel magnetized zigzag edges and makes the electrical conductance significantly spin polarized. Simultaneously the gate voltage worsens the thermoelectric performance. Estimated room-temperature figures of merit for the aforementioned nanoribbons can exceed a value of 3 if phonon thermal conductances are adequately reduced.

We study transport through a Weyl semimetal quantum dot sandwiched between an $s$-wave superconductor and a normal lead. The conductance peaks at regular intervals and exhibits double periodicity with respect to two characteristic frequencies of the system, one that originates from Klein tunneling in the system and the other coming from the chiral nature of the excitations. Using a scattering matrix approach as well as a lattice simulation, we demonstrate the universal features of the conductance through the system and discuss the feasibility of observing them in experiments.

Graphs and recently hypergraphs have been known as an important tool for considering different properties of quantum many-body systems. In this paper, we study a mapping between an important class of quantum systems, namely quantum Calderbank-Shor-Steane (CSS) codes, and Ising-like systems by using hypergraphs. We show that the Hamiltonian corresponding to a CSS code on a hypergraph $H$ which is perturbed by a uniform magnetic field is mapped to Hamiltonian of a Ising-like system on dual hypergraph $\tilde{H}$ in a transverse field. Interestingly, we show that a strong regime of couplings in one of the systems is mapped to a weak regime of couplings in another one. We also give some applications for such a mapping where we study robustness of different topological CSS codes against a uniform magnetic field including Kitaev's toric codes defined on graphs and color codes in different dimensions. We show that a perturbed Kitaev's toric code on an arbitrary graph is mapped to an Ising model in a transverse field on the same graph and a perturbed color code on a $D$ colex is mapped to a Ising-like model on a $D$-simplicial lattice in a transverse field. In particular, we use these results to explicitly compare the robustness of toric codes to uniform magnetic-field perturbations on different graphs. Interestingly, our results show that the robustness of such topological codes defined on graphs decreases with increasing dimension. Furthermore, we also use the duality mapping for some self-dual models where we exactly derive the point of phase transition.

The realization of topological superconductors (SCs) in one or two dimensions is a highly pursued goal. Prominent proposed realization schemes include semiconductor/superconductor heterostructures and set stringent constraints on the chemical potential of the system. However, the ability to keep the chemical potential in the required range while in the presence of an adjacent SC and its accompanied screening effects is a great experimental challenge. In this paper, we study a SC lattice structure in which the SC is deposited periodically on a one- or two-dimensional sample. We demonstrate that this realization platform overcomes the challenge of controlling the chemical potential in the presence of the superconductor's electrostatic screening. We show how Majorana bound states emerge at the ends of a one-dimensional system proximity coupled to a one-dimensional SC lattice and move on to present a SC-lattice-based realization of the two-dimensional $p_x+i{p}_y$ SC, hosting chiral Majorana modes at its edges. In particular, we establish that even when assuming the worst case of absolute screening, in which the chemical potential under the SC is completely unaffected by the external gate potential, the topological phase can be reached by tuning the chemical potential in the area not covered by the SC. Finally, we briefly discuss possible effects of Coulomb blockade on the properties of the system.

The topological semimetal $\beta \text{−}{\mathrm{Ag}}_2\mathrm{Se}$ features a Kramers-Weyl node at the origin in momentum space and a quadruplet of spinless Weyl nodes, which are annihilated by spin-orbit coupling. We show that single-crystalline $\beta \text{−}{\mathrm{Ag}}_2\mathrm{Se}$ manifests giant Shubnikov–de Haas oscillations in the longitudinal magnetoresistance, which stem from a small electron pocket that can be driven beyond the quantum limit by a field less than 9 T. This small electron pocket is a remainder of the spin-orbit annihilated Weyl nodes and thus encloses a Berry-phase structure. Moreover, we observed a negative longitudinal magnetoresistance when the magnetic field is beyond the quantum limit. Our experimental findings are complemented by thorough theoretical band-structure analyses of this Kramers-Weyl semimetal candidate, including first-principles calculations and an effective $\mathit{k}·\mathit{p}$ model.

A magnon Nernst effect, an antiferromagnetic analog of the magnon Hall effect in ferromagnetic insulators, has been studied experimentally for the layered antiferromagnetic insulator ${\mathrm{MnPS}}_3$ in contact with two Pt strips. Thermoelectric voltage in the Pt strips grown on ${\mathrm{MnPS}}_3$ single crystals exhibits nonmonotonic temperature dependence at low temperatures, which is unlikely to be explained by electronic origins in Pt but can be ascribed to the inverse spin Hall voltage induced by a magnon Nernst effect. Control of antiferromagnetic domains in the ${\mathrm{MnPS}}_3$ crystal by magnetoelectric cooling is found to modulate the low-temperature thermoelectric voltage in Pt, which is evidence consistent with the emergence of the magnon Nernst effect in Pt-${\mathrm{MnPS}}_3$ hybrid structures.

We investigate magnetic ordering and the quantum anomalous Hall effect (QAHE) in Cr-doped topological insulators using systematic first-principles calculations, explaining the mechanism responsible for ferromagnetic order and the reason why $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ is a better QAHE host than $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ or $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$. We conclude that these magnetic topological insulators have relatively long-range exchange interactions within quintuple layers and weak interactions between quintuple layers. Our analyses for the spin splitting of the topological surface states suggest that the temperature at which the QAHE occurs in these materials can be enhanced significantly by Mo-Cr co-doping.

We numerically demonstrate an ultrafast method to create single skyrmions in a collinear ferromagnetic sample by applying a picosecond (effective) magnetic field pulse in the presence of Dzyaloshinskii-Moriya interaction. For small samples the applied magnetic field pulse could be either spatially uniform or nonuniform while for large samples a nonuniform and localized field is more effective. We examine the phase diagram of pulse width and amplitude for the nucleation. Our finding could ultimately be used to design future skyrmion-based devices.

We present a Rapid Communication on the impact of $\mathrm{M}{\mathrm{n}}^{3+}$ substitution in the geometrically frustrated Ising garnet $\mathrm{H}{\mathrm{o}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$ using bulk magnetic measurements and low-temperature powder neutron diffraction. We find that the transition temperature $T_{\mathrm{N}}=5.8\mathrm{K}$ for $\mathrm{H}{\mathrm{o}}_3\mathrm{MnG}{\mathrm{a}}_4{\mathrm{O}}_{12}$ is raised by a factor of almost 20 when compared to $\mathrm{H}{\mathrm{o}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$. Powder neutron diffraction on $\mathrm{H}{\mathrm{o}}_3\mathrm{M}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{5-x}{\mathrm{O}}_{12}$ ($x=0.5,1$) below $T_{\mathrm{N}}$ shows the formation of a long-range-ordered state with $\mathbf{k}=(0,0,0)$. $\mathrm{H}{\mathrm{o}}^{3+}$ spins are aligned antiferromagnetically along the six crystallographic axes with no resultant moment, whereas the $\mathrm{M}{\mathrm{n}}^{3+}$ spins are oriented along the body diagonals such that there is a net moment along [111]. The magnetic structure can be visualized as ten-membered rings of corner-sharing triangles of $\mathrm{H}{\mathrm{o}}^{3+}$ spins with the $\mathrm{M}{\mathrm{n}}^{3+}$ spins ferromagnetically coupled to each individual $\mathrm{H}{\mathrm{o}}^{3+}$ spin in the triangle. Substitution of $\mathrm{M}{\mathrm{n}}^{3+}$ completely relieves the magnetic frustration with $f={\theta }_{\mathrm{CW}}/T_{\mathrm{N}}\sim 1.1$ for $\mathrm{H}{\mathrm{o}}_3\mathrm{MnG}{\mathrm{a}}_4{\mathrm{O}}_{12}$.

We consider massless Dirac fermions in a graphene monolayer in the ballistic limit, subject to both a perpendicular magnetic field $B$ and a proximity-induced pairing gap $\mathrm{\Delta }$. When the chemical potential is at the Dirac point, our exact solution of the Bogoliubov–de Gennes equation yields $\mathrm{\Delta }$-independent relativistic Landau levels. Since eigenstates depend on $\mathrm{\Delta }$, many observables nevertheless are sensitive to pairing, e.g., the local density of states or the edge state spectrum. By solving the problem with an additional in-plane electric field, we also discuss how snake states are influenced by a pairing gap.

The critical properties of the single-crystalline van der Waals bonded ferromagnet ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ were investigated by bulk dc magnetization around the paramagnetic to ferromagnetic (FM) phase transition. The ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ single crystals grown by self-flux method with Fe deficiency $x\approx 0.36$ exhibit bulk FM ordering below $T_c=152$ K. The Mössbauer spectroscopy was used to provide information on defects and local atomic environment in such crystals. Critical exponents $\beta =0.372\left(4\right)$ with a critical temperature $T_c=151.25\left(5\right)$ K and $\gamma =1.265\left(15\right)$ with $T_c=151.17\left(12\right)$ K are obtained by the Kouvel-Fisher method, whereas $\delta =4.50\left(1\right)$ is obtained by a critical isotherm analysis at $T_c=151$ K. These critical exponents obey the Widom scaling relation $\delta =1+\gamma /\beta$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M\left(H\right)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m=f_{\pm }\left(h\right)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The exponents determined in this study are close to those calculated from the results of the renormalization group approach for a heuristic model of three-dimensional Heisenberg ($d=3,n=3$) spins coupled with the attractive long-range interactions between spins that decay as $J\left(r\right)\approx r^{-(3+\sigma )}$ with $\sigma =1.89$.

Ferromagnetic Heusler alloys with low magnetic damping are highly promising materials for magnonic devices, which rely on the excitation and detection of spin waves. Using all-electrical spin-wave spectroscopy we report spin-wave propagation in sputtered ${\mathrm{Co}}_2\left({\mathrm{Mn}}_{0.6}{\mathrm{Fe}}_{0.4}\right)\text{Si}$ Heusler alloy thin films with a thickness of 50 nm. We integrated a nanostructured microwave antenna to locally excite and detect propagating spin waves in a Damon-Eshbach configuration. We estimate the group velocity to be up to 12.0 km/s and we observe spin-wave propagation with a frequency band as broad as 15 GHz. From the experimental frequency dependence of group velocity we calculate the spin-wave dispersion. Our results show that all-electrical measurements are a powerful method for determining the fundamental spin-wave characteristics of Heusler alloys, over a broad and tunable range of frequencies, and with group velocities an order of magnitude higher than in conventional materials.

The thermoelectric transport coefficients are calculated in a generic lattice model of multi-Weyl semimetals with a broken time-reversal symmetry by using the Kubo's linear response theory. The contributions connected with the Berry curvature-induced electromagnetic orbital and heat magnetizations are systematically taken into account. It is shown that the thermoelectric transport is profoundly affected by the nontrivial topology of multi-Weyl semimetals. In particular, the calculation reveals a number of thermal coefficients of the topological origin which describe the anomalous Nernst and thermal Hall effects in the absence of background magnetic fields. Similarly to the anomalous Hall effect, all anomalous thermoelectric coefficients are proportional to the integer topological charge of the Weyl nodes. The dependence of the thermoelectric coefficients on the chemical potential and temperature is also studied.

We investigate the interplay between confinement and the fermion doubling problem in Dirac-like Hamiltonians. Individually, both features are well known. First, simple electrostatic gates do not confine electrons due to the Klein tunneling. Second, a typical lattice discretization of the first-order derivative $k\to -i{\partial }_x$ skips the central point and allow spurious low-energy, highly oscillating solutions known as fermion doublers. While a no-go theorem states that the doublers cannot be eliminated without artificially breaking a symmetry, here we show that the symmetry broken by the Wilson's mass approach is equivalent to the enforcement of hard-wall boundary conditions, thus making the no-go theorem irrelevant when confinement is foreseen. We illustrate our arguments by calculating the following: (i) the band structure and transport properties across thin films of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, for which we use ab initio density functional theory calculations to justify the model; and (ii) the band structure of zigzag graphene nanoribbons.