Abstract: The recent discovery of axion states in materials such as antiferromagnetic topological insulators has boosted investigations of the magnetoelectric response in topological insulators and their promise towards realizing dissipationless topological electronics. In this paper, we develop a tight-binding methodology to explore the emergence of axion states in Bi2Se3 in proximity to magnetic insulators on the top and bottom surfaces. The topological protection of the surface states is lifted by a time-reversal-breaking perturbation due to the proximity of a magnetic insulator, and a gap is opened on the surfaces, giving rise to half-quantized Hall conductance and a zero Hall plateau-evidencing an axion insulator state. We developed a real-space tight-binding Hamiltonian for Bi2Se3 using first-principles data. Transport properties of the system were obtained within the Landauer-Buttiker formalism, and we discuss the creation of axion states through Hall conductance and a zero Hall plateau at the surfaces, as a function of proximitized magnetization and corresponding potentials at the surfaces, as well as the thickness of the topological insulator.

Abstract: Two-dimensional (2D) honeycomb ferromagnets, such as monolayer chromium trihalides, are predicted to behave as topological magnon insulators, characterized by an insulating bulk and topologically protected edge states, giving rise to a thermal magnon Hall effect. Here we report the behavior of the topological magnons in monolayer CrI3 under external stimuli, including biaxial and uniaxial strain, electric gating, as well as in-plane and out-of-plane magnetic field, revealing that one can thereby tailor the magnetic states as well as the size and the topology of the magnonic bandgap. These findings broaden the perspective of using 2D magnetic materials to design topological magnonic devices.

Abstract: We investigated the electron-phonon ( e -ph ) coupling and vibrational thermal conductivity in the representative MAB compounds, namely MoAlB, WAlB, Tc 2 AlB 2 , and Cr 2 AlB 2 . The spectral distribution functions of e -ph interaction, obtained through ab initio linear-response calculations, reveal that the electron-phonon coupling values range from low (0.15) to moderate (0.58). With such e -ph coupling, out of the considered compounds, only Tc 2 AlB 2 exhibits a superconducting transition, at 4 K. We further evaluated the thermal conductivity and associated properties like scattering rates, obtained using ab initio and other methodologies. The latter included the iterative solution of the Peierls-Boltzmann transport equation, using HIPHIVE package for advanced optimization and machine learning techniques, and employing maximum likelihood estimation to approximate scattering rates from a limited set of scattering processes. We found that these methods yield nearly identical predictions for thermal conductivity values, with a significant decrease in the computational cost compared to the first-principles methods. We examined interactions arising from both three-phonon (3 ph ) and four -phonon (4 ph ) scattering processes. The 4 ph interactions demonstrated a smaller yet significant impact on the overall vibrational thermal conductivity, most notably in Tc 2 AlB 2 . Our findings indicate that Cr 2 AlB 2 has the highest thermal conductivity across all considered crystal directions, with the thermal conductivity being spatially anisotropic, most pronouncedly in Tc 2 AlB 2 . Finally, we show that empirical expressions based on Slack models are well suited for screening the thermal conductivity properties of MAB phases, and can be employed to establish upper and lower limits of their thermal conductivity.

Abstract: Having a strong electromagnetic absorption, MXene multilayers are readily envisaged for applications in electromagnetic shields and related prospective technology. However, an ab initio characterization of the optical properties of MXenes is still lacking, due in part to major difficulties with the treatment of metallicity in the first-principles approaches. Here we addressed the latter challenge, after a careful treatment of intraband transitions, to present a thorough analysis of the electronic and optical properties of a selected set of metallic MXene layers based on density functional theory (DFT) and many-body perturbation theory calculations. Our results reveal that the GW corrections are particularly important in regions of the band structure where d and p states hybridize. For some systems, we show that GW corrections open a gap between occupied states, resulting in a band structure that closely resembles that of an intrinsic transparent conductor, thereby opening an additional line of prospective applications for the MXenes family. Nevertheless, GW and Bethe-Salpeter corrections have a minimal influence on the absorption spectra, in contrast to what is typically observed in semiconductor layers. Our present results suggest that calculations within the independent particle approximation (IPA) calculations are sufficiently accurate for assessing the optical characteristics of bulk-layered MXene materials. Finally, our calculated dielectric properties and absorption spectra, in agreement with existing experimental data, confirm the potential of MXenes as effective infrared emitters.

Abstract: We explore the effect of thickness, magnetization direction, strain, and gating on the topological quantum phase transition of a thin-film magnetic topological insulator. Reducing the film thickness to the ultrathin regime couples the edge states on the two surfaces, opening a gap known as the hybridization gap, and causing a phase transition from a topological insulator to a normal insulator (NI). An out-of-plane/in-plane magnetization of size proportional to the hybridization gap triggers a phase transition from a normal insulator state to a quantum anomalous Hall (QAH)/semimetal state. A magnetization tilt by angle 0 from the out-of-plane axis influences the topological phase transition in a way that for sufficiently large 0, no phase transition from NI to QAH can be observed regardless of the sample thickness or magnetization, and for 0 close to pi /2 the system transits to a semimetal phase. Furthermore, we demonstrate that compressive/tensile strain can be used to decrease/increase the magnetization threshold for the topological phase transition. Finally, we reveal the effect of a vertical potential acting on the film, be it due to the substrate or applied gating, which breaks inversion symmetry and raises the magnetization threshold for the transition from NI to QAH state.

Abstract: Because of strong flux confinement in mesoscopic superconductors, a giant vortex may appear in the ground state of the system in an applied magnetic field. This multiquanta vortex can then split into individual vortices (and vice versa) as a function of, e.g., applied current, magnetic field, or temperature. Here we show that such transitions can be identified by calorimetry, as the formation or splitting of a giant vortex results in a clear jump in measured heat capacity versus external drive. We attribute this phenomenon to an abrupt change in the density of states of the quasiparticle excitations in the vortex core(s), and further link it to a sharp change of the magnetic susceptibility at the transitionproving that the formation of a giant vortex can also be detected by magnetometry.

Abstract: We show in the case of a superconducting Nb ladder that a mesoscopic superconductor typically exhibits magnetoresistance oscillations whose amplitude and temperature dependence are different from those stemming from the Little-Parks effect. We demonstrate that these large resistance oscillations (as well as the monotonic background on which they are superimposed) are due to current-excited moving vortices, where the applied current in competition with the oscillating Meissner currents imposes or removes the barriers for vortex motion in an increasing magnetic field. Because of the ever present current in transport measurements, this effect should be considered in parallel with the Little-Parks effect in low-critical temperature (T-c) samples, as well as with recently proposed thermal activation of dissipative vortex-antivortex pairs in high-T-c samples.

Abstract: We describe investigations of the largely unexplored field of mesoscopic type-I superconductors. Micromagnetometry and 3D Ginzburg-Landau simulations of our single crystal β-tin samples in this regime reveal size- and temperature-dependent supercritical fields whose behavior is radically different from the bulk critical field HcB. We find that complete suppression of the intermediate state in medium-size samples can result in a surprising reduction of the critical field significantly below HcB. We also reveal an evolution of the superconducting-to-normal phase transition from the expected irreversible first order at low temperatures through the previously unobserved reversible first-order to a second-order transition close to Tc, where the critical field can be many times larger than HcB. Finally, we have identified striking correlations between the mesoscopic Hc3 for nucleation of surface superconductivity and the thermodynamic Hc near Tc. All these observations are entirely unexpected in the conventional type-I picture.

Abstract: http://dx.doi.org/doi:10.1103/PhysRevLett.96.207001

Abstract: Vortex matter in mesoscopic superconductors is known to be strongly affected by the geometry of the sample. Here we show that in nanoscale superconductors with coherence length comparable to the Fermi wavelength the shape resonances of the order parameter results in an additional contribution to the quantum topological confinement-leading to unconventional vortex configurations. Our Bogoliubov-de Gennes calculations in a square geometry reveal a plethora of asymmetric, giant multivortex, and vortex-antivortex structures, stable over a wide range of parameters and which are very different from those predicted by the Ginzburg-Landau theory. These unconventional states are relevant for high-T-c nanograins, confined Bose-Einstein condensates, and graphene flakes with proximity-induced superconductivity.

Abstract: We report experimental coupling of chiral magnetism and superconductivity in [IrFeCoPt]/Nb heterostructures. The stray field of skyrmions with radius ~50nm is sufficient to nucleate antivortices in a 25nm Nb film, with unique signatures in the magnetization, critical current and flux dynamics, corroborated via simulations. We also detect a thermally-tunable Rashba-Edelstein exchange coupling in the isolated skyrmion phase. This realization of a strongly interacting skyrmion-(anti)vortex system opens a path towards controllable topological hybrid materials, unattainable to date.

Abstract: A supersolid, a counterintuitive quantum state in which a rigid lattice of particles flows without resistance, has to date not been unambiguously realized. Here we reveal a supersolid ground state of excitons in a double-layer semiconductor heterostructure over a wide range of layer separations outside the focus of recent experiments. This supersolid conforms to the original Chester supersolid with one exciton per supersolid site, as distinct from the alternative version reported in cold-atom systems of a periodic density modulation or clustering of the superfluid. We provide the phase diagram augmented by the supersolid. This new phase appears at layer separations much smaller than the predicted exciton normal solid, and it persists up to a solid-solid transition where the quantum phase coherence collapses. The ranges of layer separations and exciton densities in our phase diagram are well within reach of the current experimental capabilities.

Abstract: The autoregressive neural networks are emerging as a powerful computational tool to solve relevant problems in classical and quantum mechanics. One of their appealing functionalities is that, after they have learned a probability distribution from a dataset, they allow exact and efficient sampling of typical system configurations. Here we employ a neural autoregressive distribution estimator (NADE) to boost Markov chain Monte Carlo (MCMC) simulations of a paradigmatic classical model of spin-glass theory, namely, the two-dimensional Edwards-Anderson Hamiltonian. We show that a NADE can be trained to accurately mimic the Boltzmann distribution using unsupervised learning from system configurations generated using standard MCMC algorithms. The trained NADE is then employed as smart proposal distribution for the Metropolis-Hastings algorithm. This allows us to perform efficient MCMC simulations, which provide unbiased results even if the expectation value corresponding to the probability distribution learned by the NADE is not exact. Notably, we implement a sequential tempering procedure, whereby a NADE trained at a higher temperature is iteratively employed as proposal distribution in a MCMC simulation run at a slightly lower temperature. This allows one to efficiently simulate the spin-glass model even in the low-temperature regime, avoiding the divergent correlation times that plague MCMC simulations driven by local-update algorithms. Furthermore, we show that the NADE-driven simulations quickly sample ground-state configurations, paving the way to their future utilization to tackle binary optimization problems.