One of the hallmarks of the Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional superconductors is the universal jump of the superfluid density that can be indirectly probed via the nonlinear exponent of the current-voltage $I\text{−}V$ characteristics. Here, we compare the experimental measurements of $I\text{−}V$ characteristics in two cases, namely NbN thin films and ${\mathrm{SrTiO}}_3$-based interfaces. While the former display a paradigmatic example of BKT-like nonlinear effects, the latter do not seem to justify a BKT analysis. Rather, the observed $I\text{−}V$ characteristics can be well reproduced theoretically by modeling the effect of mesoscopic inhomogeneity of the superconducting state. Our results offer an alternative perspective on the spontaneous fragmentation of the superconducting background in confined two-dimensional systems.

We introduce a mechanism for light-induced Floquet engineering of the Fermi surface to dynamically tip the balance between competing instabilities in correlated condensed matter systems in the vicinity of a Van Hove singularity. We first calculate how the Fermi surface is deformed by an off-resonant, high-frequency light field and then determine the impact of this deformation on the ordering tendencies using an unbiased functional renormalization group approach. As a testbed, we investigate Floquet engineering in cuprates driven by light. We find that the $d$-wave superconducting ordering tendency in this system can be strongly enhanced over the Mott insulating one. This gives rise to extended regions of induced $d$-wave superconductivity in the effective phase diagram in the presence of a light field.

We performed ${}^{13}\mathrm{C}$ NMR measurements on the single-component molecular material $\mathrm{Pd}{\left(\mathrm{tmdt}\right)}_2$ to reveal the nature of the electronic phase. For the conducting state above 200 K, the analyses using the paramagnetic shift and the nuclear spin-lattice relaxation rate find large values of the Korringa ratio indicative of highly enhanced antiferromagnetic fluctuations. A clear NMR line broadening, indicative of an antiferromagnetic order, occurs at approximately 80 K with a traceable broadening extending up to 140 K, which possibly signifies a minor fraction of a higher-temperature magnetic phase in conjunction with a change in the NMR relaxation curve below 140 K. Comparing the observed spectral profile at 5 K and simulations, the magnetic moment is estimated to be $\sim 0.18{\mu }_{\mathrm{B}}/\mathrm{tmdt}$. The present results show that $\mathrm{Pd}{\left(\mathrm{tmdt}\right)}_2$ is a Mott insulator situated near the Mott transition and hosts strong exchange interactions that give an antiferromagnetic order at an extremely high transition temperature among molecular materials.

We study the role of electronic spin and valley symmetry in the quantum interference (QI) patterns of the transmission function in graphene quantum junctions. In particular, we link it to the position of the destructive QI antiresonances. When the spin or valley symmetry is preserved, electrons with opposite spin or valley display the same interference pattern. On the other hand, when a symmetry is lifted, the antiresonances are split, with a consequent dramatic differentiation of the transport properties in the respective channel. We demonstrate rigorously this link in terms of the analytical structure of the electronic Green function, which follows from the symmetries of the microscopic model, and we confirm the result with numerical calculations for graphene nanoflakes. We argue that this is a generic and robust feature that can be exploited in different ways for the realization of nanoelectronic QI devices, generalizing the recent proposal of a QI-assisted spin-filtering effect [A. Valli et al., Nano Lett. 18, 2158 (2018)].

We derive equations of motion for Green's functions of the multiorbital Anderson impurity model by differentiating symmetrically with respect to all time arguments. The resulting equations relate the one- and two-particle Green's function to correlators of up to six particles at four times. As an application we consider continuous-time quantum Monte Carlo simulations in the hybridization expansion, which hitherto suffered from notoriously high noise levels at large Matsubara frequencies. Employing the derived symmetric improved estimators overcomes this problem.

Quantum interference between optical absorption processes can excite carriers with a polarized distribution in the Brillouin zone depending on properties of the incident optical fields. The polarized distribution of carriers introduces a current that can be controlled by the phases and polarizations of the incident optical fields. Here we study the quantum interference of two- and three-photon absorption processes in AlGaAs. We present theoretical predictions for carrier and current injection rates considering different frequencies, phases, and polarizations of the incident fields. We also discuss the important features that result from only nonlinear optical processes being involved, which leads, for instance, to a sharper distribution of carriers in the Brillouin zone.

Using transition-metal dichalcogenides as an example, we show that the quantum interference arising in two- and three-photon absorption processes can lead to controllable, highly localized carrier distributions in the Brillouin zone. We contrast this with the previously studied one- and two-photon absorption, and find qualitatively different features, including changes in the relevance of interband and intraband processes according to the excitation energy. Thus, the distribution of excitations arising under certain circumstances in two- and three-photon absorption can facilitate the study of far-from-equilibrium states that are initially well localized in crystal momentum space.

We propose a picture of high-harmonic generation (HHG) in solids based on the concept of temporally changing band structures. To demonstrate the utility of this picture, we focus on the high-order sideband generation (HSG) caused by strong terahertz (THz) and weak near-infrared (NIR) light in the context of pump-probe spectroscopy. We find that the NIR frequency dependence of the HSG indicates the existence of new energy levels (subbands) around the band-gap energy, which have multiple frequencies of THz light. This subband picture explains why the HSG intensity becomes a nonmonotonic function of the THz light amplitude. The present analysis not only reveals the origin of the plateau structure in HHG spectra, but also provides a connection to other high-field phenomena.

We theoretically study the quantum spin Hall states in an Ammann-Beenker-type octagonal quasicrystal and a periodic snub-square crystal, both sharing the same basic building blocks. Although the bulk states show significant differences in localization and transport properties, the topological phases manifest similarly in the two systems. This indicates the robustness of the topological properties regardless of symmetry and periodicity. We characterize the topological nature of the two systems with a nonzero topological invariant (spin Bott index $B_s$ and ${\mathbb{Z}}_2$ invariant), robust metallic edge states, and quantized conductance. In spite of some quantitative differences, the topological phase diagram of the two systems also exhibits similar behaviors, indicating that the topological phase transition is mainly determined by similar interactions in the two systems regardless of their structural difference. This is also reflected by the observation that the transition point between the normal insulator and the quantum spin Hall state in both systems follows a universal linear scaling relation for topological phase transitions.

Ferroelectric-based photovoltaics have shown great promise as a source of renewable energy, thanks to their in-built charge separation capability, yet their efficiency is often limited by low charge-carrier mobilities. In this paper, we compare the photovoltaic prospects of various phases of the multiferroic material ${\mathrm{BiFeO}}_3$ by evaluating their charge-carrier effective masses from first-principles simulations. We identify a tetragonal phase with the promising combination of a large spontaneous polarization and relatively light charge carriers. From a systematic investigation of the octahedral distortions present in ${\mathrm{BiFeO}}_3$, we clarify the relationship between structure and effective masses. This relationship is explained in terms of changes to the orbital character and overlap at the band edges that result from changes in the geometry. Our findings suggest some design principles for how to tune effective masses in ${\mathrm{BiFeO}}_3$ and similar materials through the manipulation of their crystal structures in experimentally accessible ways.

We investigate the nonlocal transport properties in normal/superconducting/normal (NSN) junctions based on time-reversal symmetric type-$\mathrm{I}$ and type-$\mathrm{II}$ Weyl semimetals (WSMs). In the type-$\mathrm{I}$ WSM-based NSN junction, we show that a crossed-Andreev-reflection-dominated nonlocal transport can be realized within wide ranges of related parameters, which is favorable for the Cooper-pair splitting. For the type-$\mathrm{II}$ WSM-based NSN junction, we focus on the manifestations of the anisotropic aspects of type-II WSMs in the nonlocal transport. Owing to the hyperboloidlike Fermi surfaces of type-II WSMs, the scattering configurations and nonlocal differential conductance are strongly anisotropic, differing from the scenarios in similar NSN junctions based on other Dirac materials. These results enrich the understanding of the correlated transport in time-reversal symmetric WSM-based superconducting heterostructures.

We theoretically investigate the electronic and optical properties of nanostructured ${\mathrm{MoS}}_2$ bilayers, consisting of vertically stacked monolayer (ML) of pristine ${\mathrm{MoS}}_2$ and ML of periodic triangular nanoplatelets or holes. Our calculations reveal that the interplay between edge atoms in periodic nanostructures, size of nanostructures, and distances between them, control the appearance and size of the band gaps, and optical response, including the number and intensity of peaks; the nanostructured bilayers with periodic nanoplatelets in particular exhibit rich optical properties. Specifically, we find the electronic states, originating from the atoms of the ML with either periodic nanoplatelets or holes, get embedded between electronic states of pristine ${\mathrm{MoS}}_2$ ML, but now with red- and blueshifted valence and conduction bands, respectively. For a ML with periodic nanoplatelets, electronic states originate primarily from $p$ orbitals of S atoms, and for a ML with periodic holes, they originate from $d$ orbitals of Mo atoms. The electronic structure is accessed through the self-consistent tight-binding (SCTB) method, which is the extension of our tested parametrized TB model that includes nonorthogonal ${sp}^3{d}^5$ orbitals and spin-orbit coupling. We systematically examine the parameters in our SCTB to avoid possible distortion of electronic states, ensuring high quality of our predictions.

We propose an interband tunneling picture to explain and predict the interlayer twist-angle dependence of the induced spin-orbit coupling in heterostructures of graphene and monolayer transition metal dichalcogenides (TMDCs). We obtain a compact analytic formula for the induced valley Zeeman and Rashba spin-orbit coupling in terms of the TMDC band structure parameters and interlayer tunneling matrix elements. We parametrize the tunneling matrix elements with few parameters, which in our formalism are independent of the twist angle between the layers. We estimate the value of the tunneling parameters from existing density functional theory calculations at zero twist angle and we use them to predict the induced spin-orbit coupling at nonzero angles. Provided that the energy of the Dirac point of graphene is close to the TMDC conduction band, we expect a sharp increase of the induced spin-orbit coupling around a twist angle of ${18}^{\circ }$.

The origin of the phonon band gap and its controlling factors in low-symmetry crystals are studied in the monochalcogenide family of semiconductors through a combination of first-principles calculations and the introduction of quantitative measures of bond distortion in phonon modes. The phonon spectra were calculated for the recently discovered low-symmetry 64-atom cubic $\pi$-phase of the monochalcogenides in four materials: SnS, SnSe, GeS, and GeSe. The phonon spectra of all these systems exhibit phonon band gaps in the optical phonon spectrum with widths of 0.7–2 THz. Raman measurements were performed that support the existence and magnitude of the calculated phonon band gaps in $\pi$-SnS and $\pi$-SnSe. The origin of the phonon band gaps was examined through the contributions of the bond strengths and of the mass differences to the phonon spectrum in several monochalcogenide phases. An analysis of the normal modes found more rigid bond motion below the phonon gap and less rigid bond motion above the gaps as well as differences in the relative motion of the two types of atoms. These differences are identified as originating from the acoustic and optical modes of the symmetric parent rocksalt structure. The effect of the phonon band gap on the thermodynamic properties is discussed.

We consider periodic adiabatic processes of gapped many-body spinless electrons. We find an additional contribution to the orbital magnetization due to the adiabatic time evolution, dubbed geometric orbital magnetization, which can be expressed as derivative of the many-body Berry phase with respect to an external magnetic field. For two-dimensional band insulators, we show that the geometric orbital magnetization generally consists of two pieces, the topological piece that is expressed as third Chern-Simons form in $(t,k_x,k_y)$ space, and the nontopological piece that depends on Bloch states and energies of both occupied and unoccupied bands.

We present a study of the upper critical field $H_{c2}$ of pristine and proton-irradiated ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ crystals in pulsed magnetic fields of up to 65 T. The data for $H_{c2}$ reveal pronounced downwards curvature, particularly for the in-plane field orientation, and a superconducting anisotropy that decreases with decreasing temperature. These features are indicative of Pauli paramagnetic limiting. For the interpretation of these data, we use a model of a clean single-band superconductor with an open Fermi surface in the shape of a warped cylinder, which takes into account Zeeman spin splitting. Fits to the data reveal that the in-plane upper critical field is Pauli paramagnetic limited, while the out-of-plane upper critical field is orbitally limited and that the orbital and paramagnetic fields have opposite anisotropies. A consequence of this particular combination is the unusual inversion of the anisotropy $H_{c2}^{ab}<H_{c2}^c$ of the irradiated sample at temperatures below 10 K. The fits also yield an in-plane Maki parameter ${\alpha }_M^{110}\approx 2.6$ exceeding the critical value for the formation of the Fulde-Ferrell-Larkin-Ovchinnikov state. Nevertheless, the current measurements did not reveal direct evidence for the occurrence of this state.

We consider a quantum particle subject to Ohmic dissipation, moving in a bichromatic quasiperiodic potential. In a periodic potential the particle undergoes a zero-temperature localization-delocalization transition as dissipation strength is decreased. We show that the delocalized phase is absent in the quasiperiodic case, even when the deviation from periodicity is infinitesimal. Using the renormalization group, we determine how the effective localization length depends on the dissipation. We show that a similar problem can emerge in the strong-coupling limit of a mobile impurity moving in a periodic lattice and immersed in a one-dimensional quantum gas.

Two recent experiments [M. Somayazulu et al. Phys. Rev. Lett. 122, 027001 (2019) and A. P. Drozdov et al., Nature (London) 569, 528 (2019)] reported the discovery of superconductivity in the fcc phase of ${\mathrm{LaH}}_{10}$ at a critical temperature $T_{\mathrm{c}}$ between $250\text{and}260\mathrm{K}$ under a pressure of about 170 GPa. However, the dependence of $T_{\mathrm{c}}$ on pressure showed different patterns, i.e., the former experiment observed a continuous increase of $T_{\mathrm{c}}$ up to $\sim 275\mathrm{K}$ on further increase of pressure to 202 GPa, while the latter one observed an abrupt decrease of $T_{\mathrm{c}}$ with increasing pressure. Here, based on first-principles calculations, we reveal that for the $\mathrm{fcc}-{\mathrm{LaH}}_{10}$ phase, softening of the low-frequency optical phonon modes of H atoms dramatically occurs as pressure decreases, giving rise to a significant increase of the electron-phonon coupling (EPC) constant. Meanwhile, the electronic band structure near the Fermi energy is insensitive to change with respect to pressure. These results indicate that the pressure-dependent phonon softening is unlikely associated with Fermi-surface nesting, but driven by effective screening with the electronic states near the Fermi energy. It is thus demonstrated that the strong variation of EPC with respect to pressure plays a dominant role in the decrease of $T_{\mathrm{c}}$ with increasing pressure, supporting the measurements of Drozdov et al.

We train a set of restricted Boltzmann machines (RBMs) on one- and two-dimensional Ising spin configurations at various values of temperature, generated using Monte Carlo simulations. We validate the training procedure by monitoring several estimators, including measurements of the log likelihood, with the corresponding partition functions estimated using annealed importance sampling. The effects of various choices of hyperparameters on training the RBM are discussed in detail, with a generic prescription provided. Finally, we present a closed-form expression for extracting the values of couplings, for every $n$-point interaction between the visible nodes of an RBM, in a binary system such as the Ising model. We aim at using this study as the foundation for further investigations of less well-known systems.