We propose an adaptive scheme of broadening the discrete spectral data from numerical renormalization group (NRG) calculations to improve the resolution of dynamical properties at finite energies. While the conventional scheme overbroadens narrow features at large frequency by broadening discrete weights with constant width in log-frequency, our scheme broadens each discrete contribution individually based on its sensitivity to a $z$-shift in the logarithmic discretization intervals. We demonstrate that the adaptive broadening better resolves various features in noninteracting and interacting models at comparable computational cost. The resolution enhancement is more significant for coarser discretization as typically required in multiband calculations. At low frequency below the energy scale of temperature, the discrete NRG data necessarily needs to be broadened on a linear scale. Here we provide a method that minimizes transition artifacts in between these broadening kernels.

We study interaction effects on thin films of topological mirror Kondo insulators (TMKIs), where the strong interaction is expected to play an important role. Our study has led to the following results: (i) We identify a rich phase diagram of noninteracting TMKIs with different mirror Chern numbers in the monolayer and bilayer thin films; (ii) we obtain the phase diagram with interaction and identify the regimes of interaction parameters to mimic bosonic symmetry-protected topological phases with either gapless bosonic modes or spontaneous mirror symmetry breaking at the boundary; and (iii) for the spontaneous mirror symmetry-breaking boundary, we also study various domain-wall defects between different mirror symmetry-breaking order parameters at the boundary. Our results reveal that the thin-film TMKI serves as an intriguing platform for experimental studies of interacting topological phases.

We have studied extensively the band-crossing patterns of the bulk entanglement spectrum (BES) for various lattice Chern insulators. We find that only partitions with dual symmetry can have either stable nodal lines or nodal points in the BES when the system is in the topological phase of a nonzero Chern number. By deforming the Hamiltonian to lift the accidental symmetry, one can see that only nodal points are robust. They thus should bear certain topological characteristics of the BES. By studying the band-crossing patterns in detail, we conclude that the topological characteristics of the BES are inherited from the topological order of the underlying Chern insulators, and the former can have more refined topological structures. We then conjecture that the sum of the vorticities in the BES in a properly chosen reduced Brillouin zone equals the Chern number of the underlying Chern insulator. This relation is beyond the usual classification scheme of topological insulators/superconductors.

Regarding three-dimensional (3D) topological insulators and semimetals as a stack of constituent two-dimensional (2D) topological (or sometimes nontopological) systems is a useful viewpoint. Here, we perform a comparative study of the paradigmatic 3D topological phases: Weyl semimetal (WSM), strong and weak topological insulators (STI/WTI), and Chern insulator (CI). By calculating the $\mathbb{Z}$ and ${\mathbb{Z}}_2$ indices for the thin films of such 3D topological phases, we follow dimensional evolution of topological properties from 2D to 3D. It is shown that the counterparts of STI and WTI in the time-reversal symmetry broken CI system are, respectively, WSM and CI phases. The number ${\mathcal{N}}_D$ of helical Dirac cones emergent on the surface of a topological insulator is shown to be identical to the number ${\mathcal{N}}_W$ of the pairs of Weyl cones in the corresponding WSM phase: ${\mathcal{N}}_D={\mathcal{N}}_W$. To test the robustness of this scenario against disorder, we have studied the transport property of disordered WSM thin films, taking into account both the bulk and surface contributions.

We present numerical results within the one-dimensional disordered Hubbard model for several characteristic indicators of the many-body localization (MBL). Considering traditionally studied charge disorder (i.e., the same disorder strength for both spin orientations) we find that even at strong disorder all signatures consistently show that while charge degree of freedom is nonergodic, the spin is delocalized and ergodic. This indicates the absence of the full MBL in the model that has been simulated in recent cold-atom experiments. Full localization can be restored if spin-dependent disorder is used instead.

We propose a method to manipulate the state of a single electron spin in a semiconductor quantum dot (QD). The manipulation is achieved by tunnel coupling a QD, labeled $L$, and occupied with an electron to an adjacent QD, labeled $R$, which is not occupied by an electron but having an energy linearly varying in time. We identify a parameter regime in which a complete population transfer between the spin eigenstates $|L\uparrow \rangle$ and $|L\downarrow \rangle$ is achieved without occupying the adjacent QD. This method is convenient due to the fact that manipulation can be done electrically, without precise knowledge of the spin resonance condition, and is robust against Zeeman level broadening caused by nuclear spins.

Transport measurements are performed on InAs/GaSb double quantum wells at zero and finite magnetic fields applied parallel and perpendicular to the quantum wells. We investigate a sample in the inverted regime where electrons and holes coexist, and compare it with another sample in the noninverted semiconducting regime. The activated behavior in conjunction with a strong suppression of the resistance peak at the charge neutrality point in a parallel magnetic field attest to the topological hybridization gap between electron and hole bands in the inverted sample. We observe an unconventional Landau level spectrum with energy gaps modulated by the magnetic field applied perpendicular to the quantum wells. This is caused by a strong spin-orbit interaction provided jointly by the InAs and the GaSb quantum wells.

A combined nonequilibrium Green functions–Ehrenfest dynamics approach is developed that allows for a time-dependent study of the energy loss of a charged particle penetrating a strongly correlated system at zero and finite temperatures. Numerical results are presented for finite inhomogeneous two-dimensional Fermi-Hubbard models, where the many-electron dynamics in the target are treated fully quantum mechanically and the motion of the projectile is treated classically. The simulations are based on the solution of the two-time Dyson (Keldysh-Kadanoff-Baym) equations using the second-order Born, third-order, and $\mathrm{T}$-matrix approximations of the self-energy. As application, we consider protons and helium nuclei with a kinetic energy between 1 and 500 keV/u passing through planar fragments of the two-dimensional honeycomb lattice and, in particular, examine the influence of electron-electron correlations on the energy exchange between projectile and electron system. We investigate the time dependence of the projectile's kinetic energy (stopping power), the electron density, the double occupancy, and the photoemission spectrum. Finally, we show that, for a suitable choice of the Hubbard model parameters, the results for the stopping power are in fair agreement with ab initio simulations for particle irradiation of single-layer graphene.

Charged excitons called trions play an important role in the fundamental valley dynamics in newly emerging two-dimensional semiconductor materials. We use ultrafast pump-probe spectroscopy to study the valley trion dynamics in a ${\mathrm{MoSe}}_2$ monolayer grown by using chemical vapor deposition. The dynamics displays an ultrafast trion formation followed by a nonexponential decay. The measurements at different pump fluences show that the trion decay dynamics becomes slower as the excitation density increases. The observed trion dynamics and the associated density dependence are a result of the trapping by two defect states as being the dominating decay mechanism. The simulation based on a set of rate equations reproduces the experimental data for different pump fluences. Our results reveal the important trion dynamics and identify the trapping by defect states as the primary trion decay mechanism in monolayer ${\mathrm{MoSe}}_2$ under the excitation densities used in our experiment.

The performance of a giant magnetoresistance (GMR) heterostructure Fe/Ag/Fe/InAs/Ag(100) in the current-perpendicular-to-plane geometry is presented. Calculations are based on a realistic tight-banding model with full $spd$ bands and the recursive Green's function algorithm. Results show that the system's GMR can reach values above 1000%. This GMR enhancement mainly is a result of the wave-vector filtering effect imposed by the InAs layer, restricting conductance within a small region around the $\mathrm{\Gamma }$ point in the 2D Brillouin zone. Calculations also reveal that when the Fermi level sits in the InAs band gap, MR gradually saturates as a function of InAs thickness with a smooth plateau; whereas when the Fermi level is in the InAs conduction band and close to the band bottom, GMR exhibits an oscillatory behavior with a large period. GMR oscillations are also observed with respect to Ag thickness, with oscillation amplitude determined by the Fermi level position relative to the InAs conduction band edge. The oscillation periods in both cases can be well explained by the concept of quantum-well states, and are determined by the spanning vector of the Fermi surface belly of the material whose thickness is varied. The observed GMR oscillations are due to the quantum interference of conduction electrons near the $\mathrm{\Gamma }$ point. The GMR and area-resistance (RA) product profiles at a wide range of Fermi energy positions relative to InAs bands are also compared. Near the GMR peak (with GMR above 1000%) in the conduction band, RA product can be as low as 8.$8\mathrm{\Omega }\mu {\text{m}}^2$. This feature of large GMR but small RA product results from the wave-vector filtering effect of doped InAs, and it makes the structure under study distinct from conventional GMR systems (small GMR, small RA) or magnetic tunnel junctions (large GMR, large RA).

The dielectric properties of the prototypical frustrated ferromagnetic spin-chain compound ${\mathrm{PbCuSO}}_4\left(\mathrm{OH}\right){}_2$ known as linarite are studied across its strongly anisotropic magnetic phase diagram in single-crystal samples. The ferroelectric character of the principal low-field spin spiral phase is confirmed. The measured polarization is fully consistent with the previously proposed magnetic structure. Spontaneous polarization is also detected in two other field-induced phases but in some cases is incompatible with previously suggested models for the spin arrangement.

By means of first-principles calculations, we have systematically investigated how the magnetodynamic properties Gilbert damping, magnetization, and exchange stiffness are affected when permalloy (Py) (${\mathrm{Fe}}_{0.19}{\mathrm{Ni}}_{0.81}$) is doped with $4d$ or $5d$ transition metal impurities. We find that the trends in the Gilbert damping can be understood from relatively few basic parameters such as the density of states at the Fermi level, the spin-orbit coupling, and the impurity concentration. The temperature dependence of the Gilbert damping is found to be very weak which we relate to the lack of intraband transitions in alloys. Doping with $4d$ elements has no major impact on the studied Gilbert damping, apart from diluting the host. However, the $5d$ elements have a profound effect on the damping and allow it to be tuned over a large interval while maintaining the magnetization and exchange stiffness. As regards the spin stiffness, doping with early transition metals results in considerable softening, whereas late transition metals have a minor impact. Our result agree well with earlier calculations where available. In comparison to experiments, the computed Gilbert damping appears slightly underestimated, whereas the spin stiffness shows a general good agreement.

We study superconducting instability from orbital nematic fluctuations in a minimal model consisting of the $d_{xz}$ and $d_{yz}$ orbitals, and choose model parameters which capture the typical Fermi surface geometry observed in iron-based superconductors. We solve the Eliashberg equations down to low temperatures with keeping the renormalization function and a full momentum dependence of the pairing gap. When superconductivity occurs in the tetragonal phase, we find that the pairing gap exhibits a weak momentum dependence over the Fermi surfaces. The superconducting instability occurs also inside the nematic phase. When the $d_{xz}$ orbital is occupied more than the $d_{yz}$ orbital in the nematic phase, a larger (smaller) gap is realized on the Fermi-surface parts where the $d_{xz}$ $\left(d_{yz}\right)$ orbital component is dominant, leading to a substantial momentum dependence of the pairing gap on the hole Fermi surfaces. On the other hand, the momentum dependence of the gap is weak on the electron Fermi surfaces. We also find that while the leading instability is the so-called $s_{++}$-wave symmetry, the second leading one is $d_{x^2-y^2}$-wave symmetry. In particular, these two states are nearly degenerate in the tetragonal phase, whereas such quasidegeneracy is lifted in the nematic phase and the $d_{x^2-y^2}$-wave symmetry changes to highly anisotropic $s$-wave symmetry.

Mobile impurities in cold atomic gases constitute a new platform for investigating polaron physics. Here we show that when impurity atoms interact with a two-dimensional Fermi gas with quadratic band touching, the polaron picture may either hold or break down depending on the particle-hole asymmetry of the band structure. If the hole band has a smaller effective mass than the particle band, the quasiparticle is stable and its diffusion coefficient varies with temperature as $D\left(T\right)\propto {ln}^{2}T$. If the hole band has larger mass, the quasiparticle weight vanishes at low energies due to an emergent orthogonality catastrophe. In this case we map the problem onto a set of one-dimensional channels and use conformal field theory techniques to obtain $D\left(T\right)\propto T^{\nu }$ with an interaction-dependent exponent $\nu$. The different regimes can be detected in the nonequilibrium expansion dynamics of an initially confined impurity.

We consider anisotropic long-range interacting spin systems in $d$ dimensions. The interaction between the spins decays with the distance as a power law with different exponents in different directions: We consider an exponent $d_1+{\sigma }_1$ in $d_1$ directions and another exponent $d_2+{\sigma }_2$ in the remaining $d_2\equiv d-d_1$ ones. We introduce a low energy effective action with nonanalytic power of the momenta. As a function of the two exponents ${\sigma }_1$ and ${\sigma }_2$ we show the system to have three different regimes at criticality, two where it is actually anisotropic and one where the isotropy is finally restored. We determine the phase diagram and provide estimates of the critical exponents as a function of the parameters of the system, in particular considering the case where one of the two $\sigma$'s is fixed and the other varying. A discussion of the physical relevance of our results is also presented.

The pseudogap (PG) derived from the analysis of the excess conductivity ${\sigma }^{\prime }\left(T\right)$ in superlattices and double-layer films of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }{\text{-PrBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ (YBCO-PrBCO), prepared by pulsed laser deposition, is studied for the first time. The ${\sigma }^{\prime }\left(T\right)$ analysis has been performed within the local-pair (LP) model based on the assumption of the paired fermion (LPs) formation in the cuprate high-$T_c$ superconductors (cuprates) below the representative temperature $T^{*}\gg T_c$ resulting in the PG opening. Within the model, the temperature dependencies of the PG, ${\mathrm{\Delta }}^{*}\left(T\right)$, for the samples with different number of the PrBCO layers ($N_{\mathrm{Pr}}$) were analyzed in the whole temperature range from $T^{*}$ down to $T_c$. Near $T_c,{\sigma }^{\prime }\left(T\right)$ was found to be perfectly described by the Aslamazov–Larkin (AL) and Hikami–Larkin (HL) [Maki–Thompson (MT) term] fluctuation theories, suggesting the presence of superconducting fluctuations in a relatively large (up to 15 K) temperature range above $T_c$. All sample parameters were found to change with increase of $N_{\mathrm{Pr}}$, finally resulting in the appearance of the pronounced maximum of ${\mathrm{\Delta }}^{*}\left(T\right)$ at high temperatures. The result is most likely due to increasing influence of the intrinsic magnetism of PrBCO (${\mu }_{\mathrm{Pr}}\approx 4{\mu }_B$) and suggests the possibility to search in that way the change of interplay between the superconductivity and magnetism in cuprates.

We present low temperature resistivity and magnetotransport measurements conducted on pristine and electron doped ${\mathrm{SmNiO}}_3$ (SNO). The low temperature transport in both pristine and electron-doped SNO shows a Mott variable range hopping with a substantial decrease in localization length of carriers by one order in the case of doped samples. Undoped SNO films show a negative magnetoresistance (MR) at all temperatures characterized by spin fluctuations with the evolution of a positive cusp at low temperatures. In striking contrast, upon electron doping of the films via hydrogenation, we observe a crossover to a linear nonsaturating positive $\mathrm{MR}\sim 0.2\%$ at 50 K. The results signify the role of localization phenomena in tuning the magnetotransport response in doped nickelates. Ionic doping is therefore a promising approach to tune magnetotransport in correlated perovskites.