We consider states localized by electrostatic potentials in phosphorene using an atomistic tight-binding approach. From the tight-binding spectra of states confined in a parabolic potential we extract effective masses for conduction band electrons moving along the armchair and zigzag crystal directions by a fit to the harmonic oscillator spectrum. The masses derived in this way are used for a simple single-band effective-mass model which, as we find, reproduces very well the tight-binding energy spectra in an external magnetic field, the confined probability densities, and the interaction effects. We study the confined states in the conduction band and find that both methods produce Wigner crystallization with separated single-electron islands already for small quantum dots. The effective-mass model works with slightly worse precision for confined states of the valence band. The continuum approach deviates considerably from the tight-binding model only in the limit of extremely strong lateral confinement.

The dosing of layered materials with alkali metals has become a commonly used strategy in ARPES experiments. However, precisely what occurs under such conditions, both structurally and electronically, has remained a matter of debate. Here we perform a systematic study of 1T-${\mathrm{HfTe}}_2$, a prototypical semimetal of the transition metal dichalcogenide family. By utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), we have investigated the electronic structure of this material as a function of potassium (K) deposition. From the $k_z$ maps, we observe the appearance of 2D dispersive bands after electron dosing, with an increasing sharpness of the bands, consistent with the wave-function confinement at the topmost layer. In our highest-dosing cases, a monolayerlike electronic structure emerges, presumably as a result of intercalation of the alkali metal. Here, by bringing the topmost valence band below $E_F$, we can directly measure a band overlap of $\sim 0.2$ eV. However, 3D bulklike states still contribute to the spectra even after considerable dosing. Our work provides a reference point for the increasingly popular studies of the alkali metal dosing of semimetals using ARPES.

We numerically investigate the electronic transport properties between two mesoscopic graphene disks with a twist by employing the density functional theory coupled with the nonequilibrium Green's function technique. By attaching two graphene leads to upper and lower graphene layers separately, we explore systematically the dependence of electronic transport on the twist angle, Fermi energy, system size, layer stacking order, and twist axis. When choosing different twist axes for either AA- or AB-stacked bilayer graphene, we find that the dependence of conductance on twist angle displays qualitatively distinction, i.e., the systems with “top” axis exhibit finite conductance oscillating as a function of the twist angle, while the ones with “hollow” axis exhibit nearly vanishing conductance for different twist angles near the charge neutrality point. These findings suggest that the choice of twist axis can effectively tune the interlayer conductance, making it a crucial factor in designing of nanodevices with the twisted van der Waals multilayers.

We investigate the transient behavior of the quantum transport in mesoscopic systems under a quantum quench within the Caroli scheme. Using the nonequilibrium Green's function approach, an exact solution of the transient electric current, energy current, and their fluctuations in the presence of both external bias and temperature gradient are presented that goes beyond the wide-band limit. The exact solution of the time-dependent Seebeck coefficient in the linear response regime is also obtained. This formalism is applied to study the transient behavior of a single-level quantum dot with Lorentzian linewidth induced by the temperature gradient. The damped oscillatory behavior is found in the transient electric and energy currents, as well as their fluctuations. The oscillation frequency of electric and energy currents increases with the increasing energy level of quantum dot and the decay rate of oscillation decreases as the bandwidth increases. A significantly enhanced Seebeck coefficient is generated in the transient regime. We find the maximum value of the time-dependent Seebeck coefficient can be enhanced by increasing the energy level of the quantum dot and the reference temperature of leads.

Within the ultimate goal of classifying universality in quantum many-body dynamics, understanding the relation between out-of-equilibrium and equilibrium criticality is a crucial objective. Models with power-law interactions exhibit rich well-understood critical behavior in equilibrium, but the out-of-equilibrium picture has remained incomplete, despite recent experimental progress. We construct the rich dynamical phase diagram of free-fermionic chains with power-law hopping and pairing and provide analytic and numerical evidence showing a direct connection between nonanalyticities of the return rate and zero crossings of the string order parameter. Our results may explain the experimental observation of so-called accidental dynamical vortices, which appear for quenches within the same topological phase of the Haldane model, as reported by Fläschner et al. [Nat. Phys. 14, 265 (2018)]. Our work is readily applicable to modern ultracold-atom experiments, not least because state-of-the-art quantum gas microscopes can now reliably measure the string order parameter, which, as we show, can serve as an indicator of dynamical criticality.

There is considerable recent interest in the phenomenon of anisotropic electroresistivity of correlated metals. While some interesting work has been done on the iron-based superconducting systems, not much is known for the cuprate materials. Here we study the anisotropy of elastoresistivity for cuprates in the normal state. We present theoretical results for the effect of strain on resistivity and additionally on the optical weight and local density of states. We use the recently developed extremely strongly correlated Fermi liquid theory in two dimensions, which accounts quantitatively for the unstrained resistivities for three families of single-layer cuprates. The strained hoppings of a tight-binding model are roughly modeled analogously to strained transition metals. The strained resistivity for a two-dimensional $t\text{−}{t}^{\prime }\text{−}J$ model are then obtained, using the equations developed in recent work. Our quantitative predictions for these quantities have the prospect of experimental tests in the near future, for strongly correlated materials such as the hole-doped and electron-doped high-$T_c$ materials.

We present a comprehensive experimental investigation of Yb magnetic dimers in ${\mathrm{Yb}}_{0.04}{\mathrm{Y}}_{0.96}{\mathrm{AlO}}_3$, an Yb-doped yttrium aluminum perovskite ${\mathrm{YAlO}}_3$, by means of specific heat, magnetization, and high-resolution inelastic neutron scattering (INS) measurements. In our sample, the Yb ions are randomly distributed over the lattice and $\sim 7\%$ of Yb ions form quantum dimers due to nearest-neighbor antiferromagnetic coupling along the $c$ axis. At zero field, the dimer formation manifests itself in an appearance of an inelastic peak at $\mathrm{\Delta }\approx 0.2\mathrm{meV}$ in the INS spectrum and a Schottky-like anomaly in the specific heat. The structure factor of the INS peak exhibits a cosine modulation along the $L$ direction, in agreement with the $c$-axis nearest-neighbor intradimer coupling. A careful fitting of the low-temperature specific heat shows that the excited state is a degenerate triplet, which indicates a surprisingly small anisotropy of the effective Yb-Yb exchange interaction despite the low crystal symmetry and anisotropic magnetic dipole contribution, in agreement with previous reports for the Yb parent compound, ${\mathrm{YbAlO}}_3$ [L. Wu et al., Nat. Commun. 10, 698 (2019), L. S. Wu et al., Phys. Rev. B 99, 195117 (2019)], and in contrast to ${\mathrm{Yb}}_2{\mathrm{Pt}}_2\mathrm{Pb}$ [L. S. Wu et al., Science 352, 1206 (2016), W. J. Gannon et al., Nat. Commun. 10, 1123 (2019)]. The obtained results are precisely reproduced by analytical calculations for the Yb dimers.

We theoretically study charge Kondo effects in a quadruple quantum dot. This system has been realized in a carbon nanotube [Nature (London) 535, 395 (2016)] by A. Hamo et al., while it can be also formed in a two-dimensional electron gas (2DEG). The system is in a particular situation where a quadruple dot has twofold degenerate ground states of $(n_{\text{A}}=1,n_{\text{B}}=1,n_{\text{C}}=1,n_{\text{D}}=0)$ and (0,0,0,1) charge configurations, where $n_{\lambda }$ is the electron occupation number of the individual dot $\lambda =\text{A,B,C,D}$ of the quadruple dot. The two charge states behave as the pseudospin-1/2 states of a Kondo impurity. In the spinless regime, where the real spin of electrons is frozen, for example, by an external magnetic field, the quadruple dot can exhibit a single-channel charge Kondo effect in which coherent charge fluctuations massively occur between the two charge states with the help of electron tunneling between the quadruple dot and its electron reservoirs. The origin of the charge Kondo effect is similar to that of a negative-$U$ Anderson impurity. In the spinful regime, on the other hand, the real spin and charge degrees of freedom couple each other due to interdot electron tunneling between the dots A and B so that the spin singlet is formed in the charge state (1,1,1,0). In this regime, the low-energy Hamiltonian of the quadruple dot system can be mapped onto a two-channel Kondo Hamiltonian having channel anisotropy. In realistic situations of carbon nanotubes or GaAs 2DEGs, the channel anisotropy is so large that the quadruple dot shows a single-channel charge Kondo effect also in the spinful regime at experimentally available temperatures. We compute the temperature dependence of electron transport through the quadruple dot, which is useful for identifying the charge Kondo effects.

In chiral magnets, localized topological magnetic whirls, magnetic skyrmions, can be moved by spin polarized electric currents. Upon increasing the current strength, with prospects for high-speed skyrmion motion for spintronics applications in mind, isolated skyrmions deform away from their typical circular shape. We analyze the influence of spin-transfer torques on the shape of a single skyrmion, including its stability upon adiabatically increasing the strength of the applied electric current. For rather compact skyrmions at uniaxial anisotropies well above the critical anisotropy for domain wall formation, we find for high current densities that the skyrmion assumes a noncircular shape with a tail, reminiscent of a shooting star. For larger and hence softer skyrmions close to the critical anisotropy, in turn, we observe a critical current density above which skyrmions become unstable. We show that above a second critical current density the shooting star solution can be recovered also for these skyrmions.

High tunability of two-dimensional magnetic materials (by strain, gating, heterostructuring, or otherwise) provides unique conditions for studying versatile magnetic properties and controlling emergent magnetic phases. Expanding the scope of achievable magnetic phenomena in such materials is important for both fundamental and technological advances. Here we perform atomistic spin-dynamics simulations to explore the (chiral) magnetic phases of atomic monolayers in the limit of suppressed first-neighbors exchange interaction. We report the rich phase diagram of exotic magnetic configurations, obtained for both square and honeycomb lattice symmetries, comprising coexistence of ferromagnetic and antiferromagnetic spin cycloids, as well as multiple types of magnetic skyrmions. We perform a minimum-energy path analysis for the skyrmion collapse to evaluate the stability of such topological objects and reveal that magnetic monolayers could be good candidates to host the antiferromagnetic skyrmions that are experimentally evasive to date.

We report the electronic and magnetic properties of stoichiometric ${\mathrm{CeAuBi}}_2$ single crystals. At ambient pressure, ${\mathrm{CeAuBi}}_2$ orders antiferromagnetically below a Néel temperature ($T_N$) of 19 K. Neutron diffraction experiments revealed an antiferromagnetic propagation vector $\widehat{\tau }=[0,0,1/2]$, which doubles the paramagnetic unit cell along the $c$ axis. At low temperatures several metamagnetic transitions are induced by the application of fields parallel to the $c$ axis, suggesting that the magnetic structure of ${\mathrm{CeAuBi}}_2$ changes as a function of field. At low temperatures, a linear positive magnetoresistance may indicate the presence of band crossings near the Fermi level. Finally, the application of external pressure favors the antiferromagnetic state, indicating that the $4f$ electrons become more localized.

We study phonon delocalization in disordered media in the presence of nonlinearity. By considering the Fermi-Pasta-Ulam $\beta$-model, we show that regardless of whether the initial state of the linear system is localized or not, the final state will be an extended mode after turning on the nonlinear term. We report on the results of an extensive dynamical simulation of a disordered nonlinear system, which show that, independent of the initial mode frequency, in the final state the energy spectrum is excited according to the Kolmogorov spectrum $E\left(\omega \right)\sim {\omega }^{-5/3}$. Finally, we show that disorder will not cause delocalization of intrinsic localized modes.

We study the relaxation dynamics of an extended Fermi-Hubbard chain with a strong Wannier-Stark potential tilt coupled to a bath. When the system is subjected to dephasing noise, starting from a pure initial state the system's total von Neumann entropy is found to grow monotonously. The scenario becomes rather different when the system is coupled to a thermal bath of finite temperature. Here, for sufficiently large field gradients and initial energies, the entropy peaks in time and almost reaches its largest possible value (corresponding to the maximally mixed state), long before the system relaxes to thermal equilibrium. This entropy peak signals an effective prethermal memory loss and, relative to the time where it occurs, the system is found to exhibit a simple scaling behavior in space and time. By comparing the system's dynamics to that of a simplified model, the underlying mechanism is found to be related to the localization property of the Wannier-Stark system, which favors dissipative coupling between eigenstates that are close in energy.

While the vast majority of known physical realizations of the Tomonaga-Luttinger liquid (TLL) have repulsive interactions defined with the dimensionless interaction parameter $K_{\mathrm{c}}<1$, we here report that ${\mathrm{Rb}}_2{\mathrm{Mo}}_3{\mathrm{As}}_3$ is in the opposite TLL regime of attractive interactions. This is concluded from a TLL-characteristic power-law temperature dependence of the ${}^{87}\mathrm{Rb}$ spin-lattice relaxation rates over a broad temperature range yielding the TLL interaction parameter for charge collective modes $K_{\mathrm{c}}=1.4$. The TLL of the one-dimensional band can be traced almost down to $T_{\mathrm{c}}=10.4$ K, where the bulk superconducting state is stabilized by the presence of a three-dimensional band and characterized by the ${}^{87}\mathrm{Rb}$ temperature-independent Knight shift and the absence of the Hebel-Slichter coherence peak in the relaxation rates. The small superconducting gap measured in high magnetic fields reflects either the importance of the vortex core relaxation or the uniqueness of the superconducting state stemming from the attractive interactions defining the precursor TLL.

Using density-functional theory (DFT) band-structure calculations, we study the crystal structure, lattice dynamics, and magnetic interactions in the Shastry-Sutherland magnet ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ under pressure, concentrating on experimentally relevant pressures up to 4 GPa. We first check that a ferromagnetic spin alignment shortens the nearest-neighbor Cu-Cu distance and reduces the Cu-O-Cu angle compared to the state with the antiferromagnetic spin alignment in the dimers, in qualitative agreement with the structural changes observed at ambient pressure as a function of temperature and applied field. Next, we determine the optimal crystal structures corresponding to the magnetic structures consistent with, respectively, the dimer phase realized at ambient pressure, the Néel ordered phase realized at high pressure, and two candidates for the intermediate phase with two types of dimers and different stackings. For each phase, we performed a systematic study as a function of pressure, and we determined the exchange interactions and the frequencies of several experimentally relevant phonon modes. In the dimer phase, the ratio of the inter- to intradimer couplings is found to increase with pressure, in qualitative agreement with various experiments. This increase is mostly due to the decrease of the intradimer coupling due to the reduction of the Cu-O-Cu angle under pressure. The phonon frequency of the pantograph mode is also found to increase with pressure, in qualitative agreement with recent Raman experiments. In the Néel phase, the frequency of the pantograph mode is larger than the extrapolated value from the dimer phase, again in agreement with the experimental results, and accordingly the intradimer coupling is smaller than the extrapolated value from the dimer phase. Finally, all tendencies inside the candidate intermediate phases are thoroughly worked out, including specific predictions for some Raman active phonon modes that could be used to pin down the nature of the intermediate phase.

Higgs oscillations in nonequilibrium superconductors provide a unique tool to obtain information about the underlying order parameter. Several properties like the absolute value, existence of multiple gaps, and the symmetry of the order parameter can be encoded in the Higgs oscillation spectrum. Studying Higgs oscillations with time-resolved angle-resolved photoemission spectroscopy (ARPES) has the advantage over optical measurements that a momentum-resolved analysis of the condensate dynamic is possible. In this paper, we investigate the time-resolved spectral function measured in ARPES for different quench protocols. We find that analyzing amplitude oscillations of the ARPES intensity in the whole Brillouin zone allows to understand how the condensate dynamic contributes to the emerging of collective Higgs oscillations. Furthermore, by evaluating the phase of these oscillations, the symmetry deformation dynamic of the condensate can be revealed, which gives insight about the ground state symmetry of the system. With such an analysis, time-resolved ARPES experiments might be used in the future as a powerful tool in the field of Higgs spectroscopy.

A few years ago we predicted theoretically that in systems with nesting of the Fermi surface the spin-valley half-metal has lower energy than the spin density wave state. In this paper we suggest a possible way to distinguish these phases experimentally. We calculate the dynamical spin susceptibility tensor for both states in the framework of the Kubo formalism. Discussed phases have different numbers of bands: four bands in the spin-valley half-metal and only two bands in the spin density wave. Therefore, their susceptibilities, as functions of frequency, have different numbers of peaks. Besides, the spin-valley half-metal does not have rotational symmetry, thus, in general the off-diagonal components of the susceptibility tensor are nonzero. The spin density wave obeys robust rotational symmetry and off-diagonal components of the susceptibility tensor are zero. These characteristic features can be observed in experiments with inelastic neutron scattering.

We discuss the treatment of spin-orbit coupling (SOC) in time-reversal-symmetry-broken periodic systems for relativistic electronic structure calculations of materials within the generalized noncollinear Kohn-Sham density functional theory (GKSDFT). We treat SOC self-consistently and express the GKS orbitals in a two-component spinor basis. Crucially, we present a methodology (and its corresponding implementation) for the simultaneous self-consistent treatment of SOC and exact nonlocal Fock exchange operators. The many advantages of the inclusion of nonlocal Fock exchange in the self-consistent treatment of SOC, as practically done in hybrid exchange-correlation functionals, are both formally derived and illustrated through numerical examples: (i) it imparts a local magnetic torque (i.e., the ability of the two-electron potential to locally rotate the magnetization with respect to a starting guess configuration) that is key to converge to the right solution in noncollinear DFT regardless of the initial guess for the magnetization; (ii) because of the local magnetic torque, it improves the rotational invariance of noncollinear formulations of the DFT; (iii) it introduces the dependence on specific pieces of the spinors (i.e., those mapped onto otherwise missing spin blocks of the complex density matrix) into the two-electron potential, which are key to the correct description of the orbital- and spin-current densities and their coupling with the magnetization; and (iv) when space-inversion symmetry is broken, it allows for the full breaking of time-reversal symmetry in momentum space, which would otherwise be constrained by a sum rule linking the electronic band structure at opposite points in the Brillouin zone ($\underline{\mathbf{k}}$ and $-\underline{\mathbf{k}}$). The presented methodology is implemented in a developmental version of the public crystal code. Numerical tests are performed on the model system of an infinite radical chain of ${\mathrm{Ge}}_2\mathrm{H}$ with both space-inversion and time-reversal symmetries broken, which allows to highlight all the above-mentioned effects.