Which is the best single-particle basis to express a Hubbard-like lattice model? A rigorous variational answer to this question leads to equations the solution of which depends in a self-consistent manner on the lattice ground state. Contrary to naive expectations, for arbitrary small interactions, the optimized orbitals differ from the noninteracting ones, leading also to substantial changes in the model parameters as shown analytically and in an explicit numerical solution for a simple double-well one-dimensional case. At strong coupling, we obtain the direct exchange interaction with a very large renormalization with important consequences for the explanation of ferromagnetism with model Hamiltonians. Moreover, in the case of two atoms and two fermions we show that the optimization equations are closely related to reduced density-matrix functional theory, thus establishing an unsuspected correspondence between continuum and lattice approaches.

We introduce an equilibrium formulation of the functional renormalization group (fRG) for inhomogeneous systems capable of dealing with spatially finite-ranged interactions. In the general third-order truncated form of fRG, the dependence of the two-particle vertex is described by $\mathcal{O}\left(N^4\right)$ independent variables, where $N$ is the dimension of the single-particle system. In a previous paper [Bauer et al., Phys. Rev. B 89, 045128 (2014)], the so-called coupled-ladder approximation (CLA) was introduced and shown to admit a consistent treatment for models with a purely onsite interaction, reducing the vertex to $\mathcal{O}\left(N^2\right)$ independent variables. In this work, we introduce an extended version of this scheme, called the extended coupled ladder approximation (eCLA), which includes a spatially extended feedback between the individual channels, measured by a feedback length $L$, using $\mathcal{O}\left(N^2{L}^2\right)$ independent variables for the vertex. We apply the eCLA in a static approximation and at zero temperature to three types of one-dimensional model systems, focusing on obtaining the linear response conductance. First, we study a model of a quantum point contact (QPC) with a parabolic barrier top and on-site interactions. In our setup, where the characteristic length $l_x$ of the QPC ranges between approximately 4–10 sites, eCLA achieves convergence once $L$ becomes comparable to $l_x$. It also turns out that the additional feedback stabilizes the fRG flow. This enables us, second, to study the geometric crossover between a QPC and a quantum dot, again for a one-dimensional model with on-site interactions. Third, the enlarged feedback also enables the treatment of a finite-ranged interaction extending over up to $L$ sites. Using a simple estimate for the form of such a finite-ranged interaction in a QPC with a parabolic barrier top, we study its effects on the conductance and the density. We find that for low densities and sufficiently large interaction ranges the conductance develops additional features, and the corresponding density shows some fluctuations that can be interpreted as Friedel oscillations arising from a renormalized barrier shape with a rather flat top and steep flanks.

We report on electrical spin injection from (Ga,Mn)As into a high-mobility two-dimensional electron gas confined at an (Al,Ga)As/GaAs interface. Besides standard nonlocal electrical detection, we use a magneto-optical approach which provides cross-sectional images of the spin accumulation at the cleaved edge of the sample, yielding spin decay lengths on the order of $2\mu \mathrm{m}$. In some cases we find a nonmonotonic bias voltage dependence of the spin signal, which may be linked to ballistic tunneling effects during spin injection. We observe a clear Hanle depolarization using a technique which is free of dynamic nuclear polarization effects. Fitting the data with the standard drift-diffusion model of spin injection suggests averaged in-plane spin lifetimes on the order of 1 ns.

Clean topological insulators exposed to a magnetic field develop Landau levels accompanied by a nonzero Hall conductivity for the infinite slab geometry. In this work we consider the case of disordered topological insulator nanowires and find, in contrast, that a zero Hall plateau emerges within a broad energy window close to the Dirac point. We numerically calculate the conductance and its distribution for a statistical ensemble of disordered nanowires, and use the conductance fluctuations to study the dependence of the insulating phase on system parameters, such as the nanowire length, disorder strength, and the magnetic field.

We discuss in detail algorithms for implementing tensor network renormalization (TNR) for the study of classical statistical and quantum many-body systems. First, we recall established techniques for how the partition function of a $2D$ classical many-body system or the Euclidean path integral of a $1D$ quantum system can be represented as a network of tensors, before describing how TNR can be implemented to efficiently contract the network via a sequence of coarse-graining transformations. The efficacy of the TNR approach is then benchmarked for the $2D$ classical statistical and $1D$ quantum Ising models; in particular the ability of TNR to maintain a high level of accuracy over sustained coarse-graining transformations, even at a critical point, is demonstrated.

We introduce an effective theory with manifest particle-vortex symmetry for disordered thin films undergoing a magnetic field-tuned superconductor-insulator transition. The theory may enable one to access both the critical properties of the strong-disorder limit, which has recently been confirmed by Breznay et al. [Proc. Natl. Acad. Sci. USA 113, 280 (2016)] to exhibit particle-vortex symmetric electrical response, and the nearby metallic phase discovered earlier by Mason and Kapitulnik [Phys. Rev. Lett. 82, 5341 (1999)] in less disordered samples. Within the effective theory, the Cooper-pair and field-induced vortex degrees of freedom are simultaneously incorporated into an electrically neutral Dirac fermion minimally coupled to a (emergent) Chern-Simons gauge field. A derivation of the theory follows upon mapping the superconductor-insulator transition to the integer quantum Hall plateau transition and the subsequent use of Son's particle-hole symmetric composite Fermi liquid. Remarkably, particle-vortex symmetric response does not require the introduction of disorder; rather, it results when the Dirac fermions exhibit vanishing Hall effect. The theory predicts approximately equal (diagonal) thermopower and Nernst signal with a deviation parameterized by the measured electrical Hall response at the symmetric point.

We investigate the gap in the single-electron spectrum of twisted bilayer graphene. In a perfect infinite lattice of a twisted bilayer, the gap varies exponentially in response to weak changes of the twist angle. Such a large sensitivity makes theoretical predictions of the gap nearly impossible, since experimentally the twist angle is always known with finite accuracy. To address this issue, we numerically study finite clusters of twisted bilayer graphene. For finite systems, changing the twist angle causes a gradual crossover between gapless and gapped regimes. The crossover occurs when the finite-size quantization energy becomes comparable to the matrix elements responsible for the generation of the gap. We further argue that disorder scattering can induce similar crossover, in which the mean-free path plays the same role as the system size for the finite clusters. It is demonstrated that to observe the gap experimentally, it is necessary to have a sample of suitable purity and to possess the ability to tune the twist angle accurately.

The doping of semiconductors with magnetic impurities gives rise not only to a spin-spin interaction between quasifree carriers and magnetic impurities but also to a local spin-independent disorder potential for the carriers. Based on a quantum kinetic theory for the carrier and impurity density matrices as well as the magnetic and nonmagnetic carrier-impurity correlations, the influence of the nonmagnetic scattering potential on the spin dynamics in DMS after optical excitation with circularly polarized light is investigated using the example of Mn-doped CdTe. It is shown that non-Markovian effects, which are predicted in calculations where only the magnetic carrier-impurity interaction is accounted for, can be strongly suppressed in the presence of nonmagnetic impurity scattering. This effect can be traced back to a significant redistribution of carriers in $\mathbf{k}\text{-space}$ which is enabled by the build-up of large carrier-impurity correlation energies. A comparison with the Markov limit of the quantum kinetic theory shows that, in the presence of an external magnetic field parallel to the initial carrier polarization, the asymptotic value of the spin polarization at long times is significantly different in the quantum kinetic and the Markovian calculations. This effect can also be attributed to the formation of strong correlations, which invalidates the semiclassical Markovian picture and it is stronger when the nonmagnetic carrier-impurity interaction is accounted for. In an external magnetic field perpendicular to the initial carrier spin, the correlations are also responsible for a renormalization of the carrier spin precession frequency. Considering only the magnetic carrier-impurity interaction, a significant renormalization is predicted for a very limited set of material parameters and excitation conditions. Accounting also for the nonmagnetic interaction, a relevant renormalization of the precession frequency is found to be more ubiquitous.

Photoinduced magnetic polarons in EuTe, with a magnetic moment of several hundred Bohr magnetons, were investigated as a function of pump intensity and temperature by pump-probe Faraday rotation. The quantum efficiency for optical generation of magnetic polarons is found to be 0.09. The pump-intensity dependence of the photoinduced Faraday rotation shows a sublinear increase, from which we deduce that the population of photoexcited polarons is limited by a maximum value of $4.5\times {10}^{15}{\mathrm{cm}}^{-3}$. This is four orders of magnitude less than the concentration of polarons that would completely fill the crystal, which suggests that the photoexcited polarons are anchored by defects. In addition to the generation of polarons, at high pump densities the modulated pump light also causes a small alternating heating of the illuminated region. The temperature dependence of the polaron magnetic moment is well described by the Curie–Weiss law. Above 100 K, polarons are thermally quenched with an activation energy of 11 meV.

We present experimental results on the light-to-sound transduction by a superlattice embedded in an optical microcavity, using picosecond ultrasonics. Experiments on the ability to generate and detect phonons at 0.3 and 0.6 THz were performed at low temperature. Finally, we demonstrate that such a device can detect the acoustic burst that it has generated and which has propagated through the substrate, been reflected on the backside of the substrate, and has returned to the device. We study the efficiency of this detection by varying independently the laser wavelength of the pump and probe, by means of asynchronous optical sampling.

Using novel instrumentation to combine extreme conditions of intense pulsed magnetic field up to 60 T and high pressure up to 4 GPa, we have established the three-dimensional (3D) magnetic field-pressure-temperature phase diagram of a pure stoichiometric heavy-fermion antiferromagnet (${\mathrm{CeRh}}_2{\mathrm{Si}}_2$). We find a temperature- and pressure-dependent decoupling of the critical and pseudometamagnetic fields at the borderlines of antiferromagnetism and strongly-correlated paramagnetism. This 3D phase diagram is representative of a class of heavy-fermion Ising antiferromagnets, where long-range magnetic ordering is decoupled from a maximum in the magnetic susceptibility. The combination of extreme conditions enabled us to characterize different quantum phase transitions, where peculiar quantum critical properties are revealed. The interest to couple the effects of magnetic field and pressure on quantum-critical correlated-electron systems is stressed.

Using a simple numerical model of skyrmions in a two-dimensional system interacting with a quasi-one-dimensional periodic substrate under combined dc and ac drives where the dc drive is applied perpendicular to the substrate periodicity, we show that a rich variety of novel phase-locking dynamics can occur due to the influence of the Magnus term on the skyrmion dynamics. Instead of Shapiro steps, the velocity response in the direction of the dc drive exhibits a series of spikes, including extended dc drive intervals over which the skyrmions move in the direction opposite to the dc drive, producing negative mobility. There are also specific dc drive values at which the skyrmions move exactly perpendicular to the dc drive direction, giving a condition of absolute transverse mobility.

By using the spin pumping process, we show that it is possible to transport a pure spin current across layers of conducting polyaniline (PANI) with several hundred nanometers sandwiched between a film of the ferrimagnetic insulator yttrium iron garnet (YIG) and a thin layer of platinum. The spin current generated by microwave-driven ferromagnetic resonance of the YIG film, injected through the YIG/PANI interface, crosses the whole PANI layer and then is injected into the Pt layer. By means of the inverse spin Hall effect in the Pt, the spin current is converted into charge current and electrically detected as a dc voltage. We measured a spin diffusion length in PANI of 590 ± 40 nm, which is very large compared with normal metals, demonstrating that PANI can be used as an efficient spin current conductor and poor charge current conductor, opening the path towards spintronics applications based in this very attractive material.

In pulsed magnetic fields up to 65 T and at temperatures below the Néel transition, our magnetization and magnetostriction measurements reveal a field-induced metamagneticlike transition that is suggestive of an antiferromagnetic to ferrimagnetic ordering. Our data also suggest a change in the nature of this metamagneticlike transition from second- to first-order-like near a tricritical point at $T_{\mathrm{tc}}\sim 145\mathrm{K}$ and $H_{\mathrm{c}}\sim 52\mathrm{T}$. At high fields for $H>H_{\mathrm{c}}$ we found a decreased magnetic moment roughly half of the moment determined by neutron powder diffraction. We propose that the decreased moment and lack of saturation at high fields indicate the presence of a field-induced ferrimagnetic state above the tricritical point of the H-T phase diagram for $\mathrm{US}{\mathrm{b}}_2$.

We examine the ultrafast demagnetization process of iron-based materials, namely, ${\mathrm{Fe}}_6$ clusters and bulk bcc Fe, with time-dependent spin-density functional theory (TDSDFT). The magnetization continuity equation is reformulated and the torque due to the spin-current divergence is written in terms of an effective time-dependent kinetic magnetic field, an object already introduced in the literature. Its time evolution, as extracted from the TDSDFT simulations, is identified as one of the main sources of the local out-of-equilibrium spin dynamics and it plays a major role in the demagnetization process in combination with the spin orbit interaction. Such demagnetization is particularly strong in hot spots where the kinetic torque is maximized. Finally, we find the rate of demagnetization in ${\mathrm{Fe}}_6$ to be strongly dependent on the direction of polarization of the exciting electric field and this can be linked to the out-of-equilibrium distribution of the kinetic field in two comparative cases.

We have performed first-order reversal curve (FORC) measurements to investigate the irreversible magnetization processes in the low-temperature phase of MnBi. Using temperature-dependent FORC analysis, we are able to provide a clear insight into the effects of microstructural parameters such as grain diameter, shape, and surface composition on the coercivity of nucleation hardened permanent magnet MnBi. FORC diagrams of MnBi show a unique broadening and narrowing of the coercive field distribution with increasing temperature. We were able to microscopically identify the reason for this behavior, based on the shift in the single domain critical diameter from nearly 1 to 2 $\mu \text{m}$, thereby changing the dependence of coercivity with particle size. This is based on a strong increase in the uniaxial anisotropy constant with increasing temperature. Furthermore, the results also give an additional confirmation that the magnetic hardening in low-temperature phase MnBi occurs due to nucleation mechanisms. In our case, we show that temperature-dependent FORC measurements provide a powerful tool for the microscopic understanding of high-performance permanent magnet systems.

Recently, Tao and Mo developed a semilocal exchange-correlation density functional. The exchange part of this functional is derived from a density-matrix expansion corrected to reproduce the fourth-order gradient expansion of the exchange energy in the slowly-varying-density limit, while the correlation part is based on the Tao-Perdew-Staroverov-Scuseria (TPSS) correlation functional, with a modification for the low-density limit. In the present paper, the Tao-Mo (TM) functional is assessed by computing various properties of solids and jellium surfaces. This includes 22 lattice constants and bulk moduli, 30 band gaps, seven cohesive energies, and jellium surface exchange and correlation energies for the density parameter $r_s$ in the range from 2 to 3 bohr. Our calculations show that the TM approximation can yield consistently high accuracy for most properties considered here, with mean absolute errors (MAEs) of 0.025 Å for lattice constants, 7.0 GPa for bulk moduli, 0.08 eV/atom for cohesive energies, and $35\mathrm{erg}/\mathrm{c}{\mathrm{m}}^2$ for surface exchange-correlation energies. The MAE in band gaps is larger than that of TPSS, but slightly smaller than the errors of the local spin-density approximation, Perdew-Burke-Ernzerhof generalized gradient approximation, and revised TPSS. However, band gaps are still underestimated, particularly for large-gap semiconductors, compared to the Heyd-Scuseria-Ernzerhof nonlocal screened hybrid functional.