We introduce a microscopic model to describe the ultrafast response of magnetic materials with two sublattices to heating by a femtosecond laser pulse. Even though the model is based on a set of simple Hamiltonians, it readily reproduces experimental observations such as ultrafast reversal of ferrimagnets and delayed demagnetization of one sublattice in ferromagnetic alloys. The calculations give insight into the microscopic mechanisms and thermodynamics governing the complex dynamics of these multisublattice magnets.

We assess the potential of the ferrimagnetic spinel ferrites CoFe${}_2$O${}_4$ and NiFe${}_2$O${}_4$ to act as spin filtering barriers in magnetic tunnel junctions. Our study is based on the electronic structure calculated by means of first-principles density functional theory within different approximations for the exchange correlation energy. We show that, in agreement with previous calculations, the density of states suggests a lower tunneling barrier for minority spin electrons, and thus a negative spin-filter effect. However, a more detailed analysis based on the complex band structure reveals that both signs for the spin-filtering efficiency are possible, depending on the band alignment between the electrode and the barrier materials and depending on the specific wave-function symmetry of the relevant bands within the electrode.

We present a study of entropy transport in Bi${}_2$Se${}_3$ at low temperatures and high magnetic fields. In the zero-temperature limit, the magnitude of the Seebeck coefficient quantitatively tracks the Fermi temperature of the three-dimensional Fermi surface at the $\Gamma$ point as the carrier concentration changes by two orders of magnitude (10${}^{17}$ to 10${}^{19}$ cm${}^{-3}$). In high magnetic fields, the Nernst response displays giant quantum oscillations indicating that this feature is not exclusive to compensated semimetals. A comprehensive analysis of the Landau level spectrum firmly establishes a large $g$ factor in this material and a substantial decrease of the Fermi energy with increasing magnetic field across the quantum limit. Thus, the presence of bulk carriers significantly affects the spectrum of the intensively debated surface states in Bi${}_2$Se${}_3$ and related materials.

The optical polarization properties in nonpolar $\left(1\overline{1}00\right)$ Al${}_x$Ga${}_{1-x}$N/AlN quantum wells (QWs) are investigated over the full range of Al compositions. The $\mathit{k}·\mathit{p}$ calculation predicts that the lowest energy optical transition in $\left(1\overline{1}00\right)$ Al${}_x$Ga${}_{1-x}$N layers pseudomorphically grown on unstrained AlN is dipole allowed for the electric field vector parallel to the [0001] direction, and that the degree of polarization is hardly affected by the Al${}_x$Ga${}_{1-x}$N QW width regardless of the Al composition. Experimentally, Al${}_x$Ga${}_{1-x}$N/AlN QWs ($0\le x\le 0.81$) are homoepitaxially grown on AlN substrates. Photoluminescence spectroscopy reveals that the emissions of all QWs, including GaN/AlN QWs, are strongly polarized along the [0001] direction. This result agrees with the theoretical prediction.

Low-energy reflectivity of electrons from single- and multilayer graphene is examined both theoretically and experimentally. A series of minima in the reflectivity over the energy range of 0–8 eV are found, with the number of minima depending on the number of graphene layers. Using first-principles computations, it is demonstrated that a freestanding $n$-layer graphene slab produces $n-1$ reflectivity minima. This same result is also found experimentally for graphene supported on SiO${}_2$. For graphene bonded onto other substrates it is argued that a similar series of reflectivity minima is expected, although in certain cases an additional minimum occurs, at an energy that depends on the graphene-substrate separation and the effective potential in that space.

We postulate the existence of universal crossover functions connecting the universal parts of the entanglement entropy to the low-temperature thermal entropy in gapless quantum many-body systems. These scaling functions encode the intuition that the same low-energy degrees of freedom which control low-temperature thermal physics are also responsible for the long-range entanglement in the quantum ground state. We demonstrate the correctness of the proposed scaling form and determine the scaling function for certain classes of gapless systems whose low-energy physics is described by a conformal field theory. We also use our crossover formalism to argue that local systems which are “natural” can violate the boundary law at most logarithmically. In particular, we show that several non-Fermi-liquid phases of matter have entanglement entropy that is at most of order $L^{d-1}\log \left(L\right)$ for a region of linear size $L$ thereby confirming various earlier suggestions in the literature. We also briefly apply our crossover formalism to the study of fluctuations in conserved quantities and discuss some subtleties that occur in systems that spontaneously break a continuous symmetry.

We determine the relative thermodynamic stability of competing homogeneously and inhomogeneously ordered ferroelectric phases of PbTiO${}_3$ using its free-energy landscape, obtained from constrained-polarization molecular-dynamics simulations with a first-principles effective Hamiltonian and thermodynamic integration. While we find that the tetragonal structure is thermodynamically most stable at temperatures below the ferroelectric transition temperature ($T_0=660$ K), the free energy of an “orthorhombic-like” 90${}^{\circ }$ domain phase relative to the tetragonal phase almost vanishes at $T=540$ K and remains small at all temperatures below $T_0$. In contrast to BaTiO${}_3$, the 90${}^{\circ }$ domain structure is an order of magnitude lower in energy than the one with 180${}^{\circ }$ domains. We show that the dielectric response contains signatures of such a domain structure and is significantly stronger than that of the uniformly polarized tetragonal phase of PbTiO${}_3$.

An oscillating object can stretch quantized vortices attached to it in superfluids due to the relative superflow, steadily generating quantum turbulence, even in the zero-temperature limit. We report the emission and propagation of quantized vortices from quantum turbulence generated in superfluid ${}^4$He at low temperatures. A vortex-free vibrating wire enables us to detect the first collision of vortex rings and therefore to measure the time-of-flight of a vortex emitted from a generator to a detector. The detection times from the start of turbulence generation exhibit an exponential distribution, suggesting that the detection is a Poisson process. Vortices are emitted continuously, but each vortex has a random flight velocity and direction. We estimated the nondetection time and mean detection period from the distribution for two flight distances. By estimating the flight velocity, we find that only vortices with velocities lower than the detector velocity can be detected, even if the sizes of the emitted vortices are smaller than the wire thickness or the vibration amplitude. The ratio of the detection rate as a function of vortex velocity suggests anisotropic emission of vortices from the quantum turbulence.

Electronic frustration and strong correlations may lead to large Seebeck coefficients. To understand this physics on general grounds, we compute the thermopower of the one-band Hubbard model on the three-dimensional fcc lattice over the whole range of fillings for intermediate and large interaction strengths. Dynamical mean-field theory shows that when the density approaches half-filling, the fcc lattice at strong coupling exhibits a large low-temperature Seebeck coefficient $S$. The largest effect occurs as one approaches $n=1$ from dopings where electronic frustration is maximized. The high-frequency limit of the thermopower and the Kelvin limit are both used to provide physical insight as well as practical tools to estimate the thermopower. The high-frequency limit gives a reliable estimate of the dc limit at low temperature when the metal becomes coherent. By contrast, the Kelvin approach is useful in the strongly interacting case at high temperature when transport is incoherent. The latter result shows that in doped Mott insulators at high temperature and strong coupling, the thermopower can be understood on entropic grounds.

It is shown that under certain conditions, three-component superconductors (and, in particular, three-band systems) allow stable topological defects different from vortices. We demonstrate the existence of these excitations, characterized by a $\mathbb{C}{P}^2$ topological invariant, in models for three-component superconductors with broken time-reversal symmetry. We term these topological defects “chiral $G{L}^{\left(3\right)}$ skyrmions,” where “chiral” refers to the fact that due to broken time-reversal symmetry, these defects come in inequivalent left- and right-handed versions. In certain cases, these objects are energetically cheaper than vortices and should be induced by an applied magnetic field. In other situations, these skyrmions are metastable states, which can be produced by a quench. Observation of these defects can signal broken time-reversal symmetry in three-band superconductors or in Josephson-coupled bilayers of $s_{\pm }$ and $s$-wave superconductors.

Using an electronic double layer transistor we have systematically studied the superconductor-to-insulator transition in La${}_2$CuO${}_{4+\delta }$ thin films grown by ozone-assisted molecular-beam epitaxy. We have confirmed the high crystalline quality of the cuprate films and have demonstrated the suitability of the electronic double layer technique to continuously vary the charge density in a system that is otherwise characterized by the presence of miscibility gaps. The transport and magnetotransport results highlight the role of electron-electron interactions in the mechanism of the transition due to the proximity of the Mott-insulating state.

We study the single-particle entanglement spectrum in 2D topological insulators which possess $n$-fold rotation symmetry. By defining a series of special choices of subsystems on which the entanglement is calculated, or real space cuts, we find that the number of protected in-gap states for each type of these real space cuts is a quantum number indexing (if any) nontrivial topology in these insulators. We explicitly show that the number of protected in-gap states is determined by a $Z^n$ index $(z_1,...,z_n)$, where $z_m$ is the number of occupied states that transform according to $m$th one-dimensional representation of the $C_n$ point group. We find that for a space cut separating $1/p$th of the system, the entanglement spectrum contains in-gap states pinned in an interval of entanglement eigenvalues $[1/p,1-1/p]$. We determine the number of such in-gap states for an exhaustive variety of cuts, in terms of the $Z^n$ index. Furthermore, we show that in a homogeneous system, the $Z^n$ index can be determined through an evaluation of the eigenvalues of point-group symmetry operators at all high-symmetry points in the Brillouin zone. When disordered $n$-fold rotationally symmetric systems are considered, we find that the number of protected in-gap states is identical to that in the clean limit as long as the disorder preserves the underlying point-group symmetry and does not close the bulk insulating gap.

Using fermionic representation of spin degrees of freedom within the Popov-Fedotov approach, we develop an algorithm for Monte Carlo sampling of skeleton Feynman diagrams for Heisenberg-type models. Our scheme works without modifications for any dimension of space, lattice geometry, and interaction range, i.e., it is suitable for dealing with frustrated magnetic systems at finite temperature. As a practical application, we compute uniform magnetic susceptibility of the antiferromagnetic Heisenberg model on the triangular lattice and compare our results with the best available high-temperature expansions. We also report results for the momentum dependence of the static magnetic susceptibility throughout the Brillouin zone.

Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the structural and magnetic properties of cobalt-intercalated graphene on Ir(111). The cobalt forms monolayer islands being pseudomorphic with the Ir(111) beneath the graphene. The strong bonding between graphene and cobalt leads to a high corrugation within the Moiré pattern which arises due to the lattice mismatch between the graphene and the Co on Ir(111). The intercalation regions exhibit an out-of-plane easy axis with an extremely high switching field, which surpasses the significant values reported for uncovered cobalt islands on Ir(111). Within the Moiré unit cell of the intercalation regions, we observe a site-dependent variation of the local effective spin polarization. State-of-the-art first-principles calculations show that the origin of this variation is a site-dependent magnetization of the graphene: At top sites the graphene is coupled ferromagnetically to the cobalt underneath, while it is antiferromagnetically coupled at fcc and hcp sites.

In our numerical study, we identify the best conditions for efficient domain-wall motion by spin-orbit torques originating from the spin Hall effect or Rashba effect. We demonstrate that the effect depends critically on the domain-wall configuration, the current injection scheme, and the symmetry of the spin-orbit torque. The best identified configuration corresponds to a Néel wall driven by the spin Hall effect in a narrow strip with perpendicular magnetic anisotropy. In this case, the domain-wall velocity can be a factor of 10 larger than that for the conventional current-in-plane spin-transfer torque.

We performed phase-sensitive terahertz (0.12–1.2 THz) transmission measurements of Ga-enriched layers in silicon. Below the superconducting transition $T_c^{\mathrm{middle}}=6.7$ K we find clear signatures of the formation of a superconducting condensate and of the opening of an energy gap in the optical spectra. The London penetration depth $\lambda \left(T\right)$ and the condensate density $n_s={\lambda }^2\left(0\right)/{\lambda }^2\left(T\right)$ as functions of temperature demonstrate behavior typical for conventional superconductors with $\lambda \left(0\right)=1.8\mu$m. The terahertz spectra can be well described within the framework of Eliashberg theory with strong electron-phonon coupling: the zero-temperature energy gap is $2\Delta \left(0\right)=2.64$ meV and $2\Delta \left(0\right)/k_B{T}_c=4.6$, consistent with the amorphous state of Ga. At temperatures just above $T_c$, the optical spectra demonstrate Drude behavior.

Optical orientation experiments have been performed in GaAs epilayers with photoexcitation energies in the 3 eV region, yielding the photogeneration of spin-polarized electrons in the satellite $L$ valley. We demonstrate that a significant fraction of the electron spin memory can be conserved when the electron is scattered from the $L$ to the $\Gamma$ valley following an energy relaxation of several hundreds of meV. Combining these high energy photoexcitation experiments with time-resolved photoluminescence spectroscopy of $\Gamma$-valley spin-polarized photogenerated electrons allows us to deduce a typical $L$-valley electron spin relaxation time of 200 fs, in agreement with theoretical calculations.

We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. We find that for properly heat-treated crystals with modest Co concentration, $x$ $=$ 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for $T\le$ $T_{s,N}$) and superconducting ($T\le T_c$) states against variation of pressure, disclosing pressure coefficients of $d{T}_{s,N}/dP=-(1100\pm 50)$ K/GPa and $d{T}_c/dP=-(60\pm 3)$ K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly first-order character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.

The primary factors controlling defect stability in phase-field crystal (PFC) models are examined, with illustrative examples involving several existing variations of the model. Guidelines are presented for constructing models with stable defect structures that maintain high numerical efficiency. The general framework combines both long-range elastic fields and basic features of atomic-level core structures, with defect dynamics operable over diffusive time scales. Fundamental elements of the resulting defect physics are characterized for the case of fcc crystals. Stacking faults and split Shockley partial dislocations are stabilized for the first time within the PFC formalism, and various properties of associated defect structures are characterized. These include the dissociation width of perfect edge and screw dislocations, the effect of applied stresses on dissociation, Peierls strains for glide, and dynamic contraction of gliding pairs of partials. Our results in general are shown to compare favorably with continuum elastic theories and experimental findings.

We investigate the collective spontaneous emission in a system of two identical quantum dots (QDs) strongly coupled through the dipole-dipole ($d-d$) interaction. The QDs are modeled as two-level quantum objects, while the $d-d$ interaction is described as the exchange of a virtual photon through the photonic reservoir. The master equation approach is used in the analysis. The main attention is focused on antenna characteristics of the two-QD system—the radiation intensity dependence on the meridian and azimuthal angles of observation. We show that the radiation pattern of such a system is nonstationary and its temporal behavior depends on the initial quantum state. In particular, for entangled initial states the radiative pattern exhibits oscillations on the frequency which corresponds to the $d-d$ interaction energy. We also analyze spectral properties of the directional diagram. The comparison of radiation patterns is carried out for two QDs and two classical dipoles. The concept of quantum nanoantenna is proposed based on collective spontaneous emission in QD ensembles.