The long-time thermal relaxation of (TMTTF)${}_2\mathrm{Br},{\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ and ${\mathrm{Sr}}_2{\mathrm{Ca}}_{12}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ single crystals at temperatures below 1 K and magnetic field up to 10 T is investigated. The data allow us to determine the relaxation-time spectrum of the low-energy excitations caused by the charge-density wave or spin-density wave (SDW). The relaxation time is mainly determined by a thermal activated process for all investigated materials. The maximum relaxation time increases with increasing magnetic field. The distribution of barrier heights corresponds to one or two Gaussian functions. The doping of ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ with Ca leads to a drastic shift of the relaxation-time spectrum to longer time. The maximum relaxation time changes from 50 s $\left(x=0\right)$ to 3000 s $\left(x=12\right)$ at 0.1 K and 10 T. The observed thermal relaxation at $x=12$ clearly indicates the formation of the SDW ground state at low temperatures.

The presence and character of local integrals of motion—quasilocal operators that commute with the Hamiltonian—encode valuable information about the dynamics of a quantum system. In particular, strongly disordered many-body systems can generically avoid thermalization when there are extensively many such operators. In this work, we explicitly construct local conserved operators in one-dimensional spin chains by directly minimizing their commutator with the Hamiltonian. We demonstrate the existence of an extensively large set of local integrals of motion in the many-body localized phase of the disordered XXZ spin chain. These operators are shown to have exponentially decaying tails, in contrast to the ergodic phase where the decay is (at best) polynomial in the size of the subsystem. We study the algebraic properties of localized operators and confirm that in the many-body localized phase, they are well described by “dressed” spin operators.

In crystals, molecules thermally vibrate around the periodic lattice sites. Vibrational motions are well understood in terms of phonons, which carry heat and control heat transport. The situation is notably different in disordered solids, where vibrational excitations are not phonons and can be even localized. Recent numerical work has established the concept of elastic heterogeneity: disordered solids show inhomogeneous local mechanical response. Clearly, the heterogeneous nature of elastic properties strongly influences vibrational and thermal properties, and it is expected to be the origin of anomalous features, including boson peak, vibrational localization, and temperature dependence of thermal conductivity. These are all crucial long-standing problems in materials physics, which we address in the present work. We have considered a toy model able to stabilize different states of matter, by introducing an increasing amount of size disorder. The phase diagram generated by molecular dynamics simulations encompasses the perfect crystalline state with a spatially homogeneous elastic moduli distribution, multiple defective phases with increasing moduli heterogeneities, and eventually a series of amorphous states. We have established clear correlations among the heterogeneous local mechanical response, vibrational states, and thermal conductivity. We provide evidence that elastic heterogeneity controls both vibrational and thermal properties, and is a key concept to understand the anomalous puzzling features of disordered solids.

The anomalous softening of the acoustic phonon peak in FeSi has recently received considerable experimental attention. In our work, we investigate the effect of thermal disorder on the lattice dynamics and the filling of the narrow band gap of FeSi using density functional theory. We show, by comparing the phonon density of states from temperature-independent and temperature-dependent force constants, that thermal structural disorder together with thermal expansion explains the anomalously strong renormalization of the acoustic phonons. Furthermore, we find an intricate interplay between thermal disorder and volume in gap closure.

Vorticity is a concept well established in fluid dynamics to describe the local tendency of a fluid to rotate. Here, we explore the vorticity of electron waves and show that it can be used to qualitatively estimate the strength of an electron magnetic circular dichroism (EMCD) signal, without resorting to expensive inelastic electron scattering calculations. We discuss the properties of vorticity, its relationship with orbital angular momentum, and how it can be used to investigate the characteristics of electron beams.

Since the rapid technological progress demands for ever smaller storage units, the emergence of stable magnetic order in nanomaterials down to the single-atom regime has attracted huge scientific attention to date. Electronic transport spectroscopy has been proven to be a versatile tool for the investigation of electronic, magnetic, and mechanical properties of atomic contacts. Here we report a comprehensive experimental study of the magnetoconductance and electronic properties of Pd atomic contacts at low temperature. The analysis of electronic transport $\left(dI/dV\right)$ spectra and the magnetoconductance curves yields a diverse behavior of Pd single-atom contacts, which is attributed to different contact configurations. The magnetoconductance shows a nonmonotonous but mostly continuous behavior, comparable to those found in atomic contacts of band ferromagnets. In the $dI/dV$ spectra, frequently, a pronounced zero-bias anomaly (ZBA) as well as an aperiodic and nonsymmetric fluctuation pattern are observed. While the ZBA can be interpreted as a sign of the Kondo effect, suggesting the presence of magnetic impurity, the fluctuations are evaluated in the framework of conductance fluctuations in relation to the magnetoconductance traces and to previous findings in Au atomic contacts. This thorough analysis reveals that the magnetoconductance and transport spectrum of Au atomic contacts can completely be accounted for by conductance fluctuations, while in Pd contacts the presence of local magnetic order is required.

We study laser-induced torques in bcc Fe, hcp Co, and $L{1}_0$ FePt based on first-principles electronic structure calculations and the Keldysh nonequilibrium formalism. We find that the torques have two contributions, one from the inverse Faraday effect (IFE) and one from the optical spin-transfer torque (OSTT). Depending on the ferromagnet at hand and on the quasiparticle broadening the two contributions may be of similar magnitude, or one contribution may dominate over the other. Additionally, we determine the nonequilibrium spin polarization in order to investigate its relation to the torque. We find the torques and the perpendicular component of the nonequilibrium spin polarization to be odd in the helicity of the laser light, while the spin polarization that is induced parallel to the magnetization is helicity independent. The parallel component of the nonequilibrium spin polarization is orders of magnitude larger than the perpendicular component. In the case of hcp Co we find good agreement between the calculated laser-induced torque and a recent experiment.

We present a method to calculate from first principles the spin Seebeck effect on finite systems. Our method, which is suited for all ab initio, quantum-chemistry-based results, is demonstrated quasianalytically on the ${\mathrm{Na}}_2$ dimer. To this end we start from the analytical solutions of the many-body wave function for the minimal ${\mathrm{Na}}_2$ molecule and propagate it numerically in time using the Liouville-von Neumann equation of motion. The system is coupled to two baths with different temperatures, described with a Lindblad superoperator. We mainly focus on the concept of how to divide any operator into several spatially localized contributions and show that the spatial localization of the virtual excitations (i.e., splitting of the ladder operators into two sets of localized operators with different eigenbases) is the underlying reason for the spin Seebeck effect. Last but not least, we analyze the entanglement of the system and find that the maxima of the Laplacian of the negativity coincide with the change of the direction of the spin Seebeck effect.

Single crystals of $R{\mathrm{Mg}}_2{\mathrm{Cu}}_9$ ($R=\mathrm{Y}$, Ce-Nd, Gd-Dy, Yb) were grown using a high-temperature solution growth technique and were characterized by measurements of room-temperature x-ray diffraction, temperature-dependent specific heat, and temperature- and field-dependent resistivity and anisotropic magnetization. ${\mathrm{YMg}}_2{\mathrm{Cu}}_9$ is a non-local-moment-bearing metal with an electronic specific heat coefficient, $\gamma \sim 15$ mJ/mol ${\mathrm{K}}^2$. Yb is divalent and basically non-moment-bearing in ${\mathrm{YbMg}}_2{\mathrm{Cu}}_9$. Ce is trivalent in ${\mathrm{CeMg}}_2{\mathrm{Cu}}_9$ with two magnetic transitions being observed at 2.1 K and 1.5 K. ${\mathrm{PrMg}}_2{\mathrm{Cu}}_9$ does not exhibit any magnetic phase transition down to 0.5 K. The other members being studied ($R=\mathrm{Nd}$, Gd-Dy) all exhibit antiferromagnetic transitions at low temperatures ranging from 3.2 K for ${\mathrm{NdMg}}_2{\mathrm{Cu}}_9$ to 11.9 K for ${\mathrm{TbMg}}_2{\mathrm{Cu}}_9$. Whereas ${\mathrm{GdMg}}_2{\mathrm{Cu}}_9$ is isotropic in its paramagnetic state due to zero angular momentum ($L=0)$, all the other local-moment-bearing members manifest an anisotropic, planar magnetization in their paramagnetic states. To further study this planar anisotropy, detailed angular-dependent magnetization was carried out on magnetically diluted (${\mathrm{Y}}_{0.99}{\mathrm{Tb}}_{0.01}$)${\mathrm{Mg}}_2{\mathrm{Cu}}_9$ and (${\mathrm{Y}}_{0.99}{\mathrm{Dy}}_{0.01}$)${\mathrm{Mg}}_2{\mathrm{Cu}}_9$. Despite the strong, planar magnetization anisotropy, the in-plane magnetic anisotropy is weak and field-dependent. A set of crystal electric field parameters are proposed to explain the observed magnetic anisotropy.

The multiferroic compound CuO exhibits low-temperature magnetic properties similar to antiferromagnetic iron oxides, while the electronic properties have much more in common with the high-$T_c$ cuprate superconductors. This suggests novel possibilities for the ultrafast optical excitation of magnetism. On the basis of atomistic spin dynamics simulations, we study the effect of phonon-assisted multimagnon absorption and photodoping on the spin dynamics in the vicinity of the first-order phase transition from collinear to spin-spiral magnetic order. Similar as in recent experiments, we find that for both excitations the phase transition can proceed on the picosecond timescale. Interestingly, however, these excitation mechanisms display very distinct dynamics. Following photodoping, the spin system first cools down on subpicosecond time scales, which we explain as an ultrafast magnetocaloric effect. Opposed to this, following phonon-assisted multimagnon excitation, the spin systems rapidly heats up and subsequently evolves to the noncollinear phase even under the influence of isotropic exchange interactions alone.

To get a reliable ab initio value for the magnetocrystalline anisotropy (MCA) energy of FePt, we employ the full-potential linearized augmented plane wave (FLAPW) method and the full-potential Korringa-Kohn-Rostoker (KKR) Green function method. The MCA energies calculated by both methods are in good agreement with each other. As the calculated MCA energy significantly differs from experiment, it is clear that many-body effects beyond the local density approximation are essential. It is not really important whether relativistic effects for FePt are accounted for by solving the full Dirac equation or whether the spin-orbit coupling (SOC) is treated as a correction to the scalar-relativistic Hamiltonian. From the analysis of the dependence of the MCA energy on the magnetization angle and on the SOC strength it follows that the main mechanism of MCA in FePt can be described within second order perturbation theory. However, a distinct contribution not accountable for by second order perturbation theory is present as well.

Ring-exchange interactions have been proposed as a possible mechanism for a Bose-liquid phase at zero temperature, a phase that is compressible with no superfluidity. Using the stochastic Green function algorithm (SGF), we study the effect of these interactions for bosons on a two-dimensional triangular lattice. We show that the supersolid phase that is known to exist in the ground state for a wide range of densities is rapidly destroyed as the ring-exchange interactions are turned on. We establish the ground-state phase diagram of the system, which is characterized by the absence of the expected Bose-liquid phase.

We report the electronic structure, synthesis, and measurements of the magnetic, transport, and thermal properties of the polycrystalline noncentrosymmetric compound ${\mathrm{Re}}_6\mathrm{Zr}$. We observed a bulk superconducting transition at temperature $T_c\sim 6.7$ K, and measured the resistivity, heat capacity, thermal conductivity, and the London penetration depth below the transition, as well as performed doping and pressure studies. From these measurements we extracted the critical field and the superconducting parameters of ${\mathrm{Re}}_6\mathrm{Zr}$. Our measurements indicate a relatively weak to moderate contribution from a triplet component to the order parameter, and favor a full superconducting gap, although we cannot exclude the existence of point nodes based on our data.

We report a high-pressure transport study of the upper-critical field $B_{c2}\left(T\right)$ of the topological superconductor ${\mathrm{Sr}}_{0.15}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ $(T_c=3.0$ K). $B_{c2}\left(T\right)$ was measured for magnetic fields directed along two orthogonal directions, $a$ and $a^{*}$, in the trigonal basal plane. While superconductivity is rapidly suppressed at the critical pressure $p_c\sim 3.5$ GPa, the pronounced two-fold basal-plane anisotropy $B_{c2}^a/B_{c2}^{a^{*}}=3.2$ at $T=0.3$ K, recently reported at ambient pressure [Pan et al., Sci. Rep. 6, 28632 (2016)], is reinforced and attains a value of $\sim 5$ at the highest pressure (2.2 GPa). The data reveal that the unconventional superconducting state with broken rotational symmetry is robust under pressure.

Pyrochlore oxides show several fascinating phenomena, such as the formation of heavy fermions and the thermal Hall effect. Although a key to understanding some phenomena may be the Dzyaloshinsky-Moriya (DM) interaction, its microscopic origin is unclear. To clarify the microscopic origin, we constructed a $t_{2g}$-orbital model with the kinetic energy, the trigonal-distortion potential, the multiorbital Hubbard interactions, and the $LS$ coupling, and derived the low-energy effective Hamiltonian for a $d^1$ Mott insulator with the weak $LS$ coupling. We first show that lack of the inversion center of each nearest-neighbor V-V bond causes the odd-mirror interorbital hopping integrals. Those are qualitatively different from the even-mirror hopping integrals, existing even with the inversion center. We next show that the second-order perturbation using the kinetic terms leads to the ferromagnetic and the antiferromagnetic superexchange interactions, whose competition is controllable by tuning the Hubbard interactions. Then, we show the most important result: the third-order perturbation terms using the combination of the even-mirror hopping integral, the odd-mirror hopping integral, and the $LS$ coupling causes the DM interaction due to the mirror-mixing effect, where those hopping integrals are necessary to obtain the antisymmetric kinetic exchange and the $LS$ coupling is necessary to excite the orbital angular momentum at one of two sites. We also show that the magnitude and sign of the DM interaction can be controlled by changing the positions of the O ions and the strength of the Hubbard interactions. We discuss the advantages in comparison with the phenomenological theory and Moriya's microscopic theory, applicability of our mechanism, and the similarities and differences between our case and the strong-$LS$-coupling case.

We present a method for reconstructing the phonon relaxation time distribution ${\tau }_{\omega }=\tau \left(\omega \right)$ (including polarization) in a material from thermal spectroscopy data. The distinguishing feature of this approach is that it does not make use of the effective thermal conductivity concept and associated approximations. The reconstruction is posed as an optimization problem in which the relaxation times ${\tau }_{\omega }=\tau \left(\omega \right)$ are determined by minimizing the discrepancy between the experimental relaxation traces and solutions of the Boltzmann transport equation for the same problem. The latter may be analytical, in which case the procedure is very efficient, or numerical. The proposed method is illustrated using Monte Carlo solutions of thermal grating relaxation as synthetic experimental data. The reconstruction is shown to agree very well with the relaxation times used to generate the synthetic Monte Carlo data and remains robust in the presence of uncertainty (noise).

Spin-polarized electron energy loss spectroscopy was applied to study the intensity asymmetry upon the reversal of the incident spin polarization of elastically and inelastically scattered polarized electrons from an epitaxial ferromagnetic Fe layer on W(110). The polarization of the incident beam was always collinear with the magnetic moment (magnetization) of the sample, and the asymmetries were measured for two opposite magnetizations of the sample. They allowed extracting the exchange and the spin-orbit components from the measured asymmetry. A strong asymmetry of Stoner excitations in an Fe film on W(110) is observed, as expected, at about 3 eV energy loss. The value of the asymmetry declines but is still observable when the surface is contaminated. The asymmetry of elastic scattering for normal incidence and a 50° detection angle is close to zero in contrast to the Stoner excitation asymmetry (inelastic scattering). However, the asymmetry of elastic scattering increases substantially at two specular geometries with 25° and 72° angles of incidence, compared to the normal incidence, and may be even larger than the asymmetry of Stoner excitations. The sign of elastic scattering asymmetry changes upon the reversal of the sample magnetization, indicating the magnetic origin of this asymmetry, i.e., spin-dependent electron-electron scattering with electron exchange. The calculation of the asymmetry of elastic spin-polarized electron scattering from a spin-dependent surface potential barrier shows a qualitative agreement with the measurements.

Graphene, being essentially a surface, can borrow some properties of an insulating substrate (such as exchange or spin-orbit couplings) while still preserving a great degree of autonomy of its electronic structure. Such derived properties are commonly labeled as proximity. Here we perform systematic first-principles calculations of the proximity exchange coupling, induced by cobalt (Co) and nickel (Ni) in graphene, via a few (up to three) layers of hexagonal boron nitride (hBN). We find that the induced spin splitting of the graphene bands is of the order of 10 meV for a monolayer of hBN, decreasing in magnitude but alternating in sign by adding each new insulating layer. We find that the proximity exchange can be giant if there is a resonant $d$ level of the transition metal close to the Dirac point. Our calculations suggest that this effect could be present in Co heterostructures, in which a $d$ level strongly hybridizes with the valence-band orbitals of graphene. Since this hybridization is spin dependent, the proximity spin splitting is unusually large, about 10 meV even for two layers of hBN. An external electric field can change the offset of the graphene and transition-metal orbitals and can lead to a reversal of the sign of the exchange parameter. This we predict to happen for the case of two monolayers of hBN, enabling electrical control of proximity spin polarization (but also spin injection) in graphene/hBN/Co structures. Nickel-based heterostructures show weaker proximity effects than cobalt heterostructures. We introduce two phenomenological models to describe the first-principles data. The minimal model comprises the graphene (effective) $p_z$ orbitals and can be used to study transport in graphene with proximity exchange, while the $p_z\text{−}d$ model also includes hybridization with $d$ orbitals, which is important to capture the giant proximity exchange. Crucial to both models is the pseudospin-dependent exchange coupling, needed to describe the different spin splittings of the valence and conduction bands.

The pressure dependences of the electronic and crystal structures of ${\mathrm{LaCo}}_5$ have been studied with high-resolution x-ray absorption spectroscopy (XAS) and x-ray diffraction comprehensively. The lattice constant ratio of $a/c$ shows an anomaly around 12–18 GPa without change in the lattice symmetry. In the XAS spectra with partial fluorescence mode as well as in the integrated absolute difference values of the Co $K\beta$ emission spectra, a change in the electronic structure was found at around a similar pressure range as the lattice constant anomaly. Density functional theory calculations confirm the presence of a magnetic anomaly around the same pressure region. No temperature dependence of the electronic structure for ${\mathrm{LaCo}}_5$ and ${\mathrm{YCo}}_5$ was observed.

We present results on the magnetic field dependence of the stripe order in ${\mathrm{La}}_{1.64}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_4$ (LESCO). Using resonant soft x-ray scattering at the oxygen $K$ edge to probe the (0.259,0,0.648) superlattice reflection, which is commonly associated to charge stripes, we found no pronounced difference in the wave vector, peak widths, and integrated intensity for magnetic fields up to $B=6$ T. This is in strong contrast to the behavior observed for ${\mathrm{La}}_{1.875}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$, where a stabilization of the charge modulation in high magnetic fields has been demonstrated.