We introduce the Loschmidt amplitude as a powerful tool to perform spectroscopy of generic many-body wave functions and use it to interrogate the wave function obtained after ramping the transverse field quantum Ising model through its quantum critical point. Previous results are confirmed and a more complete understanding of the population of defects and of the effects of magnon-magnon interaction or finite-size corrections is obtained. The influence of quantum coherence is clarified.

We report the observation of a two-dimensional electron system (2DES) at the (110) surface of the transparent bulk insulator ${\mathrm{SnO}}_2$ and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions—an extraordinary fact recalling the well known catalytic properties ${\mathrm{SnO}}_2$. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of $n$-type conductivity in ${\mathrm{SnO}}_2$, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band.

We study the impact of non-Hermiticity due to strong correlations in $f$-electron materials. One of the most remarkable phenomena occurring in non-Hermitian systems is the emergence of exceptional points at which the effective non-Hermitian Hamiltonian cannot be diagonalized. We here demonstrate that the temperature at which exceptional points appear around the Fermi energy is related to the Kondo temperature. For this purpose, we study the periodic Anderson model with local and nonlocal hybridization in the insulating and metallic regimes. By analyzing the effective non-Hermitian Hamiltonian, which describes the single-particle spectral function, and the temperature dependence of the magnetic moment, we show that exceptional points appear at the temperature at which the magnetic moment is screened. This temperature corresponds to the Kondo temperature. These results suggest that the well-known crossover between localized and itinerant $f$ electrons in these materials is related to the emergence of exceptional points in the single-particle spectral function at the Fermi energy. Viewing exceptional points in the combined momentum-frequency space, we observe that the exceptional points in the effective Hamiltonian form a one-dimensional manifold which changes its structure around the Kondo temperature.

We have performed pressure-dependent x-ray diffraction and resonant x-ray emission spectroscopy experiments on ${\mathrm{USb}}_2$ to further characterize the antiferromagnetic-ferromagnetic transition occurring near 8 GPa. We have found the magnetic transition coincides with a tetragonal to orthorhombic transition resulting in a 17% volume collapse as well as a transient $f$-occupation enhancement. Compared to ${\mathrm{UAs}}_2$ and UAsS, ${\mathrm{USb}}_2$ shows a reduced bulk modulus and transition pressure and an increased volume collapse at the structural transition. Except for an enhancement across the transition region, the $f$ occupancy decreases steadily from 1.96 to 1.75.

We analyze the possible existence of topological phases in two-legged spin ladders, considering a staggered interaction in both chains. When the staggered interaction in one chain is shifted by one site with respect to the other chain, the model can be mapped, in the continuum limit, into a nonlinear sigma model $\mathrm{NL}\sigma \mathrm{M}$ plus a topological term which is nonvanishing when the number of legs is two. This implies the existence of a critical point which distinguishes two phases. We perform a numerical analysis of energy levels, parity, and string nonlocal order parameters, correlation functions between $x,y,z$ components of spins at the edges of an open ladder, the degeneracy of the entanglement spectrum, and the entanglement entropy to characterize these two different phases. We identify one phase with a Mott insulator and the other one with a Haldane insulator.

We present a detailed study of the phase diagram of the Kitaev-Hubbard chain, that is the Kitaev chain in the presence of a nearest-neighbor density-density interaction, using both analytical techniques as well as DMRG. In the case of a moderate attractive interaction, the model has the same phases as the noninteracting chain, a trivial and a topological phase. For repulsive interactions, the phase diagram is more interesting. Apart from the previously observed topological, incommensurate, and charge density wave phases, we identify the “excited state charge density wave” phase. In this phase, the ground state resembles an excited state of an ordinary charge density phase, but is lower in energy due to the frustrated nature of the model. We find that the dynamical critical exponent takes the value $z\simeq 1.8$. Interestingly, this phase only appears for even system sizes, and is sensitive to the chemical potential on the edges of the chain. For the topological phase, we present an argument that excludes the presence of a strong zero mode for a large part of the topological phase. For the remaining region, we study the time dependence of the edge magnetization (using the bosonic incarnation of the model). These results further expand the region where a strong zero mode does not occur.

We investigate the effects of the competition between exchange ($J$) and dipolar ($D$) interactions on the magnetization reversal mechanisms of ferromagnetic nanotubes. Using first atomistic Monte Carlo simulations for a model with Heisenberg spins on a cylindrical surface, we compute hysteresis loops for a wide range of the $\gamma =D/J$ parameter, characterizing the reversal behavior in terms of the cylindrical magnetization components along the tube length. For $\gamma$'s close to the value for which helical ($H$) states are energetically favorable at zero applied field, we show that the hysteresis loops can occur in four different classes that are combinations of two reversal modes with well-differentiated coercivities with probabilities that depend on the tube length and radius. This variety in the reversal modes is found to be linked to the metastability of the $H$ states during the reversal that induces different paths followed along the energy landscape as the field is changed. We further demonstrate that reversal by either of the two modes can be induced by tailoring the nanotube initial state so circular states with equal or contrary chirality are formed at the ends, thus achieving low or high coercive fields at will without changing $\gamma$. Finally, the results of additional micromagnetic simulations performed on tubes with a similar aspect ratio show that dual switching modes and its tailoring can also be observed in tubes of microscopic dimensions.

Nonequilibrium of electrons, phonons, and magnons in metals is a fundamental phenomenon in condensed-matter physics and serves as an important driver in the field of ultrafast magnetism. In this work, we demonstrate that the magnetization of a subnanometer-thick Co layer with perpendicular magnetic anisotropy can effectively serve as a thermometer to monitor nonequilibrium dynamics in adjacent metals, Pt and Ru, via time-resolved magneto-optic Kerr effect. The temperature evolutions of the Co thermometer embedded in Pt layers of different thicknesses, 6–46 nm, are adequately described by a phenomenological three-temperature model with a consistent set of materials parameters. We do not observe any systematic deviations between the model and the data that can be caused by a nonthermal distribution of electronic excitations. We attribute the consistently good agreement between the model and the data to strong electron-electron interaction in Pt. By using Pt/Co/Pt and Pt/Co/Pt/Ru structures, we determine the electron-phonon coupling parameters of Pt and Ru, $g_{ep}\left(\mathrm{Pt}\right)=\left(6\pm 1\right)\times {10}^{17}\mathrm{W}{\mathrm{m}}^{–3}{\mathrm{K}}^{–1}$ and $g_{ep}\left(\mathrm{Ru}\right)=\left(9\pm 2\right)\times {10}^{17}\mathrm{W}{\mathrm{m}}^{–3}{\mathrm{K}}^{–1}$. We also find that the length scales of nonequilibrium between electrons and phonons are $l_{ep}={\left({\mathrm{\Lambda }}_e/g_{ep}\right)}^{1/2}\approx 9\mathrm{nm}$ for Pt and $\approx 7\mathrm{nm}$ for Ru, shorter than their optical absorption depths, 11 and 13 nm, respectively. Therefore, the optically thick Pt and Ru layers show two steps of temperature rise: The initial jump of electron temperature that occurs within 1 ps is caused by direct optical excitation and electronic heat transport within a distance $l_{ep}$ for the Co layer. The second temperature rise is caused by heat transport by electrons and phonons that are near thermal equilibrium. We contrast two-temperature modeling of heat transport in Pt an Ru films to calculations for Cu, which has a much longer nonequilibrium length scale, $l_{ep}\approx 63\mathrm{nm}$.

Spin current generated in a ferromagnetic metal (FM) can be divided into two types. While one is magnetization dependent and induced by the well-known anomalous Hall effect, the other is a magnetization-independent spin Hall effect which is similar to that in a paramagnetic heavy metal (HM). Here, we study the magnetization-independent spin Hall current in YIG/FM (NiFe and CoFeB) via spin-torque ferromagnetic resonance (ST-FMR) technique. Our experiments reveal the existence of a magnetization-independent spin current. Although there is a strong exchange interaction in FM, the spin current does not dephase as quickly as expected. Furthermore, we estimate the spin-torque efficiency $\xi$ of NiFe was 0.009, which is about $25\%$ of the spin-torque efficiency of Pt. These results indicate that the spin Hall effect of FM should also be taken into account when investigating FM/HM heterostructures, and furthermore this effect can also benefit from the development of spin-orbit torque devices.

For orbital-free ab initio molecular dynamics, especially on systems in extreme thermodynamic conditions, we provide an innovative pseudopotential-adapted generalized gradient approximation (GGA) functional for the noninteracting free energy. This is achieved by systematic finite-temperature extension of our recent LKT ground-state noninteracting kinetic energy GGA functional [Phys. Rev. B 98, 041111(R) (2018)]. We test the performance of the functional first via static lattice calculations on crystalline aluminum and silicon. Then we compare deuterium equation of state results against both path-integral Monte Carlo and conventional (orbital-dependent) Kohn-Sham results. The functional, denoted LKTF, outperforms the previous best semilocal free energy functional VT84F [Phys. Rev. B 88, 161108(R) (2013)], and provides modestly faster simulations. We also discuss subtleties of identification of kinetic and entropic contributions to noninteracting free energy functionals obtained by extension from ground-state orbital-free kinetic energy functionals.

We have studied the magnetization process of the new insulating ferromagnetic semiconductor (Al,Fe)Sb by means of x-ray magnetic circular dichroism. For an optimally doped sample with 10% Fe, a magnetization was found to rapidly increase at low magnetic fields and to saturate at high magnetic fields at room temperature, well above the Curie temperature of 40 K. We attribute this behavior to the existence of nanoscale Fe-rich ferromagnetic domains acting as superparamagnets. By fitting the magnetization curves using the Langevin function representing superparamagnetism plus the paramagnetic linear function, we estimated the average magnetic moment of the nanoscale ferromagnetic domain to be $300{\mu }_{\mathrm{B}}-400{\mu }_{\mathrm{B}}$ and the fraction of Fe atoms participating in the nanoscale ferromagnetism to be $\sim 50\%$. Such behavior was also reported for (In,Fe)As:Be and Ge:Fe and seems to be a universal characteristic of the Fe-doped ferromagnetic semiconductors. Further Fe doping up to 14% led to the weakening of the ferromagnetism, probably because antiferromagnetic superexchange interaction between nearest-neighbor Fe-Fe pairs becomes dominant.

Previous methods for the evaluation of the exfoliation of two-dimensional (2D) layered materials have drawbacks in computational efficiency and are unable to describe cases with semi-infinite substrates. Based on a Green's function surface (GFS) model, here we develop an approach to efficiently determine the tendency of exfoliation of 2D materials from their bulk crystals or semi-infinite substrates. By constructing appropriate surface configurations, we may calculate the exfoliation energy more precisely and quickly than the traditional way with the slab model. Furthermore, the GFS approach can provide angle-resolved photoemission spectroscopy of surface systems for direct comparison with experimental data. Our findings indicate that the GFS approach is powerful for studies of 2D materials and various surface problems.

Unconventional features of relativistic Dirac/Weyl quasiparticles in topological materials are most evidently manifested in the two-dimensional quantum Hall effect (QHE), whose variety is further enriched by their spin and/or valley polarization. Although its extension to three dimensions has been long sought and inspired theoretical proposals, material candidates have been lacking. Here, we have discovered valley-contrasting spin-polarized Dirac fermions in a multilayer form in the bulk antiferromagnet ${\mathrm{BaMnSb}}_2$, where out-of-plane Zeeman-type spin splitting is induced by in-plane inversion symmetry breaking and spin-orbit coupling in the distorted Sb square net. Furthermore, we have observed well-defined quantized Hall plateaus together with vanishing interlayer conductivity at low temperatures as a hallmark of the half-integer QHE in a bulk form. The Hall conductance of each layer is found to be nearly quantized to $2(N+1/2)e^2/h$, with $N$ being the Landau index, which is consistent with two spin-polarized Dirac valleys protected by the strong spin-valley coupling.

Here we propose an exact formalism, off-shell effective energy theory (OET), which provides a thermodynamic description of a generic quantum Hamiltonian. The OET is based on a partitioning of the Hamiltonian and a corresponding density matrix ansatz constructed from an off-shell extension of the equilibrium density matrix; and there are dual realizations based on a given partitioning. To approximate OET, we introduce the central point expansion (CPE), which is an expansion of the density matrix ansatz, and we renormalize the CPE using a standard expansion of the ground-state energy. We showcase the OET for the one-band Hubbard model in $d=1$, 2, and $\infty$, using a partitioning between kinetic and potential energy, yielding two realizations denoted as $\mathcal{K}$ and $\mathcal{X}$. OET shows favorable agreement with exact or state-of-the-art results over all parameter space, and has a negligible computational cost. Physically, $\mathcal{K}$ describes the Fermi liquid, while $\mathcal{X}$ gives an analogous description of both the Luttinger liquid and the Mott insulator. Our approach should find broad applicability in lattice model Hamiltonians, in addition to real materials systems.

${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ has recently been proposed to be a Kitaev-like honeycomb magnet. To assess how close it is to realizing Kitaev quantum spin liquids, we have measured magnetization and specific heat on high-quality single crystals in magnetic fields applied along high-symmetry directions. Small training fields reveal a weak but canonical ferrimagnetic behavior below 27 K, which suggests that the previously established zigzag antiferromagnetic order, with collinear moments pointing along the zigzag chains, must be supplemented by $\mathbf{q}=0$ (e.g., Néel type in the absence of structural modulations) moment canting. Moderate fields in the honeycomb plane suppress the thermal transition at 27 K, and seem to partly reverse the moment canting when applied perpendicular to the zigzag chains. In contrast, out-of-plane fields leave the transition largely unaffected, but promotes another transition below 10 K, possibly also related to canting reversal. The magnetism in ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ is highly anisotropic and close to tipping points between competing phases.

The composition-dependent properties and phase stability of ${\mathrm{Co}}_{2}Y\text{Ga}(Y=\text{Cr},\text{V})$-based shape-memory alloys are investigated by using the first-principles exact muffin-tin-orbital method in combination with the coherent potential approximation. It is shown that both ${\mathrm{Co}}_2\mathrm{CrGa}$ and ${\mathrm{Co}}_2\mathrm{VGa}$ alloys possess $L{2}_1$ structures at the ferromagnetic (FM) state but tetragonal structures at the paramagnetic (PM) state. In off-stoichiometric ${\mathrm{Co}}_{2}Y\mathrm{Ga}$ alloys, the excess Ga atom has a strong tendency to take the $Y$ sublattice, whereas the excess Co atom tends to take the $Y$ site in the FM $L{2}_1$ phase when $Y=\text{Cr}$ but the Ga site in the other phases when $Y=\text{V}$. The off-stoichiometric Cr78 and Cr79 alloys can occur in the normal martensitic transformation (MT) at the PM state but the reentrant martensitic transformation (RMT) at the FM state, whereas only the normal MT is obtained in the V50 and V15 alloys, where the small difference of the magnetization between the austenite and martensite is supposed to suppress the RMT. In all four groups of off-stoichiometric alloys, the correlation between the composition-dependent properties with the experimental $T_M$ values for the normal MT has been established. In the Cr78 and Cr79 alloys, $T_M$ for the RMT is also predicted to decrease with increasing $e/a$. The PM ordering tends to soften the elastic constants of all these $L{2}_1$ alloys. In the four groups of FM $L{2}_1$ alloys, $C^{\prime },G,E,\mathrm{\Theta }$, and G/B decrease whereas $A$ increases with increasing $e/a$, which favors $T_M$ for their RMT decreasing but that for their normal MT increasing with $e/a$. The composition dependence of the MT of these alloys could be explained from their minority density of states around the Fermi level of the $L{2}_1$ phase by means of the Jahn-Teller effect.

We show that an acoustic metamaterial consisting of an array of spinning cylindrical inclusions can possess many unusual properties, including folded bulk bands and interface-state bands. The folding of bands inside the first Brillouin zone is made possible by a rotation-induced antiresonance of compressibility and the rotational Doppler effect. Both bulk and interface-state band dispersions exhibit remarkable filling-fraction-dependent features such as the emergence of a cutoff frequency when the filling fraction exceeds a critical value. Robust one-way transport properties are supported by nondegenerate interface states, but within the same band, interface states at different frequencies can have different propagation directions.

We formulate a dynamical model to describe a photoinduced charge density wave (CDW) quench transition and apply it to recent multiprobe experiments on ${\mathrm{LaTe}}_3$ [A. Zong et al., Nat. Phys. 15, 27 (2019)]. Our approach relies on coupled time-dependent Ginzburg-Landau equations tracking two order parameters that represent the modulations of the electronic density and the ionic positions. We aim at describing the amplitude of the order parameters under the assumption that they are homogeneous in space. This description is supplemented by a three-temperature model, which treats separately the electronic temperature, temperature of the lattice phonons with stronger couplings to the electronic subsystem, and temperature of all other phonons. The broad scope of available data for ${\mathrm{LaTe}}_3$ and similar materials as well as the synergy between different time-resolved spectroscopies allow us to extract model parameters. The resulting calculations are in good agreement with ultrafast electron diffraction experiments, reproducing qualitative and quantitative features of the CDW amplitude evolution during the initial few picoseconds after photoexcitation.

We examine the tunneling spectroscopy of three-dimensional normal-$\mathrm{metal}/{\mathrm{Sr}}_2{\mathrm{RuO}}_4$ junctions as an experimental means to identify pairing symmetry in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. In particular, we consider three different possible pairing states in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$: spin-singlet chiral $d$-wave, spin-triplet helical $p$-wave, and spin-nematic $f$-wave ones, all of which are consistent with recent nuclear-magnetic-resonance experiments [A. Pustogow et al., Nature (London) 574, 72 (2019)]. The Blonder-Tinkham-Klapwijk theory is employed to calculate the tunneling conductance, and the cylindrical two-dimensional Fermi surface of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is properly taken into account as an anisotropic effective mass and a cutoff in the momentum integration. It is pointed out that the chiral $d$-wave pairing state is inconsistent with previous tunneling conductance experiments along the $c$ axis. We also find that the remaining candidates, the spin-triplet helical $p$-wave pairing state and the spin-nematic $f$-wave ones, can be distinguished from each other by the in-plane tunneling spectroscopy along the $a$ and $b$ axes.