Spin and orbital magnetic moments of cationic iron, cobalt, and nickel clusters have been determined from x-ray magnetic circular dichroism spectroscopy. In the size regime of $n=10–15$ atoms, iron and cobalt clusters show fully spin-polarized unoccupied $3d$ states with maximized spin magnetic moments of $1{\mu }_B$ per hole because of completely filled $3d$ majority-spin bands. The notable exception is ${\mathrm{Fe}}_{13}^{+}$ where an unusually low average spin magnetic moment of $0.73\pm 0.12$ ${\mu }_B$ per unoccupied $3d$ state is detected, an effect which is neither observed for ${\mathrm{Co}}_{13}^{+}$ nor ${\mathrm{Ni}}_{13}^{+}$. This distinct behavior can be linked to the existence and accessibility of antiferromagnetic, paramagnetic, or nonmagnetic phases in the respective bulk phase diagrams of iron, cobalt, and nickel. Compared to the experimental data, available density functional theory calculations generally seem to underestimate the spin magnetic moments significantly. In all clusters investigated, the orbital magnetic moment is quenched to $5\%–25\%$ of the atomic value by the reduced symmetry of the crystal field. The magnetic anisotropy energy in this size range is well below $65\mu \mathrm{eV}$ per atom.

We calculate the spectral function of the one-dimensional Hubbard-Holstein model using the time-dependent density matrix renormalization group, focusing on the regime of large local Coulomb repulsion, and away from electronic half-filling. We argue that, from weak to intermediate electron-phonon coupling, phonons interact only with the electronic charge, and not with the spin degrees of freedom. For strong electron-phonon interaction, spinon and holon bands are not discernible anymore and the system is well described by a spinless polaronic liquid. In this regime, we observe multiple peaks in the spectrum with an energy separation corresponding to the energy of the lattice vibrations (i.e., phonons). We support the numerical results by introducing a well controlled analytical approach based on Ogata-Shiba's factorized wave function, showing that the spectrum can be understood as a convolution of three contributions, originating from charge, spin, and lattice sectors. We recognize and interpret these signatures in the spectral properties and discuss the experimental implications.

We describe an approach for calculations of phonon contributions to the electron spectral function, including both quasiparticle properties and satellites. The method is based on a cumulant expansion for the retarded one-electron Green's function and a many-pole model for the electron self-energy. Pole models are also used for the phonon density of states and the Eliashberg functions. Our calculations incorporate ab initio dynamical matrices and electron-phonon couplings from the density functional theory. Illustrative results are presented for several elemental metals and for Einstein and Debye models with a range of coupling constants. These are compared with experiment and other theoretical models. Estimates of corrections to Migdal's theorem are obtained by comparing with leading order contributions to the self-energy, and are found to be significant only for large electron-phonon couplings and low temperatures.

${\mathrm{IrTe}}_2$ exhibits a structural phase transition at $T_s\sim 280$ K. Utilizing the density functional theory, we have simulated optical spectra above and below $T_s$. Simulated optical absorption spectra for a specific (5X) structure show great agreement with experimental data below $T_s$, which provides the structural identification of ${\mathrm{IrTe}}_2$ at low temperature. Comparison between experimental and calculated optical spectra also indicates that the Ir-Ir dimer formation governs the optical transition over the wide energy window of the low frequency part. The analysis of Fermi surfaces and band dispersions supports the effects of dimerization on the optical spectra at the low frequency region. The conductivity calculation based on the Boltzmann transport theory within the constant relaxation time approximation reveals that the dimensional reduction as well as the change of conducting plane takes place during the structural transition of ${\mathrm{IrTe}}_2$.

In this paper, we investigate the occurrence of quantum phase transitions in topological systems out of equilibrium. More specifically, we consider graphene with a sizable spin-orbit coupling, irradiated by circularly polarized light. In the absence of light, the spin-orbit coupling drives a quantum spin Hall phase where edge currents with opposite spins counterpropagate. On the other hand, the light generates a time-dependent vector potential, which leads to a hopping parameter with staggered time-dependent phases around the benzene ring. The model is a dynamical version of the Haldane model, which considers a static staggered flux with zero total flux through each plaquette. Since the light breaks time-reversal symmetry, a quantum Hall (QH) phase protected by an integer topological invariant arises. An important difference with the static QH phase is the existence of counterpropagating edge states at different momenta, which are made possible by zero- and two-photon resonances. By numerically solving the complete problem, with spin-orbit coupling and light, and investigating different values of the driving frequency $\omega$, we show that the spectrum exhibits nontrivial gaps not only at zero energy but also at $\omega /2$. This additional gap is created by photon resonances between the valence and conduction band of graphene, and the symmetry of the spectrum forces it to lie at $\omega /2$. By increasing the intensity of the irradiation, the topological state in the zero energy gap undergoes a dynamical phase transition from a quantum spin Hall to a quantum Hall phase, whereas the gap around $\omega /2$ remains in the quantum Hall regime.

We study the effect of Coulomb interactions on the low-energy band structure of single-layer transition metal dichalcogenide semiconductors using an effective low-energy model. We show how a large conduction band spin splitting and a spin dependent Fermi velocity are generated in ${\mathrm{MoS}}_2$, as a consequence of the difference between the gaps of the two spin projections induced by the spin-orbit interaction. The conduction band and Fermi velocity spin splitting found are in agreement with the optical absorption energies of the excitonic peaks A, B measured in the experiments.

We propose that bilayer graphene can provide an experimental realization of deconfined criticality. Current experiments indicate the presence of Néel order in the presence of a moderate magnetic field. The Néel order can be destabilized by application of a transverse electric field. The resulting electric field induced state is likely to have valence bond solid order, and the transition can acquire the emergent fractionalized and gauge excitations of deconfined criticality.

In two-dimensional crystals of transition-metal dichalcogenides (TMDC) having strong spin-orbit interaction such as monolayer ${\mathrm{WSe}}_2$, quantum states can be labeled by a valley index $\tau$ defined in the reciprocal space and the spin index $s$. We developed a first-principles theoretical formalism to both qualitatively and quantitatively predict nonequilibrium quantum transport of valley-polarized currents. We propose a ${\mathrm{WSe}}_2$ TMDC transistor to selectively deliver net valley- and spin-polarized current $I_{\tau ,s}$ to the source or drain by circularly polarized light under external bias. Due to the lack of translational symmetry of the real-space device, we predict a depolarization effect that increases with the decrease of the channel length of the transistor.

The crossover from an exciton gas to an electron-hole plasma is studied in a GaN/(Al,Ga)N single quantum well by means of combined time-resolved and continuous-wave photoluminescence measurements. The two-dimensional Mott transition is found to be of continuous type and to be accompanied by a characteristic modification of the quantum well emission spectrum. Beyond the critical density, the latter is strongly influenced by band-gap renormalization and Fermi filling of continuum states. Owing to the large binding energy of excitons in III-nitride heterostructures, their injection-induced dissociation could be tracked over a wide range of temperatures, i.e., from 4 to $150\mathrm{K}$. Various criteria defining the Mott transition are examined, which, however, do not lead to any clear trend with rising temperature: the critical carrier density remains invariant around ${10}^{12}{\mathrm{cm}}^{-2}$.

Motivated by the recent synthesis of two insulating ${\mathrm{Li}}_2{\mathrm{IrO}}_3$ polymorphs, where ${\mathrm{Ir}}^{4+}$ $S_{\mathrm{eff}}=1/2$ moments form 3D (“harmonic”) honeycomb structures with threefold coordination, we study magnetic Hamiltonians on the resulting $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ hyperhoneycomb lattice and $\gamma \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ stripyhoneycomb lattice. Experimentally measured magnetic susceptibilities suggest that Kitaev interactions, predicted for the ideal ${90}^{\circ }$ Ir-O-Ir bonds, are sizable in these materials. We first consider pure Kitaev interactions, which lead to an exactly soluble 3D quantum spin liquid (QSL) with emergent Majorana fermions and ${\mathrm{Z}}_2$ flux loops. Unlike 2D QSLs, the 3D QSL is stable to finite temperature, with $T_c\approx \left|K\right|/100$. On including Heisenberg couplings, exact solubility is lost. However, by noting that the shortest closed loop $\ell$ is relatively large in these structures, we construct an $\ell \to \infty$ approximation by defining the model on the Bethe lattice. The phase diagram of the Kitaev-Heisenberg model on this lattice is obtained directly in the thermodynamic limit, using tensor network states and the infinite-system time-evolving-block-decimation (iTEBD) algorithm. Both magnetically ordered and gapped QSL phases are found, the latter being identified by an entanglement fingerprint.

Floquet Majorana fermions appear as steady states at the boundary of time-periodic topological phases of matter. In this work, we theoretically study the main features of these exotic topological phases in the periodically driven one-dimensional Kitaev model. By controlling the ac fields, we can predict topological phase transitions that should give rise to signatures of Majorana states in experiments. Moreover, the knowledge of the time dependence of these Majorana states allows one to manipulate them. Our work contains a complete analysis of the monochromatic driving in different frequency regimes.

Transition-metal dichalcogenides (TMD) share the same global layered structure, but distinct polymorphs are characterized by different local coordinations of the transition-metal atoms. Here we compared the $1T$ and $2H$ families of metallic TMD, both in the bulk and in the two-dimensional forms. By means of first-principles time-dependent density functional calculations of the loss function, we established the direct connection between the low-energy plasmon properties and the crystal-structure symmetry. The different atomic environments affect the $d-d$ electron-hole excitations, which are prominent at low energies, resulting in distinct in-plane plasmon dispersions in the two families. Conversely, the different periodicity of the plasmon reappearance along the $c$ axis perpendicular to the layers can be used to distinguish the various crystal structures of TMD.

We revisit an antiferromagnetic quantum phase transition with $\mathit{Q}=2{\mathit{k}}_F$, where $\mathit{Q}$ is an ordering wave vector and ${\mathit{k}}_F$ is a Fermi momentum. Reformulating the Hertz-Moriya-Millis theory within the strong-coupling approach to diagonalize the spin-fermion coupling term and performing the scaling analysis for an effective-field theory with quantum corrections in the Eliashberg approximation, we propose an interacting fixed point for this antiferromagnetic quantum phase transition, where antiferromagnetic spin fluctuations become locally critical to interact with renormalized electrons. The emergence of local quantum criticality suggests a mechanism of $\omega /T$ scaling for antiferromagnetic quantum criticality, generally forbidden in the context of the Hertz-Moriya-Millis theory.

Initial phase shift in a precessional motion of resident electron-spin polarization is studied in a ${\mathrm{CdTe}/\mathrm{Cd}}_{0.85}{\mathrm{Mg}}_{0.15}\mathrm{Te}$ single quantum well using a time-resolved Kerr rotation technique. The generation dynamics of resident electron-spin polarization involve the formation and transformation of the associated optically excited states and are complicated particularly in the early time region. A careful analysis of the phase shift gives a deep understanding of the generation processes. In the experiments, the negative phase shift of the resident electron-spin polarization is observed, and the mechanism associated especially with a quick hole spin flip in negative trions is studied through the dependences on excitation power and magnetic field strength.

A wealth of atomistic information is contained within a self-assembled quantum dot (QD), associated with its chemical composition and the growth history. In the presence of quadrupolar nuclei, as in InGaAs QDs, much of this is inherited to nuclear spins via the coupling between the strain within the polar lattice and the electric quadrupole moments of the nuclei. Here, we present a computational study of the recently introduced inverse spectra nuclear magnetic resonance technique to assess its suitability for extracting such structural information. We observe marked spectral differences between the compound InAs and alloy InGaAs QDs. These are linked to the local biaxial and shear strains, and the local bonding configurations. The cation alloying plays a crucial role especially for the arsenic nuclei. The isotopic line profiles also largely differ among nuclear species: While the central transition of the gallium isotopes have a narrow linewidth, those of arsenic and indium are much broader and oppositely skewed with respect to each other. The statistical distributions of electric field gradient (EFG) parameters of the nuclei within the QD are analyzed. The consequences of various EFG axial orientation characteristics are discussed. Finally, the possibility of suppressing the first-order quadrupolar shifts is demonstrated by simply tilting the sample with respect to the static magnetic field.

The phase diagram of the metal-insulator transition in a three-dimensional quantum percolation problem is investigated numerically based on the multifractal analysis of the eigenstates. The large-scale numerical simulation has been performed on systems with linear sizes up to $L=140$. The multifractal dimensions, exponents $D_q$ and ${\alpha }_q$, have been determined in the range of $0\le q\le 1$. Our results confirm that this problem belongs to the same universality class as the three-dimensional Anderson model; the critical exponent of the localization length was found to be $\nu =1.622\pm 0.035$. However, the multifractal function $f\left(\alpha \right)$ and the exponents $D_q$ and ${\alpha }_q$ produced anomalous variations along the phase boundary, $p_c^Q\left(E\right)$.

An applied electric current through a space-inversion asymmetric magnet induces spin-orbit torques (SOTs) on the magnetic moments, which holds much promise for future memory devices. We discuss general Green's function expressions suitable to compute the linear-response SOT in disordered ferromagnets. The SOT can be decomposed into an even and an odd component with respect to magnetization reversal, where in the limit of vanishing disorder the even SOT is given by the constant Berry curvature of the occupied states, while the odd part exhibits a divergence with respect to disorder strength. Within this formalism, we perform first-principles density-functional theory calculations of the SOT in Co/Pt(111) and Mn/W(001) magnetic bilayers. We find the even and odd torque components to be of comparable magnitude. Moreover, the odd torque depends strongly on an additional capping layer, while the even torque is less sensitive. We show that the even torque is nearly entirely mediated by spin currents in contrast to the odd torque, which can contain an important contribution not due to spin transfer. Our results are in agreement with experiments, showing that our linear-response theory is well-suited for the description of SOTs in complex ferromagnets.

We present an in-depth examination of the composition and magnetism of cobalt $\left({\mathrm{Co}}^{2+}\right)$-doped iron-oxide nanoparticles encapsulated in Pyrococcus furiosus ferritin shells. We show that the ${\mathrm{Co}}^{2+}$ dopant ions were incorporated into the $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3/{\mathrm{Fe}}_3{\mathrm{O}}_4$ core, with small paramagnetic-like clusters likely residing on the surface of the nanoparticle that were observed for all cobalt-doped samples. In addition, element-specific characterization using Mössbauer spectroscopy and polarized x-ray absorption indicated that ${\mathrm{Co}}^{2+}$ was incorporated exclusively into the octahedral B sites of the spinel-oxide nanoparticle. Comparable superparamagnetic blocking temperatures, coercivities, and effective anisotropies were obtained for 7%, 10%, and 12% cobalt-doped nanoparticles, and were only slightly reduced for 3% cobalt, indicating a strong effect of cobalt incorporation, with a lesser effect of cobalt content. Due to the regular particle size and separation that result from the use of the ferritin cage, a comparison of the effects of interparticle interactions on the disordered assembly of nanoparticles was also obtained that indicated significantly different behaviors between undoped and cobalt-doped nanoparticles.