The $ab$-plane resistivity and Hall effect are studied in ${\mathrm{Fe}}_{1-y}{M}_y{\mathrm{Te}}_{0.65}{\mathrm{Se}}_{0.35}$ single crystals doped with two transition-metal elements, M = Co or Ni, over a wide doping range, $0\le y\le 0.2$. The superconducting transition temperature, $T_c$, reaches zero for Co at $y\simeq 0.14$ and for Ni at $y\simeq 0.032$, while the resistivity at the $T_c$ onset increases weakly with Co doping, and strongly with Ni doping. The Hall coefficient $R_H$, positive for $y=0$, remains so at high temperatures for all $y$, while it changes sign to negative at low $T$ for $y>0.135$ (Co) and $y>0.06$ (Ni). The analysis based on a two-band model suggests that at high $T$ residual hole pockets survive the doping, but holes get localized upon the lowering of $T$, so that the effect of the electron doping on the transport becomes evident. The suppression of the $T_c$ by Co impurity is related to electron doping, while in the case of the Ni impurity strong electron localization most likely contributes to fast decrease of the $T_c$.

We introduce a class of two-dimensional nonresonant single-phase phononic materials and investigate its peculiar dispersion characteristics. The material consists of a thin platelike structure with an embedded periodic lattice of acoustic black holes. The use of these periodic tapers allows achieving remarkable dispersion properties such as zero group velocity in the fundamental modes, negative group refraction index, birefraction, Dirac-like cones, and mode anisotropy. The dispersion properties are numerically investigated using both a three-dimensional supercell plane wave expansion method and finite element analysis. The effect on the dispersion and propagation characteristics of key geometric parameters of the black hole, such as the taper profile and the residual thickness, are also explored.

We present ab initio calculations of the temperature-dependent exchange of energy between a classical charged point particle and the phonons of a crystalline material. The phonons, which are computed using density functional perturbation theory (DFPT) methods, interact with the moving particle via the Coulomb interaction between the density induced in the material by phonon excitation and the charge of the classical particle. Energy relaxation rates are computed using time-dependent perturbation theory. The method, which is applicable wherever DFPT is, is illustrated with results for CsI, an important scintillator whose performance is affected by electron thermalization. We discuss the influence of the form assumed for quasiparticle dispersion on theoretical estimates of electron cooling rates.

Quantization of spin-wave excitations necessitates the reconsideration of the classical fluctuation-dissipation relation (FDR) used for temperature control in spin-lattice dynamics simulations of ferromagnetic metals. In this paper, Bose-Einstein statistics is used to reinterpret the Langevin dynamics of both lattice and spins, allowing quantum statistics to be mimicked in canonical molecular dynamics simulations. The resulting quantum heat baths are tested by calculating the specific heats and magnetization over a wide temperature range, from 0 K to above the Curie temperature, with molecular dynamics (MD), spin dynamics (SD), and spin-lattice dynamics (SLD) simulations. The results are verified with experimental data and available theoretical analysis. Comparison with classical results also shows the importance of quantization effects for spin excitations in all the ferromagnetically ordered configurations.

Transition metal-rare earth (TM-RE) Fe/Tb-multilayer systems have been known to show exchange-bias-like shifts in the form of double hysteresis loop (DHL) along and opposite to the field cooling axis. Planar domain walls, with opposite handedness at the interfaces, are held responsible for such DHL. Here, we report on the formation of nanoparticulated Fe layers in the Cu-matrix within a Fe-Cu/Tb multilayer and their eventual low-temperature characteristics. AC susceptibility measurements indicate that these diluted magnetic clusters have a superspin-glass-type of freezing behavior. Eventually, this Fe-cluster/Tb interlayer interaction, which is conjectured to be mediated by the pinned moments within the individual clusters, has helped in increasing the exchange bias field in the system to a high value of $\approx 1.3$ kOe, which gradually vanishes around 50 K. Polarized neutron reflectivity confirms a very strong antiferromagnetic (AF) coupling between the individual layers. The magnitude of the magnetic moment of each of the individual Tb or Fe-Cu layer remains similar, but due to the strong AF-coupling at the interfaces, the entire ferrimagnetic Fe-Cu/Tb entity flips its direction at a compensation field of around 3.7 kOe. This study shows that magnetic dilution can be an effective way to manipulate the possible domain walls or the clusters in TM and thereby the exchange bias in TM-RE systems.

The dynamical transverse magnetic Kohn-Sham susceptibility calculated within time-dependent density functional theory shows a fairly linear behavior for a finite energy window. This observation is used to propose a scheme where the computation of this quantity is greatly simplified. Regular simulations based on static density functional theory can be used to extract the dynamical behavior of the magnetic response function. Besides the ability to calculate elegantly damping of magnetic excitations due to electron-hole excitations, we derive along the way useful equations giving the main characteristics of these excitations: effective $g$ factors and the resonance frequencies that can be accessed experimentally using inelastic scanning tunneling spectroscopy or spin-polarized electron energy loss spectroscopy.

We calculate conductance and noise for quantum transport at the nodal point for arbitrarily tilted and anisotropic Dirac or Weyl cones. Tilted and anisotropic dispersions are generic in the absence of certain discrete symmetries, such as particle-hole and lattice point group symmetries. Whereas anisotropy affects the conductance $g$, but leaves the Fano factor $F$ (the ratio of shot noise power and current) unchanged, a tilt affects both $g$ and $F$. Since $F$ is a universal number in many other situations, this finding is remarkable. We apply our general considerations to specific lattice models of strained graphene and a pyrochlore Weyl semimetal.

We present numerical investigations based on the Luttinger-Kohn four-band $k·p$ theory and, accordingly, establish a quantitatively valid model of the excitonic fine structures of droplet epitaxial GaAs/AlGaAs quantum dots under uniaxial stress control. In the formalisms, stressing a photoexcited quantum dot is equivalent creating a pseudomagnetic field that is directly coupled to the pseudospin of the exciton doublet and tunable to tailor the polarized fine structure of exciton. The latter feature is associated with the valence-band mixing of exciton, which is especially sensitive to external stress in inherently unstrained droplet epitaxial GaAs/AlGaAs quantum dots and allows us to mechanically design and prepare any desired exciton states of QD photon sources prior to the photon generation.

Owing to their long phonon mean free paths (MFPs) and high thermal conductivity, carbon nanotubes (CNTs) are ideal candidates for, e.g., removing heat from electronic devices. It is unknown, however, how the intrinsic phonon MFPs depend on vibrational frequency in nonequilibrium. We determine the spectrally resolved phonon MFPs in isotopically pure CNTs from the spectral phonon transmission function calculated using nonequilibrium molecular dynamics, fully accounting for the resistive phonon-phonon scattering processes through the anharmonic terms of the interatomic potential energy function. Our results show that the effective room temperature MFPs of low-frequency phonons ($f<0.5$ THz) exceed $10\mu \mathrm{m}$, while the MFP of high-frequency phonons ($f\gtrsim 20$ THz) is in the range 10–100 nm. Because the determined MFPs directly reflect the resistance to energy flow, they can be used to accurately predict the thermal conductivity for arbitrary tube lengths by calculating a single frequency integral. The presented results and methods are expected to significantly improve the understanding of nonequilibrium thermal transport in low-dimensional nanostructures.

We demonstrate that the evolution of a two-dimensional electron gas system at an interface of a metal and the model topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ can be controlled by choosing an appropriate kind of metal element and by applying a low temperature evaporation procedure. In particular, we find that only topological surface states (TSSs) can exist at a Mn/${\mathrm{Bi}}_2{\mathrm{Se}}_3$ interface, which would be useful for implementing a TI-based device with surface current channels only. The existence of TSSs alone at the interface is confirmed by angle-resolved photoemission spectroscopy (ARPES). Based on the ARPES and core-level x-ray photoemission spectroscopy measurements, we propose a cation intercalation model to explain our findings.

By using Majorana's stellar representation, we give a clear geometrical interpretation of the topological phases of inversion-symmetric polymerized models by mapping the Bloch states of multiband systems to Majorana stars on the Bloch sphere. While trajectories of Majorana stars of a filled Bloch band exhibit quite different geometrical structures for topologically trivial and nontrivial phases, we further demonstrate that these structures are uniquely determined by distributions of Majorana stars of two high-symmetrical momentum states, which have different parities for topologically different states.

In this paper we present an accurate numerical scheme for extracting interatomic exchange parameters ($J_{ij}$) of strongly correlated systems, based on first-principles full-potential electronic structure theory. The electronic structure is modeled with the help of a full-potential linear muffin-tin orbital method. The effects of strong electron correlations are considered within the charge self-consistent density functional theory plus dynamical mean-field theory. The exchange parameters are then extracted using the magnetic force theorem; hence all the calculations are performed within a single computational framework. The method allows us to investigate how the $J_{ij}$ parameters are affected by dynamical electron correlations. In addition to describing the formalism and details of the implementation, we also present magnetic properties of a few commonly discussed systems, characterized by different degrees of electron localization. In bcc Fe, treated as a moderately correlated metal, we found a minor renormalization of the $J_{ij}$ interactions once the dynamical correlations are introduced. However, generally, if the magnetic coupling has several competing contributions from different orbitals, the redistribution of the spectral weight and changes in the exchange splitting of these states can lead to a dramatic modification of the total interaction parameter. In NiO we found that both static and dynamical mean-field results provide an adequate description of the exchange interactions, which is somewhat surprising given the fact that these two methods result in quite different electronic structures. By employing the Hubbard-I approximation for the treatment of the $4f$ states in hcp Gd we reproduce the experimentally observed multiplet structure. The calculated exchange parameters result in being rather close to the ones obtained by treating the $4f$ electrons as noninteracting core states.

We discuss the dephasing of quantum beat oscillation related to the intersubband transition between heavy-hole and light-hole excitons in a GaAs/AlAs multiple quantum well as measured by a pump-probe technique. We investigate the dependence of dephasing on temperature and pump energy. The analysis of the time-domain signals reveals that a strong quantum beat induces the rapid dephasing related to intersubband transition; the dependencies of the dephasing rate correspond to those of the amplitude. This implies that a quantum beat is useful in applications utilizing an ultrafast optical response.

We present a theoretical study of the evolution of spatial coherence during intraband relaxation between exciton states in a pair of vertically stacked semiconductor quantum dots coupled to acoustic phonons. We show that spatial coherence can be transferred to the ground state even in a system of uncoupled nonidentical quantum dots if a particular kind of degeneracy between the interlevel energy splittings is present. The phonon-assisted mechanism of coherence transfer leads to a dependence of the amount of the resulting coherence on the interdot distance and temperature. We analyze also the impact of carrier-phonon dynamics on a coupled system, where spatial coherence is present in the delocalized ground state.

We present experimental, numerical, and theoretical evidence for an unusual mode of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering experiments on 8-nm particles of hematite display a loss of diffraction intensity with temperature, the intensity vanishing around 150 K. However, the signal from inelastic neutron scattering remains above that temperature, indicating a magnetic system in constant motion. In addition, the precession frequency of the inelastic magnetic signal shows an increase above 100 K. Numerical Langevin simulations of spin dynamics reproduce all measured neutron data and reveal that thermally activated spin canting gives rise to an unusual type of coherent magnetic precession mode. This “rotor” mode can be seen as a high-temperature version of superparamagnetism and is driven by exchange interactions between the two magnetic sublattices. The frequency of the rotor mode behaves in fair agreement with a simple analytical model, based on a high-temperature approximation of the generally accepted Hamiltonian of the system. The extracted model parameters, such as the magnetic interaction and the axial anisotropy, are in excellent agreement with results from Mössbauer spectroscopy.

In a single-component Ginzburg-Landau model which possesses thermodynamically stable vortex excitations, the zero-field superconducting phase transition is second order even when fluctuations are included. Beyond the mean-field approximation the transition is described in terms of proliferation of vortex loops. Here we determine the order of the superconducting transition in an effective 3D vortex-loop model for the recently proposed multiband type-1.5 superconductors. The vortex interaction is nonmonotonic, i.e., exponentially screened and attractive at large separations, and short-range repulsive. We show that the details of the vortex interaction, despite its short-range nature, can lead to very different properties of the superconducting transition than found in type-1 and type-2 systems. Namely, the type-1.5 regime with nonmonotonic intervortex interaction can have a first-order vortex-driven phase transition not found in the single-band case.

The ferrimagnetic insulator yttrium iron garnet (YIG) has become an important material in the emergent field of spin caloritronics. Despite this and the fact that this material has been studied for over half a century, the thermal properties of magnons in YIG have not been sufficiently characterized, mainly because at not very low temperatures, they are overwhelmed by the contribution of phonons. Experimental attempts to characterize the magnon specific heat and thermal conductivity in YIG make use of large magnetic fields to freeze the magnon contributions and isolate those of phonons relative to their behavior at zero field. Here we present calculations of the magnon thermal properties in YIG under elevated magnetic fields using spin-wave theory. We show that at a temperature of $10\mathrm{K}$, a field of at least $300\mathrm{kOe}$ is necessary to decrease the magnon contributions to 10% of their zero-field values. With the results of the calculations, we reinterpret recent measurements of the magnon thermal properties in YIG at temperatures up to 20 K and a field of 70 kOe, and suggest a procedure to determine their values at room temperature with the use of a field of 300 kOe.

The rich phase diagrams of magnetically frustrated pyrochlores have maintained a high level of interest over the past 20 years. To experimentally explore these phase diagrams requires a means of tuning the relevant interactions. One approach to achieve this is chemical pressure, that is, varying the size of the nonmagnetic cation. Here, we report on the family of lead-based pyrochlores $A_2{\text{Pb}}_2{\text{O}}_7$ ($A$ = Pr, Nd, Gd), which we have characterized with magnetic susceptibility and specific heat. Lead is the largest known possible $B$-site cation for the pyrochlore lattice. Thus, these materials significantly expand the phase space of the frustrated pyrochlores. ${\text{Pr}}_2{\text{Pb}}_2{\text{O}}_7$ has an absence of long-range magnetic order down to 400 mK and a spin-ice-like heat capacity anomaly at 1.2 K. Thus, ${\text{Pr}}_2{\text{Pb}}_2{\text{O}}_7$ is a candidate for a quantum spin ice state, despite weaker exchange. ${\text{Nd}}_2{\text{Pb}}_2{\text{O}}_7$ transitions to a magnetically ordered state at 0.41 K. The Weiss temperature for ${\text{Nd}}_2{\text{Pb}}_2{\text{O}}_7$ is ${\theta }_{\mathrm{CW}}=-0.06$ K, indicating close competition between ferromagnetic and antiferromagnetic interactions. ${\text{Gd}}_2{\text{Pb}}_2{\text{O}}_7$ is a Heisenberg antiferromagnet that transitions to long-range magnetic order at 0.81 K, in spite of significant site mixing. Below its ordering transition, we find a $T^{3/2}$ heat capacity dependence in ${\text{Gd}}_2{\text{Pb}}_2{\text{O}}_7$, indication of a ground state that is distinct from other gadolinium pyrochlores. These lead-based pyrochlores provide insight into the effects of weakened exchange on highly frustrated lattices and represent further realizations of several exotic magnetic ground states which can test theoretical models.

We perform an extensive density-matrix renormalization-group study of the ground-state phase diagram of the spin-$1/2J_1\text{−}{J}_2$ Heisenberg model on the kagome lattice. We focus on the region of the phase diagram around the kagome Heisenberg antiferromagnet, i.e., at $J_2=0$. We investigate the static spin structure factor, the magnetic correlation lengths, and the spin gaps. Our results are consistent with the absence of magnetic order in a narrow region around $J_2\approx 0$, although strong finite-size effects do not allow us to accurately determine the phase boundaries. This result is in agreement with the presence of an extended spin-liquid region, as it has been proposed recently. Outside the disordered region, we find that for ferromagnetic and antiferromagnetic $J_2$, the ground state displays signatures of the magnetic order of the $\sqrt{3}\times \sqrt{3}$ and the $q=0$ type, respectively. Finally, we focus on the structure of the entanglement spectrum (ES) in the $q=0$ ordered phase. We discuss the importance of the choice of the bipartition on the finite-size structure of the ES.