We have laid out the results of a rigorous theoretical investigation into the response of electron dressed states, i.e., interacting Floquet states arising from the off-resonant coupling of Dirac pseudospin-1 electrons in $\alpha \text{−}{\mathrm{T}}_3$ lattices, to external radiation with various polarizations. Specifically, we have examined the role played by the hopping-scale parameter $\alpha$ that is a measure of the coupling strength with an additional atom at the center of the honeycomb graphene lattice and which, when varied, continuously gives rise to different Berry phases. We have found that the electronic properties of the $\alpha \text{−}{\text{T}}_3$ model (consisting of a flat band and two cones) could be significantly modified depending on the polarization of the imposed irradiation. We have demonstrated that under elliptically polarized light the low-energy band structure of such lattices will directly depend on the valley index. We have obtained and analyzed the wave functions, their symmetries, and the corresponding Berry phases, connections, and curvatures, and revealed that such field-modified geometry phases could be finite even for a dice lattice, which has not been observed in the absence of an optical dressing field. These results lead to possible radiation-induced band structure control and engineering, as well as experimental and technological realization of such optoelectronic and integrated photonic devices.

Finding new topological materials and understanding the physical essence of topology are crucial problems for researchers. We study the topological property based on several proposed Su-Schrieffer-Heeger (SSH) related models. We show that the topologically nontrivial phase (TNP) in the SSH model can exist under other model conditions and can even be caused by anisotropy; its origin has a close relation with the even-bond dimerization. Further study shows that the TNP is not determined by the periodicity of the system, but the parity of the two kinds of bonds plays a significant role for its formation. This finding reveals the topological invariant of the system and might supply potential extended topological materials for both theoretical and experimental research.

Recent experiments confirm that two-dimensional (2D) SnTe films possess strong in-plane ferroelectricity. For controlling or improving this 2D ferroelectricity, the substrate effects should be a prevailing concern. In this work first-principle calculations are performed to investigate the substrate effects on the ferroelectric polarization in this 2D system. Our results indicate that suitable substrates can significantly enhance the ferroelectricity of 2D SnTe. We investigated a series of metals, semiconductors, and insulator materials used as the substrates (Ni, Pd, Pt, Si, Ge, CaO, MgO, etc.), which have different interaction types and bonding strength with the 2D SnTe. For these SnTe/substrate heterostructures, both geometric and electronic structures are explored. It is found that in Pt- and MgO-supported SnTe films, the in-plane polar displacement is significantly enhanced, which can be understood under the theory of lone-pair electrons. Importantly, the substrates show very interesting controlling behaviors for the ferroelectricity of SnTe originating from electronic level, such as enhancing ferroelectric polarization and inducing magnetism. In SnTe/MgO(001) heterostructure, substrate-enhanced ferroelectricity is discussed in detail. In addition, substrate-induced magnetism of SnTe films is observed in Ni-supported SnTe, which indicates a possible scheme to obtain the multiferroics in 2D SnTe. This study suggests that it is a potential method to tune the ferroelectricity of 2D SnTe films by different substrates.

Neutron powder diffraction was used in operando to determine the macroscopic strain and piezoelectric coefficient as a function of applied electric field in a technically relevant actuator material. We were able to individually investigate the two coexisting phases in the material and reveal the origin of maximized strain at phase boundaries. Insight into the strain mechanisms gives evidence that, on average, the classic inverse piezoelectric effect does not apply for polycrystalline materials.

Finite-temperature disordered solid solutions and magnetic materials are difficult to study directly using first-principles calculations, due to the large unit cells and many independent samples that are required. In this work, we develop a combined cluster expansion and atomic displacement expansion, which we fit to first-principles energies, forces, and stresses. We then use the expansion to calculate thermodynamic quantities at nearly first-principles levels of accuracy. Our model naturally includes both configurational and vibrational entropy, including anharmonic contributions and structural phase transitions. In addition, we can treat coupling between atomic displacement and chemical or magnetic degrees of freedom. As examples, we use our expansion to calculate properties of ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$, magnetic MnO, Al with vacancies, and ${\mathrm{Ba}}_x{\mathrm{Sr}}_{1-x}{\mathrm{TiO}}_3$. Finally, we demonstrate that by treating all the relevant degrees of freedom explicitly, we can in some cases achieve improved convergence of fitting parameters versus distance as compared to a pure cluster expansion.

The nitrogen vacancy (NV) center in diamond is the most widely studied color center in diamond for its wide range of applications in both physics and biology. The negatively charged state ${\mathrm{NV}}^{-}$ is often favored for its better spin-optical properties, whereas the presence of the neutral charge state ${\mathrm{NV}}^0$ is often minimized or neglected by using a single optimized excitation wavelength. In many cases, the use of additional laser wavelengths has an important impact on the NV optical properties and can lead to a dramatically quenched fluorescence. Through measurements of the photophysics of the NV center we show in this work that this quenching mechanism is mediated by charge state interconversion providing additional channels for nonradiative decay. Our results show that this mechanism is also sensitive to the spin state of the ${\mathrm{NV}}^{-}$. A better understanding of this physical mechanism will help mitigate the spin and charge state depolarization effects that are detrimental for quantum technology applications.

We present ab initio calculations of the electronic temperature dependence of the magnetization, electronic energy, and specific heat of FePt ${L1}_{\text{0}}$, fcc-Ni, hcp-Co, and bcc-Fe. We find that atomic magnetic moments in each compound disappear at very high temperature, ranging from 3100 to 8100 K. However, for some compounds, the consequences of this phenomenon are noticeable on the electronic energy and specific heat even at low electronic temperature. Consequently, large deviations from the Sommerfeld approximation and from some previous work that did not take into account explicitly the dependence of the electronic structure on the electronic temperature are shown. Our results are of interest in the field of laser-induced ultrafast magnetization dynamics, since they provide a more precise estimate of the electronic specific heat that enters in the three-temperature model.

We analyze a system of two interacting spin-qubits subjected to a Landau-Majorana-Stückelberg-Zener (LMSZ) ramp. We prove that LMSZ transitions of the two spin qubits are possible without an external transverse static field since its role is played by the coupling between the spin qubits. We show how such a physical effect could be exploited to estimate the strength of the interaction between the two spin qubits and to generate entangled states of the system by appropriately setting the slope of the ramp. Moreover, the study of effects of the coupling parameters on the time behavior of the entanglement is reported. Finally, our symmetry-based approach allows us to discuss also effects stemming from the presence of a classical noise or non-Hermitian dephasing terms.

We analyze the evolution of the dynamics of a neutral $s$-wave superconductor between the BCS and BEC regimes. We consider 2d case when a BCS-BEC crossover occurs already at weak coupling as a function of the ratio of the two scales—the Fermi energy $E_F$ and the bound state energy for two fermions in a vacuum, $E_0$. The BCS and BEC limits correspond to $E_F\gg E_0$ and $E_F\ll E_0$, respectively. The chemical potential $\mu =E_F-E_0$ changes the sign between the two regimes. We use the effective action approach, derive the leading terms in the expansion of the effective action in the spatial and time derivative of the slowly varying superconducting order parameter $\mathrm{\Delta }(r,\tau )$, and express the action in terms of a derivatives of the phase $\varphi (r,\tau )$ of $\mathrm{\Delta }(r,\tau )=\mathrm{\Delta }{e}^{i\varphi (r,\tau )}$. The action contains ${\left(\nabla \varphi \right)}^2$ and ${\dot{\varphi }}^2$ terms, which determine the dispersion of collective phase fluctuations, and $i\pi A\dot{\varphi }$ term. For continuous $\varphi (r,\tau )$, the latter reduces to the contribution from the boundary and does not affect the dynamics. We show that this long-wavelength action does not change through the BCS-BEC crossover. We apply our approach to a moving vortex, for which $\varphi$ is singular at the center of the vortex core, and $i\pi A_{\mathrm{vort}}\dot{\varphi }$ term affects vortex dynamics. We find that this term has two contributions. One comes from the states away from the vortex core and has $A_{\mathrm{vort},1}=n/2$, where $n$ is the fermion density. The other comes from electronic states inside the vortex core and has $A_{\mathrm{vort},2}=-n_0/2$, where $n_0$ is the fermion density at the vortex core. This last term comes from the continuous part of the electronic spectrum and has no contribution from discrete levels inside the core; it also does not change if we add impurities. We interpret this term as the contribution to vortex dynamics in the continuum limit, when the spacing between energy levels $\omega$ is set to zero, while fermionic lifetime $\tau$ can be arbitrary. The total $A_{\mathrm{vort}}=(n-n_0)/2$ determines the transversal force acting on the vortex core, $\pi A_{\mathrm{vort}}\dot{\mathbf{R}}\times \widehat{z}$, where $\dot{\mathbf{R}}$ is the velocity of the vortex core and $\widehat{z}$ a unit vector perpendicular to the 2d sample. The difference $(n-n_0)/2$ changes through the BEC-BCS crossover as $n_0$ nearly compensates $n$ in the BCS regime, but vanishes in the BEC regime.

Decoherence processes in crystals of molecular magnets are prototypical for interacting electronic spin systems. We analyze the Landau-Zener dynamics of the archetypical ${\mathrm{TbPc}}_2$ complex diluted in a diamagnetic monocrystal. The dependence of the tunneling probability on the field sweep rate is evaluated in the framework of the recently proposed master equation in which the decoherence processes are described through a phenomenological Lindblad operator. Thus, we showcase low-temperature magnetic measurements that complement resonant techniques in determining small tunnel splittings and dephasing times.

Recent experiments revealed non-Fermi-liquid resistivity in the unconventional superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ when strain pushes one of the Fermi surfaces close to a van Hove singularity. The origin of this behavior and whether it can be understood from a picture of well-defined quasiparticles is unclear. We employ a Boltzmann transport analysis beyond the single relaxation-time approximation based on a single band which undergoes a Lifshitz transition where the Fermi surface crosses a van Hove singularity either due to uniaxial or epitaxial strain. First, analytically investigating impurity scattering, we clarify the role of the diverging density of states together with the locally flat band at the point of the Lifshitz transition. Additionally, including electron-electron scattering numerically, we find good qualitative agreement with resistivity measurements on uniaxially strained ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, including the temperature scaling and the temperature dependence of the resistivity peak. Our results imply that, even close to the Lifshitz transition, a description starting from well-defined quasiparticles holds. To test the validity of Boltzmann transport theory near a van Hove singularity, we provide further experimentally accessible parameters, such as thermal transport, the Seebeck coefficient, and Hall resistivity and compare different strain scenarios.

A complete set of infrared-active and Raman-active lattice modes is obtained from density functional theory calculations for single-crystalline centrosymmetric orthorhombic neodymium gallate. The results for infrared-active modes are compared with an analysis of the anisotropic long-wavelength properties using generalized spectroscopic ellipsometry. The frequency-dependent dielectric function tensor and dielectric loss function tensor of orthorhombic neodymium gallium oxide are reported in the spectral range of $80–1200{\mathrm{cm}}^{-1}$. A combined eigendielectric displacement vector summation and dielectric displacement loss vector summation approach augmented by considerations of lattice anharmonicity is utilized to describe the experimentally determined tensor elements. All infrared-active transverse and longitudinal optical mode pairs obtained from density functional theory calculations are identified by our generalized spectroscopic ellipsometry investigation. The results for Raman-active modes are compared to previously published experimental observations. Static and high-frequency dielectric constants from theory as well as experiment are presented and discussed in comparison with values reported previously in the literature.

While the influence of dipolar interactions on the spin-wave dispersion in ferromagnets with localized magnetic moments has been studied in detail, similar studies in itinerant electron systems are rather scarce due to experimental difficulties. Using the newly developed neutron Larmor precession technique MIEZE in a longitudinal field configuration, we succeeded to map out the spin-wave dispersion in Fe at small momentum $q$ and energy transfers $E$. The results demonstrate an excellent agreement of the magnon dispersion with the Holstein-Primakoff theory, which takes the dipolar interactions into account. At larger $q$, the data is in agreement with previous investigations by Collins et al., Phys. Rev. 179, 417 (1969). The $q$ dependence of the linewidth of the magnons is proportional to $q^{2.5}$ in agreement with dynamical scaling theory. The critical exponent for the stiffness, $\mu =0.35\pm 0.01$, agrees with field theory. The spin dynamics in Fe is now explored by neutron scattering over an energy range $15\mu \mathrm{eV}<E_{\mathrm{sw}}<120\mathrm{meV}$, i.e., over about four orders of magnitude in energy.

The influence of the amplitude of an external magnetic field ($H$) and femtosecond laser pulse fluence ($F$) on ultrafast magnetization dynamics has been investigated in a ferromagnetic ${\mathrm{Ni}}_{54.3}{\mathrm{Mn}}_{31.9}{\mathrm{Sn}}_{13.8}$ Heusler-alloy film using the time-resolved magneto-optical Kerr effect. A large slowing down of the demagnetization process was observed and characteristic parameters of magnetization precession were determined for a wide range of $H$ and $F$ values. Long demagnetization times of the order of hundreds of picoseconds have been found and explained as a result of the Curie temperature ($T_{\mathrm{C}}$) proximity in the alloy film studied. Effective magnetic anisotropy field ($H_{\text{k}}^{\text{eff}}$) and Gilbert damping parameter dependencies were determined. A significant reduction of the precession frequency versus $F$ of the uniform Kittel mode was found. A strong decrease of $H_{\text{k}}^{\text{eff}}$ with $F$ was well simulated in the frame of an extended version of the microscopic three-temperature model (eM3TM), and explained by the $T_{\mathrm{C}}$ proximity effect. The estimated low values of the eM3TM model parameters, the demagnetization rate and electron-lattice coupling constant, appeared essential to explain the slowing down effect of demagnetization. Precession amplitude dependencies were explained by a phenomenological approach taking into account $H_{\text{k}}^{\text{eff}}$ and changes of the equilibrium magnetization angles induced by pump-pulse excitation.

We study the famous example of weakly first-order phase transitions in the $\left(1+1\right)$-dimensional quantum $Q$-state Potts model with $Q>4$. We numerically show that these weakly first-order transitions have approximately conformal invariance. Specifically, we find that entanglement entropy on considerably large system sizes fits perfectly with the universal scaling law of this quantity in conformal field theories (CFTs). This supports that the weakly first-order transitions are proximate to complex fixed points, which are described by recent conjectured complex CFTs. Moreover, the central charge extracted from this fitting is close to the real part of the complex central charge of these complex CFTs. We also study the conformal towers and the drifting behaviors of these conformal data (e.g., central charge and scaling dimensions).

We investigate both the conductance and the shot-noise power of a confined chiral device that engenders subtle embedded backscattering mechanisms. We present analytical results and the correspondent numerical confirmation of the chiral electronic sublattice signal. Examples of quantum dots generating chiral symmetries include graphene sheets and topological insulators. The analytical results are universal and exhibit a robust and peculiar signal for an arbitrary number of open scattering channels. We also demonstrate a tunable mechanism of the valleytronics shot-noise power signal through perpendicular magnetic fields and/or the device symmetry edges. The results also indicate a “Fano factor” associated with the main quantum interference term with a universal value of $1/4$ for a quantum dot with symmetric contacts, regardless of external fields and the number of open channels.

The electronic structure of the clean and stoichiometric $\mathrm{GaN}\left(10\overline{1}0\right)$ cleavage surface is investigated in a comprehensive angle-resolved photoelectron spectroscopy study. A clear distinction between surface and bulk related features allows us to measure the dispersions of the occupied surface state band from the N dangling bonds as well as of the uppermost bulk valence bands and to extract the effective hole masses directly with high precision. This is performed along various directions in $k_{\parallel }$ space providing a detailed picture of the electronic band dispersion. The obtained results show three separated bulk bands without indications of a crossing of these bands as commonly predicted in theoretical works. Moreover, from the observed Fermi-level pinning we determine the position of the minimum of the empty Ga-derived surface state band, which is found deep within the fundamental band gap at $\sim 2.3$ eV above the valence band maximum.

The chiral-induced spin selectivity effect is of fundamental importance in further understanding the thermospin transport in double-strand-DNA (dsDNA) molecules. Here we propose a hybrid dsDNA-based thermoelectric device made of a chiral dsDNA molecule coupled to normal-metal and superconducting leads for exploring the spin-dependent thermoelectricity. We give a comparison of the thermoelectric responses of the dsDNA system and the quantum dot system in the presence of superconductivity, and find that the former does not rely on external means to break the particle-hole symmetry due to the intrinsic properties of dsDNA molecules. Our findings show that spin-thermoelectric coefficients can go beyond charge thermoelectric ones, and even a pure spin thermopower can be generated by adjusting the external magnetic field and the temperature, which indicates that this chiral dsDNA hybrid junction can in practice act as a pure spin-current generator. Furthermore, the sizable spin and charge thermoelectric conversion efficiencies can be achieved by tuning system parameters. Our results open opportunities for fabricating spin caloritronic devices based on chiral molecules.

We propose a realization of chiral Majorana modes propagating on the hinges of a 3D antiferromagnetic topological insulator, which was recently theoretically predicted and experimentally confirmed in the tetradymite-type ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$-related ternary chalgogenides. These materials consist of ferromagnetically ordered 2D layers, whose magnetization direction alternates between neighboring layers, forming an antiferromagnetic order. Besides surfaces with a magnetic gap, there also exist gapless surfaces with a single Dirac cone, which can be gapped out when proximity coupled to an $s$-wave superconductor. On the sharing edges between the two types of gapped surfaces, the chiral Majorana modes emerge. We further propose experimental signatures of these Majoana hinge modes in terms of two-terminal conductance measurements.