The fine structure and temperature evolution of infrared spectra have been intensively used to probe the nature of Jahn-Teller dynamics in correlated materials. At the same time, a theoretical framework to adequately extract the information on the complicated vibronic dynamics from infrared spectra is still lacking. In this work, the first-principles theory of the infrared spectra of dynamical Jahn-Teller system is developed and applied to the Mott-insulating ${\mathrm{Cs}}_3{\mathrm{C}}_{60}$. With the calculated coupling parameters for Jahn-Teller and infrared active vibrational modes, the manifestation of the dynamical Jahn-Teller effect in infrared spectra is elucidated. In particular, the temperature evolution of the infrared line shape is explained. The transformation of the latter into Fano resonance type in metallic fulleride is discussed on the basis of obtained results.

The vitrification of single-element metallic glass remains a challenging research topic and attracts a great deal of attention. It is reported that the propensity for formation of the amorphous phase increases as the size of the system decreases in experiments. However, the thermodynamic contributions to this phenomenon are still elusive. Here, we study the size effects on the thermodynamic properties of the Cu nanoparticles in both crystalline and amorphous phases using the Frenkel-Ladd methods. Smaller particles show stronger size dependences on these quantities. From the perspective of free-energy difference between the amorphous phase and the crystalline phase, the thermodynamic driving force for vitrification of single-element metallic liquids catches up with that for crystallization dramatically, given the particle diameter smaller than a critical value (i.e., $\sim 4$ nm for Cu), which provides a thermodynamic interpretation for the size dependence of the glass-forming ability.

We employ the planar Hall effect to sensitively probe the in-plane magnetization of a ferromagnetic thin film using a nearly perpendicular applied field. We establish that a significant change in coercivity occurs upon exposure to a wide variety of ligands, regardless of whether the film is capped with a diamagnetic overlayer or not. In the overlayer case, we believe that the change is due to a modified magnetic response of the diamagnetic capping metal. The size of the shift varies with ligand hardness, alkane chain length, magnetic sensor layer material, nonmagnetic overlayer material, and the thickness of each metal layer. The effect continues to be observable for thicknesses of at least 100 nm of Au deposited on 30 nm of Co. Au overlayers are shown to be more sensitive than Cu and Ag, both of which display shifts statistically indistinguishable from zero. Moreover, Au surfaces exposed to 1-dodecylthiol exhibit a larger effect than the same surface exposed to ethanol, the ligand with the greatest ligand strength examined. We demonstrate that the source of the effect is not magnetic contamination.

A comprehensive calculation of the solute formation energies, solute-solute and vacancy-solute binding energies in bcc iron is reported. An extended set of solutes with atomic numbers 1 to 54 has been considered. We used the projector augmented wave method of density functional theory with the generalized gradient approximation for the exchange-correlation energy functional. The prominent results are the following: (1) formation energies of solutes from fourth and fifth periods vary with their atomic numbers such that they reach maxima near the ends of the periods and a minimum in between, with a local increase near Cu and Ag (like a quasiparabolic valley). Solutes from second and third periods show similar trend like the elements near the ends of the fourth and fifth periods. (2) The size factors of the solutes also show similar variation with their atomic numbers like their formation energies. These trends corroborate the relatively smaller formation energies and size factors of the common alloying additions to Fe (such as $3d,4d$, and $sp$ elements) as compared to solutes that lack solubility (such as Li, Na, K, Rb, He, Ne, Ar, Kr, Xe, F, Cl, Br, I, Mg, Ca, Sr, Ag, Cd, In, Y). (3) The solubilities estimated from our formation energies are found to be in reasonable agreement with those from the phase diagram database. (4) Solute-solute and vacancy-solute binding energies are found to vary with the atomic number of the solutes in a manner inverse to solute formation energies and solute size factors, reaching strong binding energies near the ends of the periods which generally include the insoluble elements. (5) Another trend revealed by our work is that the size factors of isoelectronic sets of solutes increase down the groups with associated increase of formation energies, and strength of solute-solute and vacancy-solute binding energies. (6) A significant correlation is found between our vacancy-solute binding energies of $3d$ and $4d$ elements and the corresponding diffusion coefficients from literature whereby solutes with strong binding energies have higher diffusion coefficients and vice versa.

The crystal structures and electronic structures of lead dioxide are investigated with ab initio calculations. We find the previously known tetragonal $\beta \text{−}\mathrm{Pb}{\mathrm{O}}_2$ (space group: $P{4}_2/mnm$) is dynamically unstable at low temperature due to the existence of a vibrational soft mode under the harmonic approximation, and it becomes stable at finite temperature about 200 K attributed to the enhanced anharmonic effect. Under the guidance of the vibrational soft mode, we find an orthorhombic structure (space group: Pnnm) that is a candidate of the ground state of $\mathrm{Pb}{\mathrm{O}}_2$ at low temperature. Electronic structure calculations suggest the low-temperature orthorhombic phase is a trivial insulator, while the tetragonal high-temperature phase is a topological Dirac nodal line semimetal. Therefore, there is a topological phase transition from a trivial insulator to a topological semimetal in $\mathrm{Pb}{\mathrm{O}}_2$, strongly coupled with the stabilized soft mode and thus anharmonicity-driven structural transformation. For future experiment verification, the results for the temperature dependence of x-ray diffraction and Raman spectra are also provided. Our work demonstrates the important role of anharmonicity in describing the structural phase transition and exploits the impact of anharmonicity to the topological property of the system.

We employ an atomic-scale theory within the framework of nonaffine lattice dynamics to uncover the origin of the Johari-Goldstein (JG) $\beta$-relaxation in metallic glasses (MGs). Combining simulation and experimental data with our theoretical approach, we reveal that the large mass asymmetry between the elements in a ${\mathrm{La}}_{60}{\mathrm{Ni}}_{15}{\mathrm{Al}}_{25}$ MG leads to a clear separation in the respective relaxation timescales, giving strong evidence that JG relaxation is controlled by the lightest atomic species present. Moreover, we show that only qualitative features of the vibrational density of states determine the overall observed mechanical response of the glass, paving the way for a possible unified theory of secondary relaxations in glasses.

Electric field reversal of large magnetization has been achieved in the multiferroic hexaferrite $\mathrm{B}{\mathrm{a}}_{0.4}\mathrm{S}{\mathrm{r}}_{1.6}\mathrm{M}{\mathrm{g}}_2\mathrm{F}{\mathrm{e}}_{12}{\mathrm{O}}_{22}$ with conical spin structures. The variation of magnetization depends on the sweeping rate of the applied electric field, leading to a significant fatigue effect. Such a slow dynamical behavior is attributed to the hysteresis magnetization process facilitated by the electric field. Moreover, the $M\text{−}H$ hysteresis loop can be manipulated by external electric fields, causing either a positive or negative exchange bias phenomenon. The role of the electric field acting as a bias magnetic field on the magnetic hysteresis loop is discussed in terms of the pinning of magnetic domains by conjugated ferroelectric domains.

We study the tunneling phenomenon of a ladder system with a Peierls distortion in a magnetic flux, and the response of electrons in the insulator is investigated when the tunneling current flows on one-dimensional gapped chains along the external electric field. Without the magnetic field, the ladder system is insulated by the charge density wave order. Then, by the increase of the magnetic field, it becomes metallic with the disappearance of the distortion of the lattice. Finally, the gap appears, and it becomes an insulator. At the metallic state, the topological transition also occurs. To show this phenomenon, we consider the distortion by the phonon in the ladder model, and calculate the distortion gap and the transition probability by using both Landau-Zener formula and the instanton method. The transition to the metallic states will be applied to the current control by the magnetic field.

In transition metal dichalcogenides (TMDCs) the spin-orbit interaction affects differently the conduction and valence band energies as functions of the wave vector $k$ and the band gap is usually large except in few TMDCs that are metallic without band gaps. Consequently, when a perpendicular magnetic field $B$ is applied, the conduction and valence band Landau levels are also different and this leads to a splitting of the interband optical absorption lines in both the absence and presence of an external electric field $E_z$. When $B$ and $E_z$ are present, the peaks in the imaginary part of the Hall conductivity give two distinct contributions of opposite sign to the interband spectrum. The real part of the right- and left-handed interband conductivity, however, retains its two-peak structure but the peaks are shifted in energy and amplitude with respect to each other in contrast with graphene. The response of the intraband conductivity is significantly modified when the Fermi energy $E_F$ and the field $B$ are varied. Its optical spectral weight is found to increase with $E_F$ in contrast with the decrease observed in graphene. Further, the position and amplitude of the intraband response depend on the field $B$. The absorption peaks vary linearly with $B$ for all fields similar to bilayer graphene for low fields but in contrast with the high-field $\sqrt{B}$ dependence in it.

On the road of searching for Majorana zero modes (MZMs) in topological insulator-based Josephson junctions, a highly sought signature is the protected full transparency of electron transport through the junctions due to the existence of the MZMs, associated with complete gap closing between the electronlike and holelike Andreev bound states (ABSs). Here we present direct experimental evidence of gap closing and full transparency in single Josephson junctions constructed on the surface of three-dimensional topological insulator (3D TI) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our results demonstrate that the two-dimensional (2D) surface of 3D TIs provides a promising platform for hosting and manipulating MZMs.

Approaching a quantum critical point (QCP) has been an effective route to stabilize superconductivity. While the role of magnetic QCPs has been extensively discussed, similar exploration of a structural QCP is scarce due to the lack of suitable systems with a continuous structural transition that can be conveniently tuned to 0 K. Using inelastic x-ray scattering, we examine the phonon spectrum of the nonmagnetic quasiskutterudite $({\mathrm{Ca}}_x{\mathrm{Sr}}_{1-x}{)}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$, which represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration $x$. We unambiguously detect the softening of phonon modes around the M point on cooling towards the structural transition. Intriguingly, at $x=0.85$, the soft mode energy squared at the M point extrapolates to zero at $(-5.7\pm 7.7)$ K, providing the first compelling microscopic evidence of a structural QCP in $({\mathrm{Ca}}_x{\mathrm{Sr}}_{1-x}{)}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$. The enhanced phonon density of states at low energy provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP.

The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6–13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

We show how transitions between different Lifshitz phases in bilayer Dirac materials with and without spin-orbit coupling can be studied by driving the system. The periodic driving is induced by a laser, and the resultant phase diagram is studied in the high-frequency limit using the Brillouin-Wigner perturbation approach to leading order. The examples of such materials include bilayer graphene and spin-orbit coupled materials, such as bilayer silicene. The phase diagrams of the effective static models are analyzed to understand the interplay of topological phase transitions with changes in the Chern number and topological Lifshitz transitions with the ensuing changes in the Fermi surface. Both the topological transitions and the Lifshitz transitions are tuned by the amplitude of the drive.

A quantum system of particles can exist in a localized phase, exhibiting ergodicity breaking and maintaining forever a local memory of its initial conditions. We generalize this concept to a system of extended objects, such as strings and membranes, arguing that such a system can also exhibit localization in the presence of sufficiently strong disorder (randomness) in the Hamiltonian. We show that localization of large extended objects can be mapped to a lower-dimensional many-body localization problem. For example, motion of a string involves propagation of pointlike signals down its length to keep the different segments in causal contact. For sufficiently strong disorder, all such internal modes will exhibit many-body localization, resulting in the localization of the entire string. The eigenstates of the system can then be constructed perturbatively through a convergent “string locator expansion.” We propose a type of out-of-time-order string correlator as a diagnostic of such a string localized phase. Localization of other higher-dimensional objects, such as membranes, can also be studied through a hierarchical construction by mapping onto the localization of lower-dimensional objects. Our arguments are “asymptotic” (i.e., valid up to rare regions) but they extend the notion of localization (and localization protected order) to a host of settings where such ideas previously did not apply. These include high-dimensional ferromagnets with domain wall excitations, three-dimensional topological phases with looplike excitations, and three-dimensional type-II superconductors with flux line excitations. In type-II superconductors, localization of flux lines could stabilize superconductivity at energy densities where a normal state would arise in thermal equilibrium.

We present a detailed study of the thermal dependence of the neutron powder diffraction (NPD) of $R\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}{\mathrm{O}}_3$ perovskites ($R=\mathrm{Tm}$, Er, Ho, Dy, and Tb) combined with a complete characterization of their magnetic, electric transport and ferroelectric properties. All samples are described with an orthorhombic (Pbnm) crystallographic structure. The inverse of magnetic susceptibility at high temperature shows an effective magnetic moment with contributions of high-spin $\mathrm{C}{\mathrm{o}}^{3+}$, $\mathrm{F}{\mathrm{e}}^{3+}$, and the corresponding rare-earth cations ($R^{3+}$) moments. Negative Curie-Weiss temperatures indicate antiferromagnetic (AFM) correlations between the magnetic ions. At low temperature, the reduced magnetic saturation values are associated with crystal-field effects on $R^{3+}$ ions. Below 300 K, magnetic reflections in NPD data show that the spin configuration in the ordered state is AFM type-G with a weak ferromagnetic component along the $c$ axis (${\mathrm{\Gamma }}_4$) for transition metal ions. Spin reorientation (SR) transitions are observed, changing the irreducible representation from ${\mathrm{\Gamma }}_4$ ($G_x$) to ${\mathrm{\Gamma }}_2\left(G_z\right)$ at low temperatures, except for the Ho compound where it changes from ${\mathrm{\Gamma }}_4$ ($G_x$) to ${\mathrm{\Gamma }}_1\left(G_y\right)$. Magnetization data under field cooling (FC) and zero field cooling (ZFC) protocols show reversal magnetization for $R=\mathrm{Er}$ and Tb. A significant change of the slope of the M vs T curves is associated with the onset of magnetic order ($T_{N1}\sim 250\mathrm{K}$) of Fe/Co sublattice. The electric conductivity at room temperature shows that Tm value is ten times less than the other rare earths. The temperature dependence of the electrical conductivity can be described with a variable range hopping model ($\ln \sigma \sim T^{-1/4}$). Also, in the Tm case, the complex electrical permittivity shows a different behavior in comparison with the other members of the series. Finally, the Tm sample can be polarized when the specimen is cooled under different values of electrical field (E). Pyroelectric current can be detected while the sample is warming and depolarized, showing the ferroelectric critical temperature ($T_C$) at $T_{N1}$. A low electric polarization of $400\mu \mathrm{C}/{\mathrm{m}}^2$ was measured for $\mathrm{TmF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}{\mathrm{O}}_3$.

The magnetic proximity effect (MPE) is a well-established magnetic phenomenon that occurs at certain heavy-metal (HM)/ferromagnet (FM) interfaces. However, there is still an active debate as to whether the presence of a MPE affects spin transport through such a HM/FM interface. Here we demonstrate that the MPE at Pt/Co and $\mathrm{A}{\mathrm{u}}_{0.25}\mathrm{P}{\mathrm{t}}_{0.75}/\mathrm{Co}$ interfaces can be enhanced substantially by thermal annealing protocols. From this ability, we show that the MPE has no discernible influence on either the dampinglike or the fieldlike spin-orbit torques exerted on the FM layer due to the spin Hall effect of the HM layer, indicating a minimal role of the MPE compared to other interfacial effects, e.g., spin memory loss and spin backflow.

A quantitative descriptor of local atomic environments is often required for the analysis of atomistic data. Descriptors of the local atomic environment ideally provide physically and chemically intuitive insight. This requires descriptors that are low-dimensional representations of the interplay between atomic geometry and electronic bond formation. The moments of the local density of states relate the atomic structure to the electronic structure and bond chemistry. This makes it possible to construct electronic structure based descriptors of the local atomic environment that have an immediate relation to the binding energy. We show that a low-dimensional moments-descriptor is sufficient as the lowest moments, calculated from the closest atomic neighborhood, carry the largest contributions to the local bond energy. Here, we construct moments-descriptors that project the space of local atomic environments on a two-dimensional map. We discuss in detail the separation of various atomic environments and their connections in the map. The distances in the map may be related to energy differences between local atomic environments as we show by analytic considerations based on analytic bond-order potentials and by numerical assessment using tight-binding and density-functional theory calculations. Possible applications of the proposed moments-descriptors include the classification of local atomic environments in molecular-dynamic simulations, the selection of structure sets for developing and testing interatomic potentials, as well as the construction of descriptors for machine-learning applications.

Understanding the phase behavior of semiconductor materials is important for applications in solid state physics and nanoscience. Accurate experimental data is often difficult to obtain due to strong kinetic effects. In this work, we calculate the temperature-pressure phase diagram of lead selenide (PbSe) using the random phase approximation (RPA), an accurate wave-function-based many-body technique. We consider three crystalline phases, the low pressure B1 phase (NaCl type), the intermediate B33 phase (CrB type), and the high pressure B2 phase (CsCl type). The electronic contributions to the free energy (at $T=0\text{K}$) are calculated in the Born-Oppenheimer approximation using the RPA, whereas phononic contributions are computed in the quasiharmonic approximation using density functional theory and the PBEsol functional. At room temperature, we find transition pressures of $4.6\pm 0.3\mathrm{GPa}$ for the B1 $\leftrightarrow$ B33 transition and $18.7\pm 0.3\mathrm{GPa}$ for the B33 $\leftrightarrow$ B2 transition, in good agreement with experiments. In contrast to the interpretation of recent experiments, we observe a negative Clapeyron slope for both transitions. Gibbs free energy differences between competing structures have small gradients close to coexistence, consistent with pronounced hysteresis observed in experiments. The phase diagram presented in this work can serve as a reference for future studies of PbSe and should prove useful in the development of accurate and efficient force fields.

${\mathrm{CaFe}}_2{\mathrm{O}}_4$ possesses an intriguing crystal structure characterized by the presence of ${\mathrm{FeO}}_6$ octahedra that share both edges and corners, and featuring zigzag Fe chains that are assembled in a honeycomb tube network. The nominal ${\mathrm{CaFe}}_2{\mathrm{O}}_4$ has been identified to show a long-range antiferromagnetic spin ordering of Néel temperature $T_N\sim 184$ K. From dc and ac magnetic susceptibility measurements on both polycrystalline and single-crystal samples, a random distribution of ferrimagnetically ordered clusters of Fe spins is proposed to exist in the oxygen deficient ${\mathrm{CaFe}}_2{\mathrm{O}}_{4-\delta }$. The ferrimagnetic ordering is proposed, coming from the oxygen vacancy-induced ${\mathrm{Fe}}^{3+}$ to ${\mathrm{Fe}}^{2+}$ conversion for the antiferromagnetically coupled spins of ${\mathrm{Fe}}^{2+}$ ions in the low-spin state and the ${\mathrm{Fe}}^{3+}$ ions in the high-spin state, which leads to an incomplete cancellation of staggered magnetization below $T_N$. Current model reasonably explains the inconsistencies found in the literature regarding the persistent ferromagnetic component for ${\mathrm{CaFe}}_2{\mathrm{O}}_4$, having a confirmed antiferromagnetic long-range spin ordering from neutron diffraction studies. We calculate the antiferromagnetic spin structure and the parameters of a Heisenberg Hamiltonian via spin-polarized density functional theory, obtaining results that are consistent, to a very high degree, with our experimental results for ac and dc magnetic susceptibility.