Magneto-elastic phase transition in a linear chain within the double and super-exchange model

In this work a magneto-elastic phase transition in a linear chain was obtained due the interplay between magnetism and lattice distortion in a double and super-exchange model. We consider a linear chain consisting of classical localized spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by the anti-ferromagnetic super-exchange interaction between neighbor localized spins. Additionally, the lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. The phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.     

On the phase diagram of a two-dimensional electron–hole system

The phase diagram for a system of spatially separated electrons and holes in coupled quantum wells or graphene double layers is studied in the framework of a BCS-like mean-field approach and a Landau expansion in terms of the pairing order parameter. We find a second order transition between an electron–hole plasma and a BCS phase, as well as a first-order transition between the BCS phase and a bosonic Mott phase of tightly bound electron–hole pairs without phase coherence. The electron–hole plasma exists at low and at high densities for weak interaction, the BCS phase at moderate density and the Mott phase at high density and strong interaction.     

Novel aspects of charge and lattice dynamics in the hole-doped manganite La0.67Sr0.33MnO3

Previous infrared studies on the hole-doped manganite La_0.67Sr_0.33MnO₃ (LSMO) have analysed its charge dynamics in terms of one type of charge carrier despite evidence of both electron and hole Fermi surfaces. Here, we investigate the charge dynamics of an LSMO film with infrared and optical spectroscopy in order to provide a complete picture of metallic conduction. In the ferromagnetic metallic phase, the low-frequency optical conductivity is best explained by a two-carrier model comprising electrons and holes. The number densities, effective masses and relaxation response of the delocalized electrons and holes are quantified. We discover that only one-third of the doped charges are coherent and contribute to the dc transport. Metallic LSMO cannot be classified as a bad metal at low temperatures because the mean free path of the coherent, mobile charge carriers exceeds the Ioffe–Regel–Mott limit. The incoherent spectral response of the doped charges manifests itself as a broad mid-infrared feature. We also report the first observation of splitting of an infrared-active phonon due to local Jahn–Teller distortion in the vicinity of the thermally driven transition to the nonmetallic, paramagnetic phase in LSMO. This demonstrates that infrared spectroscopy is capable of detecting the presence of local lattice distortions in correlated electron systems.     

Quantum crystallization of two-dimensional dipole systems

An analysis is made of the region of existence of crystalline order in a system of spatially separated electrons (e) and holes (h) in two coupled quantum wells for various concentrations n, temperatures T, and distances D between the layers. A study is also made of crystallization in a system of electrons in semiconductor structures near a metal electrode for various distances d between the semiconductor and the metal. Calculations of the crystalline phase were made using variational calculations of the ground-state energy of the system allowing for pairing of quasiparticles with nonzero momentum. For a system of two coupled quantum wells, regions in (T,n,D) space are determined in which electron (or hole) charge-density waves exist in each layer and regions where these charge-density waves are in phase, in other words, indirect excitons (or pairs with spatially separated electrons and holes) interacting as electric dipoles, become crystallized. In the electron system in semiconductor structures near a metal electrode, regions of existence of an electron crystal are also obtained in (T,n,D) space, where over large distances the electrons interact as electric dipoles because of image forces. Fiz. Tverd. Tela (St. Petersburg) 40, 1350–1355 (July 1998)     

Thermoelectric power of HoBaCo2O5.5: possible evidence of the spin blockade in cobaltites

Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, HoBaCo2O5.5, in which the alternative layers of CoO6 octahedra and of [CoO(5)](2) bipyramids are occupied by Co3+ species. The T-dependent Seebeck coefficient S shows a clear change of the conduction regime at the metal-insulator (MI) transition (T(MI) approximately 285 K). The sign change of S from S<0 to S>0 can be explained assuming that a spin state transition occurs at T(MI). In the metallic state, Co2+ e(g) electrons are moving in a broad band on the background of high or intermediate spin Co3+ species. In contrast, the insulating behavior may result from the Co3+ spin state transition to a low-spin Co3+ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin Co(4+)t(2g) holes, whereas the high-spin Co2+ electrons become immobilized due to a spin blockade.     

Modeling of Nonthermal Solid‐to‐Solid Phase Transition in Diamond Irradiated with Femtosecond x‐ray FEL Pulse

In this contribution we review in detail our recently developed hybrid model able to trace simultaneously nonequilibrium electron kinetics, evolution of an electronic structure, and eventually nonthermal phase transition in solids irradiated with femtosecond free‐electron laser pulses. Diamond irradiated with an ultrashort intense x‐ray pulse serves as an example to show how an irradiated material undergoes an ultrafast phase transition on sub‐picosecond timescales. The transition of diamond into graphite is induced by an excitation of electrons from the valence band into the conduction band, which, in turn, induces a rapid change of the interatomic potential. Our theoretical model incorporates: a Monte‐Carlo method for tracing high‐energy electrons and K‐shell holes in diamond; a temperature equation for the valence‐band and low‐energy conduction‐band electrons; a tight binding method for calculation of the evolving electronic structure of the material and potential energy surfaces; and molecular dynamics propagating atomic trajectories. This unified approach predicts the damage threshold of diamond in a good agreement with experimentally measured values. It reveals a multi‐step nature of nonthermal phase transition being an interplay between electronic excitation, changes of the band structure, and atomic reordering. An effect of pulse parameters, such as photon energy and temporal pulse shape, on the phase transition is discussed in detail. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nature of e(g) electron order in La(1-x)Sr(1+x)MnO(4)

X-ray scattering measurements of the low-temperature structure of La(1-x)Sr(1+x)MnO(4) ( 0.33< or =x< or =0.67) indicate the existence of three distinct regions: a disordered phase (x<0.4), a charge-ordered phase (x> or =0.5), and a mixed phase (0.4< or =x<0.5). For x>0.5, the modulation vector associated with the charge order is incommensurate with the lattice and depends linearly on the concentration of e(g) electrons. The primary superlattice reflections are strongly suppressed along the modulation direction and the higher harmonics are weak, implying the existence of a largely transverse and nearly sinusoidal structural distortion, consistent with a charge-density wave of the e(g) electrons.     

Semimetallic electrical transport in PbTe-SnTe superlattices on KCl substrate

The electrical resistivity and Hall coefficient (R_H) in PbTe-SnTe superlattices on KCl are measured between 4.2 and 300 K. Magnetic field dependence of R_H shows a sign inversion of R_H for a specimen of PbTe-SnTe with 100-50 A at 5 K. This is due to coexistence of electrons and holes. PbTe-SnTe superlattices are of type II, where the valence band edge of SnTe is higher than the conduction band edge of PbTe. From the magnetic field dependence of R_H, the electron and hole concentrations are calculated and the band-offset between PbTe and SnTe is estimated. The possibility of the structural phase transition of these superlattices is also discussed.     

Crystallization and quantum melting of few electron system in a spherical quantum dot: quantum Monte Carlo simulation

Spherical quantum dots with a few charged Fermi particles (electrons or holes) are studied for different total spins. Simulation by quantum path integral Monte Carlo method is performed. The dependence of the electron correlations in the quantum dot is studied at different mean interelectron separation controlled by number of electrons in the quantum dot and by steepness of electron confinement (the latter parameter can be changed by the gate voltage). The ‘cold’ melting—quantum transition from Wigner crystal-like state (i.e. from regime of strongly correlated electrons) to a Fermi liquid-like state—driven by the steepness of electron confinement is studied. The pair correlation function and radial function characterizing electron quantum delocalization are analyzed.     

Collective phase-like mode and the role of lattice distortions at TN ~TC in RMn2O5 (R= Pr, Sm, Gd, Tb, Bi)

We report on electronic collective excitations in RMn(2)O(5) (R =Pr, Sm, Gd, Tb) showing condensation starting at and below ~T(N) ~T(C)~ 40-50 K. Their origin is understood as partial delocalized e(g) electron orbitals in the Jahn-Teller distortion of the pyramid dimer with strong hybridized Mn(3+)-O bonds. Our local probes, Raman, infrared, and x-ray absorption, back the conclusion that there is no structural phase transition at T(N)~T(C). Ferroelectricity is magnetically assisted by electron localization triggering lattice polarizability by unscreening. We have also found phonon hardening as the rare earth is sequentially replaced. This is understood as a consequence of lanthanide contraction. It is suggested that partially f-electron screened rare earth nuclei might be introducing a perturbation to e(g) electrons prone to delocalize as the superexchange interaction takes place.     

Dynamics of the processes of electron-hole recombination and capture of charge carriers in anatase doped with boron, carbon, or nitrogen

The first-principles investigation of the processes of nonradiative recombination of electron-hole pairs and binding of excited charge carriers with impurity atoms in anatase doped with boron, carbon, or nitrogen has been carried out using the perturbation theory method. The perturbation is provided by a dynamically screened electron-electron interaction potential calculated in the random phase approximation. It has been shown that the most probable processes occurring upon doping with boron and carbon are exchange processes in which electrons are bound with the impurity atom, whereas the most probable processes observed upon doping with nitrogen are exchange processes in which holes are bound with the impurity atom. These processes occur within a time interval of shorter than 2 fs. The next in probability are the processes of energy losses by unbound electrons and holes due to the generation of phonons. For the case of nitrogen doping, the time of this process is estimated at approximately 300 fs. For excitons formed in this case, the luminescence photon energy and the binding energy of electrons or holes with the impurity atom are estimated. The agreement between the calculated data and the results of experiments on the photocatalysis proceeding on the surface of N-doped anatase is discussed.     

EPR investigation of local paramagnetic centers in perovskite-like crystals

In ScF₃ single crystals (pure and doped) as well as in Rb₂KScF₆ and Rb₂KDyF₆ crystals with a perovskite-like structure, point nanodefects (vacancy in place of trivalent cations) have been found and studied. Electron paramagnetic resonance has been used to investigate local paramagnetic centers that are not detected using X-ray diffraction. The angular dependence of the spectra indicates a local distortion of the cubic symmetry of the crystals. An additional hyperfine structure in the observed spectra is due to the delocalization of electrons over six F^? ions forming the first coordination polyhedron around the vacancy. The crystals studied are characterized by a high electron mobility and a high electron velocity, which depends on the impurity. The high mobility of electrons of the cation center can be indirectly responsible for the structural phase transition occurring in the ScF₃ crystal under uniaxial pressure.     

Adsorption properties of chalcogen atoms on a golden buckyball Au16(-) from first principles

Using first-principles density functional theory, we investigate the adsorption properties of chalcogen elements (oxygen and sulfur) on an anionic golden nanocage Au(16)(-) and its effects on the structural and electronic properties of the golden cage. In particular, we find that when a sulfur atom is encapsulated inside Au(16)(-), its bonding character with Au atoms appears ionic due to electron transfer from sulfur to the gold nanocage. In contrast, the exohedrally adsorbed S atom tends to have strong orbital hybridization with the golden nanocage. For an oxygen adsorption case, electrons from the golden cage tend to be shared with the adsorbed O atom exhibiting strong orbital hybridization, regardless of its adsorption sites. To investigate the transition behaviors between the most stable exohedral and endohedral adsorption configurations, we calculate the activation and reaction energies in the transition. The oxygen atom experiences a lower energy barrier than the sulfur atom due to its smaller atomic radius. Finally, we explore the vibrational properties of S- or O-adsorbed Au(16)(-) buckyballs by calculating their infrared spectra.     

Manifestations of broken symmetry: the surface phases of Ca(2-x)SrxRuO4

The surface structural phases of Ca(2-x)SrxRuO4 are investigated using quantitative low energy electron diffraction. The broken symmetry at the surface enhances the structural instability against the RuO6 rotational distortion while diminishing the instability against the RuO6 tilt distortion occurring within the bulk crystal. As a result, suppressed structural and electronic surface phase transition temperatures are observed, including the appearance of an inherent Mott metal-to-insulator transition for x=0.1 and possible modifications of the surface quantum critical point near x(c) approximately 0.5.     

Anomalies of thermal expansion and electrical resistivity of layered cobaltates YBaCo<Subscript>2</Subscript>O<Subscript>5 + <Emphasis Type="Italic">x</Emphasis> </Subscript>: The role of oxygen chain ordering

Layered cobaltates YBaCo₂O_{5 + x} have been investigated in the oxygen concentration range 0.23 ≤ x ≤ 0.52. It has been revealed that the oxygen ordering plays the key role in the appearance of anomalies in temperature dependences of structural parameters and electron transport. It has been shown that the orthorhombic lattice distortion caused by oxygen chain ordering is a necessary “trigger” for the phase transition from the insulating state to the metallic state at T ≈ 290–295 K, after which the orthorhombic distortion is significantly more pronounced. In the boundary region of the cobaltate compositions, where the oxygen ordering has a partial or local character, there are additional low-temperature (100–240 K) structural and resistive features with a large hysteresis. The observed anomalies can be explained by a change in the spin state of the cobalt ions, which is extremely sensitive to parameters of the crystal field acting on the ions, as well as by the spin-transition-induced delocalization of electrons.     

The effect of band Jahn-Teller distortion on the magnetoresistivity of manganites: a model study

We present a model study of magnetoresistance through the interplay of magnetisation, structural distortion and external magnetic field for the manganite systems. The manganite system is described by the Hamiltonian which consists of the s-d type double exchange interaction, Heisenberg spin-spin interaction among the core electrons, and the static and dynamic band Jahn-Teller (JT) interaction in the e(g) band. The relaxation time of the e(g) electron is found from the imaginary part of the Green's function using the total Hamiltonian consisting of the interactions due to the electron and phonon. The calculated resistivity exhibits a peak in the pure JT distorted insulating phase separating the low temperature metallic ferromagnetic phase and the high temperature paramagnetic phase. The resistivity is suppressed with the increase of the external magnetic field. The e(g) electron band splitting and its effect on magnetoresistivity is reported here.     

Subpicosecond time-resolved Mott transition in CuCl

The Mott transition in CuCl from a metallic phase of free electrons and holes towards an insulating phase of bound particles (excitons or biexcitons) has been studied by time-resolved luminescence with a resolution slightly better than 1 ps. The phase change takes place in a very short but finite time (about 3 ps) at a carrier density N 10¹⁹ cm^-3.     

Excitonic metal–insulator phase transition of the Mott type in compressed calcium

It has been experimentally found that, under the static compression of a calcium crystal at room temperature, it undergoes a series of structural phase transitions: face-centered cubic lattice → body-centered cubic lattice → simple cubic lattice. It has been decided to investigate precisely the simple cubic lattice (because it is an alternative lattice) with the aim of elucidating the possibility of the existence of other (nonstructural) phase transitions in it by using for this purpose the Hubbard model for electrons with half-filled ns-bands and preliminarily transforming the initial electronic system into an electron–hole system by means of the known Shiba operators (applicable only to alternative lattices). This transformation leads to the fact that, in the new system of fermions, instead of the former repulsion, there is an attraction between electrons and holes. Elementary excitations of this new system are bound boson pairs—excitons. This system of fermions has been quantitatively analyzed by jointly using the equation-of-motion method and the direct algebraic method. The numerical integration of the analytically exact transcendental equations derived from the first principles for alternative (one-, two-, and three-dimensional) lattices has demonstrated that, in systems of two-species (electrons + hole) fermions, temperature-induced metal–insulator phase transitions of the Mott type are actually possible. Moreover, all these crystals are in fact excitonic insulators. This conclusion is in complete agreement with the analytically exact calculations of the ground state of a one-dimensional crystal (with half-filled bands), which were performed by Lieb and Wu with the aim to find out the Mott insulator–metal transition of another type.     

On the possible mechanism of structural transition in cuprates

A possible mechanism of tetragonal to orthorhombic transition in high-T_c cuprates based on the removal of orbital degeneracy of p states in the CuO₂ cell by electron lattice interaction is proposed. Spontaneous distortion creates a finite energy gap or a pseudogap in the density of states depending on the relative strength of the next-near and nearest neighbour hopping strengths. The gap is a function of electron density and vanishes beyond the structural transition temperature. The growth of the gap leads to a metal semiconductor transition as temperature decreases with attendant stripe and orbital ordering. The phase diagram for the distorted phase is examined in detail in the parameter space.     

Chladni figures in Andreev billiards

We study wave functions and their nodal patterns in Andreev billiards consisting of a normal-conducting (N) ballistic quantum dot in contact with a superconductor (S). The bound states in such systems feature an electron and a hole component which are coherently coupled by the scattering of electrons into holes at the S-N interface. The wave function “lives” therefore on two sheets of configuration space, each of which features, in general, distinct nodal patterns. By comparing the wave functions and their nodal patterns for holes and electrons detailed tests of semiclassical predictions become possible. One semiclassical theory based on ideal Andreev retroreflection predicts the electron- and hole eigenstates to perfectly mirror each other. We probe the limitations of validity of this model both in terms of the spectral density of the eigenstates and the shape of the wavefunctions in the electron and hole sheet. We identify cases where the Chladni figures for the electrons and holes drastically differ from each other and explain these discrepancies by limitations of the retroreflection picture.