Linear wave equations on flat-band networks host compact localized eigenstates (CLS). Nonlinear wave equations on translationally invariant flat-band networks can host compact discrete breathers-time-periodic and spatially compact localized solutions. Such solutions can appear as one-parameter families of continued linear compact eigenstates, or as discrete sets on families of non-compact discrete breathers, or even on purely dispersive networks with fine-tuned nonlinear dispersion. In all cases, their existence relies on destructive interference. We use CLS amplitude distribution properties and orthogonality conditions to derive existence criteria and stability properties for compact discrete breathers as continued CLS.
A deposition model to simulate the growth of doped rare gas crystals is used. The study involves organic molecules with a single intramolecular hydrogen bond such as malonaldehyde, 2chloromalonaldehyde and acetylacetone as impurities. Different trapping sites were obtained depending on the rare gas properties for a given impurity, and depending on the molecular size and shape for a given crystal. Simulations were carried out by using classical molecular dynamics methods including an anharmonic thermal correction, to take into account the zero point movement of the crystal. The results are correlated to spectroscopic data previously achieved for these systems by steady state IR spectroscopy.
The temperature dependence of the linear thermal expansion coefficients (LTEC) of a single crystal of α-(BEDT-TTF)2NH4Hg(SCN)4 where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene was studied by the method of precision capacitive dilatometry in the temperature range 2-250 K along the crystallographic direction b (perpendicular to the crystal layers). Negative values of LTEC were found below 4 K. Probably it is due to charge fluctuations as the temperature approaches the temperature of transition to the superconducting state Tc. It has been suggested that the bends on temperature dependence of LTEC observed in the temperature range 25-45 K are related to order-disorder arrangement of NH 4 + ions. Weak maximum of the LTEC, detected at a temperature of about 200-220 K, can be caused by the processes of charge redistribution and the associated intermolecular interaction fluctuations.
The charge carrier overheating effect was studied in the p-type Si0.4Ge0.6/Ge/Si0.4Ge0.6 heterostructure with two subband occupy. The temperature dependences of hole-phonon relaxation time τ h - ph sat weak magnetic fields demonstrated transition of the 2D system from regime of “partial inelasticity” characterized by dependence τ h - ph − 1 ∝ T2 to regime of small-angle scattering, described by dependence τ h - ph − 1 ∝ T5 with temperature increase. But in higher magnetic fields the dependence τ h - ph − 1 ∝ T3 manifests itself on dependences τh-ph(Th-ph). The possible explanations of such dependences are discussed. PACS: 72.15.Lh Relaxation times and mean free path; 72.20. My Galvanomagnetic and other magnetotransport effects; 72.20. −i Conductivity phenomena in semiconductors and insulators.
Fresh and aged melt-grown or gas-phase grown CdI2 crystals are studied by means of low-temperature photoluminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging. The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well as the changes in local surrounding of iodine atoms and the possible growth of polytypic modifications of CdI2 are taken into account when considering the diversity of optical spectra.
We report on the experimental observation of energy accumulation near the high frequency boundary of the inertial range in the spectrum of turbulence in a system of capillary waves on the surface of liquid hydrogen driven by a harmonic force. The effect is manifested as a local maximum in the spectrum of pair correlation function of the surface elevation. This phenomenon is dynamical and can be seen only during reconfiguration of the turbulent cascade caused by waves generation of below the driving frequency.
By the use of the Rayleigh method we have calculated the angular dependence of the reflectivity and the efficiencies of several other diffracted orders when the periodically corrugated surface of an isotropic elastic medium is illuminated by a volume acoustic wave of shear horizontal polarization. These dependencies display the signatures of Rayleigh and Wood anomalies, usually associated with the diffraction of light from a metallic grating. The Rayleigh anomalies occur at angles of incidence at which a diffracted order appears or disappears; the Wood anomalies here are caused by the excitation of the shear horizontal surface acoustic waves supported by the periodically corrugated surface of an isotropic elastic medium. The dispersion curves of these waves in both the nonradiative and radiative regions of the frequency-wavenumber plane are calculated, and used in predicting the angles of incidence at which the Wood anomalies are expected to occur.
The heat capacity at constant pressure of fullerite C60 has been investigated using an adiabatic calorimeter in a temperature range from 1.2 to 120 K. Our results and literature data have been analyzed in a temperature interval from 0.2 to 300 K. The contributions of the intramolecular and lattice vibrations into the heat capacity of C60 have been separated. The contribution of the intramolecular vibration becomes significant above 50 K. Below 2.3 K the experimental temperature dependence of the heat capacity of C60 is described by the linear and cubic terms. The limiting Debye temperature at T→0 K has been estimated (Θ0 = 84.4 K). In the interval from 1.2 to 30 K the experimental curve of the heat capacity of C60 describes the contributions of rotational tunnel levels, translational vibrations (in the Debye model with Θ0 = 84.4 K), and librations (in the Einstein model with ΘE,lib = 32.5 K). It is shown that the experimental temperature dependences of heat capacity and thermal expansion are proportional in the region from 5 to 60 K. The contribution of the cooperative processes of orientational disordering becomes appreciable above 180 K. In the high-temperature phase the lattice heat capacity at constant volume is close to 4.5 R, which corresponds to the high-temperature limit of translational vibrations (3 R) and the near-free rotational motion of C60 molecules (1.5 R).
Heat transport mechanisms present in 2-adamantanone and 1-cyanoadamantane crystals were investigated in a broad temperature range. To characterize scattering processes, thermal conductivity and heat capacity measurements were carried out. A particular care was paid to the cooling rate of specimen which influenced the thermal history of the samples. The experimental results led to a conclusion that under slow cooling the thermal conductivity reaches the highest values and resembles the behavior of ordered molecular crystals. As for fast cooling, the "quenching" resulted in changes in both the structure and the temperature dependence of the thermal conductivity, the latter resembling that of amorphous solids. In heat capacity measurements the thermal history made on samples did not reflect the preliminary findings known from thermal conductivity results, which could imply that the observed mechanisms are more complex. (C) 2015 AIP Publishing LLC.
An exact diagonalization method is applied to solve the quantum-mechanical problem of spinless helium atom in an external electric field of arbitrary magnitude. The basis set for two-electron problem is built from different pair combinations psi(nalama) (alpha r(a))psi(nblbmb) (alpha r(b)) of orthonormalized single-particle hydrogen-like wave functions psi(nml)(r) belonging to any possibly bound states of the individual a- and b-electrons in the Coulomb central field renormalized by the scale parameter alpha > 0. Within the selected basis the matrix elements of the total Hamiltonian allows an exact analytical representation in the form of finite numerical sums. The diagonalization procedure is performed by Jacobi algorithm for N x N square Hermitian matrix built on the basis of dimension N = 25. The systematics and the numerical values of the low-lying energy levels at zero field are in good agreement with known experimental data. The field dependences of low-lying levels (Stark effect) and polarizability in the ground state of helium atom are presented. It is shown that even extremely high external fields lead only to shifting or splitting of existing low levels, without disturbance of their systematics. Typically, no new low-energy excitation can be created under external electric field of moderate intensity. Radical reconstruction in spectrum of individual helium atoms can be expected in condensed helium phases where each atom is deeply affected by interaction fields from neighbors. This result should be taken into account at interpretation of electrodynamic experiments on superfluid helium. (C) 2014 AIP Publishing LLC.
The results of investigations of thermoluminescence dynamics during destruction of neon–helium and krypton–helium condensates containing stabilized nitrogen and oxygen atoms are presented. Spectra of the thermoluminescence of a krypton–helium condensate contained bands of N and O atoms and NO molecules. The intensities of the bands in these spectra were found to increase simultaneously during destruction processes in the temperature range 1.5–15 K. The observation of NO molecules provides clear evidence for chemical reactions in the nanoclusters comprising the sample at low temperatures. Destruction of neon–helium samples occurred in two stages. During the first stage the α-group of N atoms surrounded by Ne and N2 molecules dominated the spectra. During the second stage, the spectra contained intense bands of N and O atoms stabilized in a molecular nitrogen matrix. The unusual characteristics of the thermoluminescence spectra were observed, and their changes were explained in terms of the shell structure of impurity nanoclusters, which comprised the impurity–helium condensates.
Using one-photon excitation we studied photoluminescence of C-60 saturated with molecular hydrogen over a temperature range from 10 to 230K. Saturation of samples was done at 30 atm and at temperatures low enough (T < 250 degrees C) to exclude chemical sorption. The samples were saturated during periods of varied duration T to reach different occupancy levels. To check the reliability of our luminescence results and their interpretation, our spectra for pure C-60 were compared with data known in the art, demonstrating good compatibility. The luminescence spectra were attributed according to the approach of Akimoto and Kan'no by separating the total spectra into two components of different origin. The A-type spectra, associated with exciton transport to deep traps, become prevalent over the B-type emission above 70K. The integrated intensity I as a function of the temperature T of the luminescence measurements I(T) remained at a constant level up to the orientational vitrification point of about 100 K when the saturation times exceeded a certain value (for one, 50 h for a saturation temperature of 200 degrees C); then I(T) went down rather steeply with increasing T. However, at longer T the intensity I(T) persisted consistently to higher T (the higher, the longer tau) and then dropped with increasing T. This finding made us reexamine the lattice parameter vs. saturation time dependence for saturation temperatures of 200 and 230 degrees C. As a result, additional evidence allowed us to infer that after the completion of the single-molecule filling of O-voids (specifically, after roughly 50 h at T-sat = 200 degrees C) a slower process of double filling sets in. Double filling entails an anisotropic deformation of the octahedral cage, which modifies rotational dynamics more than single filling. Further, we argue that singlet exciton transport to traps (which is responsible for the A-type emission) can be crucially hampered by rotational jumps of one of the molecules, over which a travelling exciton is spread. Such jumps break coherence, and the exciton stops, thereby increasing the probability of emissionless deactivation. If so, then the temperature at which the rotational jumps occur sufficiently frequently may be by inference considered to be the unfreezing point for the orientational glass state (essentially coinciding with the inverse critical point T-g, where the rotational system freezes into the orientational glass). This treatment of T-g differs from that existing in the art, according to which the glass state is destroyed owing to the increased density of phonon states. Keeping to our reasoning, we conclude that the orientational glass state does not disappear but, instead, is conserved almost unchanged under one-molecule filling and persists to appreciably higher temperatures in the case of double filling, which has a stronger effect on exciton dynamics. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4746795
Experimental evidence of metastable ion dipoles in solid helium is examined. Similar quasiparticles with positive scattering lengths for injected electrons are assumed to exist in the liquid phases of cryogenic liquids. Phenomena that can be used for detecting and monitoring a dipole gas in superfluid helium (referred to as cryogenic electrolyte) are discussed. The most interesting of these phenomena are: special features of the dielectric behavior of ion dipole gases, the temperature dependence of the ion dipole gas osmotic pressure at the boundary of liquid 3He-4He solution stratification, relaxation phenomena of collective origin in cryogenic electrolytes, and the transformation of the phonon spectrum of liquid helium owing to strong interactions between phonons and heavy dipole quasiparticles.
We analyze a gap equation for the propagator of Dirac quasiparticles and conclude that in graphene in a magnetic field, the order parameters connected with the quantum Hall ferromagnetism dynamics and those connected with the magnetic catalysis dynamics necessarily coexist (the latter have the form of Dirac masses and correspond to excitonic condensates). This feature of graphene could lead to important consequences, in particular, for the existence of gapless edge states. Solutions of the gap equation corresponding to recently experimentally discovered novel plateaus in graphene in strong magnetic fields are described.
Successive magnetic-field-induced charge-density-wave transitions in the layered molecular conductor α-(BEDT-TTF)2KHg(SCN)4 are studied in a hydrostatic pressure regime in which the zero field charge-density- wave (CDW) state is completely suppressed. It is shown that the orbital effect of the magnetic field restores the density wave, while orbital quantization induces transitions between different CDW states as the field strength is varied. The latter show up as distinct anomalies in the magnetoresistance as a function of field. The interplay between the orbital and Pauli paramagnetic effects, which act, respectively, to enhance and to suppress the CDW instability, is particularly manifest in the angular dependence of the field-induced anomalies.
The intermetallic compound CeGe exhibits unusual magnetic behavior owing to the interplay between Kondo and antiferromagnetic coupling. This system is interesting because the Kondo temperature is close to the Néel temperature, so there is a close competition between the low-temperature interactions, which can be tuned by varying external parameters such as pressure and applied magnetic field. Interestingly, magnetization measurements up to 12 kbar reveal that the Néel temperature is not affected by pressure. Measurements of the electrical resistivity show, however, that the sharp upturn below TN is sensitive to pressures up to 15 kbar. This suggests that pressure may change the complex antiferromagnetic spin structure. The validity of an explanation based on the magnetic superzones seen in the rare earths is discussed here.
The temperature dependence of the current-voltage characteristics of high-quality thin films of tin from 7 to 50 mu m thick are investigated in the absence of an external magnetic field. For the first time, we have experimentally observed phase slip centres (PSCs) and phase slip lines (PSLs) on the same superconducting tin film with known parameters in the temperature intervals corresponding to the mechanisms of their formation and existence. We have shown that the states of a wide film with increasing transport current appear in the following order: the superconducting state for current less than critical; the resistive vortex state for current more than critical, but less than maximum current for the uniform flux flow (instability current); the critical state due to the onset of instability of the steady pattern of viscous motion of the vortices; a vortex-free resistive state with PSLs for current more than instability current, but less than the upper critical current; and the normal state at a current higher than the upper critical current.
We discuss a time-space symmetry in phase slippage processes in Josephson junctions and charge density wave (CDW) stacked junctions. In Josephson junctions, above a critical current a phase slip of 2π occurs periodically in time with a period T corresponding to the rate given by the Josephson relation ν = 1/ T= 2eV/h, where V is the voltage on the junction. In CDW stacks, a CDW dislocation appears in the weakest junction of the stack when the voltage on the stack exceeds a threshold value. This phase dislocation corresponds to a local phase slippage of 2π. As the voltage is increased, new dislocations show up and form a periodic array of dislocations with period L. The inverse spacing 1/L obeys an analog of Josephson equation, vF /L = 2eV/h.
An IR spectrometric investigation of the dynamic glass transition of ethanol from the rotationally disordered crystal to the orientationally disordered crystal is carried out. The samples considered are thin films formed from the gas phase at a substrate temperature of T = 16 K . The measurements are performed using the experimental apparatus which has been described in detail in our recent work. The sample thickness was d = 2 μ m , and the typical rate of annealing is approximately 10 K ∕ min . The results are compared with the phase diagram of solid ethanol proposed by M.A. Ramos et al. We observe good agreement between the temperature intervals of existence of the amorphous and crystalline states. The low-temperature amorphous phase ( 12 – 70 K ) is described by the present authors as amorphous solid ethanol by analogy with the amorphous solid water.