Vibrational and electronic properties of the amorphous systems Ni<Subscript>44</Subscript>Nb<Subscript>56</Subscript>, Ni<Subscript>62</Subscript>Nb<Subscript>38</Subscript>, and Cu<Subscript>33</Subscript>Zr<Subscript>67</Subscript> as derived from specific heat measurements

The specific heats of the amorphous systems Ni₄₄Nb₅₆, Ni₆₂Nb₃₈, and Cu₃₃Zr₆₇ were studied in the temperature range 3–273 K. The data obtained allow one to isolate the contribution due to atomic vibrations from the experimentally measured specific heat, to determine the density of electronic states at the Fermi level and the temperature dependence of the characteristic Debye parameter Θ over a broad temperature range, and to calculate a few frequency moments that characterize the vibrational spectrum. The information derived on the average characteristics of vibrational spectra is in good agreement with earlier data on inelastic neutron scattering. In transferring from Ni₄₄Nb₅₆ to Ni₆₂Nb₃₈, the density of electronic states at the Fermi level decreases and the characteristic vibrational frequencies increase. The density of electronic states at the Fermi level for Cu₃₃Zr₆₇ is close to that for Ni₆₂Nb₃₈. The characteristic frequencies of the vibrational spectrum of the Cu₃₃Zr₆₇ system are substantially lower (by 30%) than those of the Ni₄₄Nb₅₆ and Ni₆₂Nb₃₈ systems.     

Densities of phonon and electron states in V3Si and Cr3Si crystals

The regularization method has been used to solve the non-correct problem of finding out the phonon density of states from the temperature dependence of phonon heat capacity for two single crystals with A15 structure (the superconductor V₃Si and the non-superconductor Cr₃Si. The solution found agrees with the results of neutrongraphical and tunnel study. The electron density of states obtained in a narrow energy range near the Fermi level as the regularized solution of the reverse problem (from the temperature of the electron spin susceptibility) has at the Fermi energy the sharp maximum for V₃Si, but the flat minimum for Cr₃Si.     

Specific heat of small vanadium particles in the normal and superconducting state

The specific heat C of ultrafine vanadium particles of various diameters (2.9–13 nm) has been measured in the temperature interval 1.5–12 K and in magnetic fields up to 3.5 T. Both the vibrational and electronic contributions to C in the normal state are strongly enhanced as compared to the bulk behavior. For not too small particles (> ～10nm), the vibrational specific heat can be interpreted in terms of the discrete phonon spectrum of free elastically vibrating small spheres while, at low temperatures, the vibrational specific heat of the smallest particles is predominantly due to Einstein modes which are attributed to low-frequency vibrations of weakly bound surface atoms. Level quantization does not appear to play a detectable role in the electronic specific heat of the normal state. Rather, the observed enhancement must be attributed to an increased electronic density of states at (100) surfaces of bcc metals or to electronic states of substoichiometric V-oxides. The transition range to superconductivity progressively broadens with decreasing particle size due to fluctuations. In this temperature range, the electronic specific heat behaves in qualitative agreement with theoretical predictions.     

Heat capacity of La1−x SrxMnO3 single crystals in different magnetic states

The heat capacity of three single-crystal samples of La_1?x Sr_xMnO₃ (x=0, 0.2, and 0.3) is measured in the temperature range 4–400 K. It is found that the heat capacity undergoes abrupt changes due to the transitions from the antiferromagnetic phase to the paramagnetic phase (x=0) and from the ferromagnetic phase to the paramagnetic phase (x=0.2 and 0.3). The phonon contribution to the heat capacity and the Debye characteristic temperatures for the La_0.7Sr_0.3MnO₃ sample are determined over a wide range of temperatures. The electronic density of states at the Fermi level is evaluated. It is demonstrated that an increase in the strontium concentration x brings about an increase in the electronic density of states at the Fermi level. The contributions of spin waves to the heat capacity and the entropy are estimated under the assumption that the phonon spectrum remains unchanged upon doping with Sr.     

Atomic disorder and the magnetic, electrical, and optical properties of a Co2CrAl Heusler alloy

Two Co₂CrAl alloy samples subjected to different heat treatment regimes are studied. An exact distribution of atoms over the sublattices in the samples is determined by X-ray diffraction and neutron diffraction methods. These data are used to perform ab initio density of states calculations and to calculate the magnetic moments of the samples in a coherent potential approximation. The calculated magnetic moments are compared to the experimental values. The effect of atomic ordering on the electronic structure near the Fermi level is analyzed using optical methods. The possible causes of the detected temperature dependence of the electrical resistivity, unusual for metallic alloys, are discussed.     

Field-enhanced electronic specific heat of carbon nanotubes

The tight-binding model including curvature effects is used to study the effect of transverse electric field on the low-temperature electronic specific heat (C_v) for armchair and zigzag carbon nanotubes (ACNTs and ZCNTs). Electric field could effectively modulate energy dispersions of CNTs and cause a shift of electronic states toward the Fermi energy. As field strength reaches to a critical value (F_c), it induces special structures in the density of states near the Fermi energy and thus the giant specific heat. At F_cs, C_v has a value comparable to that of the phonon specific heat and reveals strongly non-linear dependence on temperature. The critical field strength and giant specific heat are closely related to nanotube's geometry. Moreover, under F_cs, the extra longitudinal magnetic flux could cause a re-enhancement in C_v for ZCNTs, whereas C_v is always diminished for ACNTs.     

Anomalous temperature dependence of the low-frequency vibrational density of states in vitreous B2O3

Inelastic neutron-scattering measurements of v-B₂O₃ reveal an upshift in energy of the first moment of the vibrational density of states for frequencies below ≈ 15 meV as temperature increases from 20 to 300 K. Such a stiffening is at odds with predictions based on some current approaches. Instead, the present result and related anomalies in specific heat and other microscopic and transport properties can be understood in terms of anharmonic motions at low frequencies. The atomic origin of the anharmonicity, and its consequence for the thermodynamical behavior are examined by molecular-dynamics simulations.     

Vibrational and electronic characteristics of Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript>, Zr<Subscript>80</Subscript>Pt<Subscript>20</Subscript> icosahedral quasicrystals and their amorphous Counterparts

The heat capacity of Zr₇₀Pd₃₀ and Zr₈₀Pt₂₀ icosahedral quasicrystals and their amorphous counterparts is studied in the temperature range 1.5–500 K in order to establish a correlation between the short-range atomic order and the physical properties of these compounds. A comparison of the data made it possible to reveal changes in the vibrational spectra within the low-and high-energy ranges, as well as in the density of states, superconducting characteristics, electron-phonon interaction, and anharmonicity of the lattice thermal vibrations and to calculate the main average frequencies (moments) characterizing the vibrational spectra. The lower superconducting transition temperature T _c of the quasicrystals as compared to that of the amorphous counterparts can be associated with the decrease in the density of states on the Fermi surface, the hardening of the phonon spectrum, and the weakening of the electron-phonon coupling.     

On rigid bands and mass enhancements in noble metal alloys

The rigid band model assumes that the electronic density of states of an alloy can be inferred from that of the host. Band calculations are usually inconsistent with this model when compared with the measured electronic specific heat of alloys. It has been suggested that the apparent discrepancy for alloys with noble metal hosts is due to the neglect of changes in the electron-phonon enhancement of the density of states at the Fermi level. A semi-empirical calculation, based on measurements of the high temperature electrical resistivity, shows that changes in the enhancement factor are much too small to account for the failure of the rigid band model.     

Electronic structure of La2CuO4

The electronic band structure of La₂CuO₄ is performed using self-consistent linear muffin-tin orbital method. The 17 band complex is found to arise mainly from the overlap between Cu-3d and O-2p wavefunctions. The calculated density of states at the Fermi energy (N E _F), the conduction band-width and the electronic specific heat coefficient are given.     

Single molecule vibrationally mediated chemistry

Tunnelling electrons may scatter inelastically with an adsorbate, releasing part of their energy through the excitation of molecular vibrations. The resolution of inelastic processes with a low temperature scanning tunnelling microscope (STM) provides a valuable tool to chemically characterize single adsorbates and their adsorption mechanisms. Here, we present a molecular scale picture of single molecule vibrational chemistry, as resolved by STM. To understand the way a reaction proceed it is needed knowledge about both the excitation and damping of a molecular vibration. The excitation is mediated by the specific coupling between electronic molecular resonances present at the Fermi level and vibrational states of the adsorbate. Thus, the two-dimensional mapping of the inelastic signal with an STM provides the spatial distribution of the adsorbate electronic states (near the Fermi level) which are predominantly coupled to the particular vibrational mode observed. The damping of the vibration follows a competition between different mechanisms, mediated via the creation of electron-hole pairs or via anharmonic coupling between vibrational states. This latter case give rise to effective energy transfer mechanisms which eventually may focus vibrational energy in a specific reaction coordinate. In this single-molecule work-bench, STM provides alternative tools to understand reactivity in the limit of low excitation rate, which demonstrate the existence of state-specific excitation strategies which may lead to selectivity in the product of a reaction.     

On the lattice dynamics of AgGaS2

Summary The vibrational spectrum and one-phonon density of states of a chalcopyrite crystal AgGaS₂ are calculated in an extended Keating’s model with two-bond-stretching and one-bond-bending force constants. Three charges of ions and three force constants are determined by a least-square fitting to experimental frequencies of long-wave phonons taken from Raman-scattering experiments. The calculated constant-volume specific heat, Debye temperature and elastic constants, of AgGaS₂ are in agreement with the experimental data of other authors.     

Structure and properties of the intercalation compound Fe x TiTe2

The Fe _x TiTe₂ system, which belongs to the class of materials with the electronic spectrum containing below the Fermi level the band of localized states with a strong temperature dependence of the band width, has been investigated experimentally. Heating of the material leads to a broadening of the band of localized states. When the top of this band crosses with the Fermi level, the effect of retrograde solubility is observed in the system; i.e., the metal precipitates to the composition ensuring the absence of increase in the Fermi energy during heating. The influence of the band of localized states on the structure of the material and its magnetic and electrical properties has been analyzed.     

Superconducting and normal state properties of dilute alloys of LaSn3 containing Ce impurities

The dependence of the superconducting transition temperature T_c on Ce impurity concentration and the specific heat jump at T_c as a function of T_c are reported for the system ($\text{La}$Ce)Sn₃. The experimental results are analyzed using a theory due to Kaiser concerning the effect on superconductivity of nonmagnetic localized resonant impurity states This analysis yields values for the intraatomic Coulomb repulsion parameter U_eff and the Ce local density of states at the Fermi level N_? (E_F). The results of low temperature normal state heat capacity and magnetic susceptibility measurements which give independent estimates of N_? (E_F) are also reported. A large pressure dependence of the T_c of ($\text{La}$Ce)Sn₃ alloys was observed for pressures up to 20 kbar. This behavior is similar to that previously observed in several other superconducting matrix-Ce impurity systems in which the Ce solute 4f electron shell undergoes a continuous-pressure induced demagnetization.     

Interrelation between electronic structure and interatomic distances for compounds

The heat capacity was studied for LaMn₂Si₂, La_0.75Y_0.25Mn₂Si₂, La_0.7Y_0.3Mn₂Si₂, YMn₂Si₂ and LaFe₂Si₂ isostructural intermetallic compounds in the temperature range 1.8–360 K. The electronic, magnetic and lattice contributions to the heat capacity of the compounds were determined and analyzed. The interrelation was found between values of the electronic contribution to the heat capacity (density of states at the Fermi level) and crystal lattice parameters of R(Mn,Fe,Ni)₂Si₂ compounds. The electronic contribution and the density of states at Fermi level increase with increasing lattice parameters of the compounds. The change of interlayer Mn–Mn exchange interactions with change of Y concentration in La_1-xY_xMn₂Si₂ compounds is not accompanied by considerable changes in the electronic contribution to the heat capacity and density of states at the Fermi level. The performed analysis of the magnetic contribution shows that no essential differences exist between the behavior of the heat capacity of the compounds with d_Mn–Mnd_c and with d_Mn–Mn<d_c upon various types of the magnetic phase transitions.     

Thermoelectric transport properties of an apparent Fermi liquid: Relation to an analytic anomaly in the density of states and application to hole‐doped delafossites

Through the motivation of the recent discovery of dispersionless regions in the band structure of the delafossites, a model density of states of free fermions including a d anomaly is studied. The resulting temperature dependence of the chemical potential is obtained both exactly and by different approximation schemes which are then discussed thoroughly. This includes the introduction of an approximation of the polylogarithm difference which is capable of accessing a parameter range neither covered by Sommerfeld expansion nor by Boltzmann approximation. It is found that the Fermi temperature and several other temperature scales may be very low, giving rise to experimentally observable behaviours differing from the one described by Fermi liquid theory. In particular, two kinds of apparent Fermi liquid behaviour emerge at intermediate temperatures. This behaviour is related to recent transport data reported for CuCr_1‐xMg_xO₂ [A. Maignan et al., Solid State Commun. 149 , 962 (2009)] and CuRh_1‐xMg_xO₂ [A. Maignan et al., Phys. Rev. B 80 , 115103 (2009)] by means of the temperature independent correlation functions ratio approximation. In this way an effective density of states as well as the effective charge carrier density of these materials are determined. Furthermore, conclusions about the specific heat of the latter material are drawn which presents particular effects of the analytical anomaly.     

Edge states of graphene bilayer strip

The electronic structure of the zig-zag bilayer strip is analyzed. The electronic spectraof the bilayer strip is computed. The dependence of the edge state band flatness on thebilayer width is found. The density of states at the Fermi level is analytically computed.It is shown that it has the singularity which depends on the width of the bilayer strip.There is also asymmetry in the density of states below and above the Fermi energy.     

Temperature-dependent ac conductivity and dielectric response of vanadium doped CaCu3Ti4O12 ceramic

Successful incorporation of vanadium dopant within the giant dielectric material CaCu₃Ti₄O₁₂ (CCTO) through a  conventional solid-state sintering process is achieved and its influence on the dielectric as well as electrical properties as a function of temperature and frequency is reported here. Proper crystalline phase formation together with dopant induced lattice constant shrinkage was confirmed through X-ray diffraction. The temperature dependence of the dielectric constant at different constant frequencies was investigated. We infer that the correlated barrier hopping (CBH) model is dominant in the conduction mechanism of the ceramic as per the temperature-dependent ac conductivity measurements. The electronic parameters such as density of the states at the Fermi level, N(E _f) and hopping distance, R _ω of the ceramic were also calculated using this model.     

Evolution with composition of the d-band density of states at the Fermi level in highly spin polarized Co1-xFexS2

Highly spin polarized (SP) and half-metallic ferromagnetic systems are of considerable current interest and of potential importance for spintronic applications. Recent work has demonstrated that Co1-xFexS2 is a highly polarized ferromagnet (FM) where the spin polarization can be tuned by alloy composition. Using 59Co FM-NMR as a probe, we have measured the low-temperature spin relaxation in this system in magnetic fields from 0 to 1.0 T for 0相似文献   

Electronic structure and lattice dynamics of CaPd3B studied by first-principles methods

Using first-principles methods, we have studied the electronic structure and lattice dynamics of CaPd₃B and compared them to isostructural MgNi₃C. CaPd₃B possesses less electronic states at the Fermi level, but more phonon modes at low frequencies, than MgNi₃C. According to the phonon density of states, low frequency acoustic modes are dominated by Pd states, corresponding to Ni in MgNi₃C. Furthermore, these Pd modes show soft phonons, which may be significant for second-order phase transitions. Based on the comparison to MgNi₃C, we suggest that the properties of these two compounds may be similar.