Unusual magnetic-field-induced phase transition in the mixed state of superconducting NbSe2

The thermal conductivity kappa in the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c axis, at several temperatures below T(c). With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H(c2), an unexpected hysteretic behavior of kappa(H) with all the generic features of a first order phase transition. The transition is not manifest in the kappa(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.     

High-field galvanomagnetic properties of graphite

Measurements of the galvanomagnetic properties of graphite show structure in a region well above the quantum limit (10–23T). The first measurements of the longitudinal magnetoresistance in this field range are reported. The conductivity anisotropy, $\text{σ}{}_{\mathrm{<mtext>xx</mtext>}}\text{σ}{}_{\mathrm{<mtext>zz</mtext>}}$, shows strong field dependence, resulting in the lowest anisotropy values ever reported for any highly oriented graphite.     

Evolution of the electronic properties of transition metal nanoclusters on graphite surface

The electronic properties of nanoclusters of transition (Ni, Co, Cr) and noble (Au, Cu) metals deposited on the surface of highly oriented pyrolytic graphite (HOPG) are studied using the method of X-ray photoelectron spectroscopy. The laws of variation of a change ΔE _b in the binding energies of core-level electrons in the initial (ΔE _i) and final (ΔE _f) states of atoms in nanoclusters, the intrinsic widths γ of photoelectron lines, and their singularity indices α as functions of the metal cluster size d are determined. A qualitative difference in behavior of the ΔE _i(d) and α(d) values in metals of the two groups (Ni, Cr versus Co, Cu) is found. The values of the final-state energy (ΔE _f < 0) and the line width (Δγ > 0) in the clusters of all metals studied vary in a similar manner. It is shown that a significant contribution to E _i is due to a transfer of the valence-shell electrons at the cluster-substrate interface, which is caused by the contact potential difference. The value of an uncompensated charge per nanocluster is determined as a function of the cluster size and the number of atoms in the cluster. The behavior of ΔE _f(d) is controlled by the Coulomb energy of a charged cluster and by a decrease in the efficiency of electron screening, which is different in the metals studied. The broadening of photoelectron lines is determined by a spread of the cluster sizes and by lower electron screening in the final Fermi system. An asymmetry of the core-level electron spectra of nanoclusters can be explained using notions about the electron-hole pair excitation near the Fermi level. The effect of the structure of the density of electron states in the d band of transition metals on the asymmetry of photoelectron lines is considered and it is concluded that this structure near the Fermi level qualitatively changes with a decrease in the nanocluster size. The obtained results indicate that the behavior of the electron subsystem of clusters of the d-metals in a size range of 2–10 nm under consideration is close to the behavior of a normal Fermi system.     

First-order phase transition in the graphite compound LiC6

Elastic neutron scattering and scanning calorimetry reveal a weakly first-order “melting” of the Li layers in the graphite intercalation compound LiC₆ at T₀ = 715.4 K, in excellent agreement with recent experiment with recent experiments by Robinson and Salamon. The first-order nature of the transition also agrees with a prediction by Bak and Domany derived from Landau expansions, but the detailed behavior of the fluctuation-induced power-law precursor differs from recent Monte Carlo simulations of the 3D?3q Potts model.     

Enhanced magnetoresistance through magnetic-field-induced phase transition in Ni2MnGa co-doped with Co and Mn

We report the observation of giant negative magnetoresistance up to −46% at 60 kOe magnetic field in Ni_1.68Co_0.32Mn_1.20Ga_0.80 alloy, which is about 5 times larger than that reported in Ni-Mn-Ga alloys. The significant change in resistivity during martensitic transformation originates from the altered electronic structure due to the change of magnetic state. The magnetic-field-induced phase transition from partially antiferromagnetic martensite to ferromagnetic austenite is responsible for the enhanced magnetoresistance.     

On the absence of purely electronic phase transition in extended Falicov-Kimball model

The Falicov-Kimball model extended by the hybridization between the localized and band electrons is considered in the approximation valid for the narrow band. The limit of infinite Coulomb repulsion G is considered. In spite of the fact that the approximation involved favour the abrupt change of the occupation number of ?-states we have not found any discontinuous solution. This suggests that the first order phase transition in SmS can not be considered as being or purely electronic nature.     

Structural phase transition of aluminum induced by electronic excitation

The dynamics of a structural phase transition induced by interband electronic excitation in aluminum is studied by determining the time evolution of the dielectric constant at 1.55 eV through the measurement of the transient reflectivity induced by an ultrafast pump pulse. The threshold fluence and the time scale for this transition are significantly less than the values necessary for ultrafast heat-induced melting, indicating that this phase change is caused by band structure collapse and lattice instability resulting from strong electronic excitation.     

On the electronic structure of graphite

Comprehending investigations of the electronic structure of polycrystalline as well as monocrystalline graphite have been performed by means of X-ray emission, self absorption and X-ray induced photoemission techniques. On the basis of these combined investigations a model for the determination of the relative cross sections for the photoemission process has been established and applied to graphite, where it yields _s / _p =32. The anisotropy and polarization of the CK-radiation of monocrystalline graphite is discussed in terms of the binding properties of the graphite lattice. The predictions are verified by measurements of the CK-emission employing a crystal monochromator which acts simultaneously as a nearly perfect analyzer for the polarization of the monochromatized radiation. By means of the self absorption technique the unoccupied part of the conduction band has been investigated.     

Two-dimensional phase transition in xenon submonolayer films adsorbed on (0001) graphite

Ultra high vacuum molecular beam techniques coupled with LEED and Auger electron spectroscopy are particularly well suited to the study of surface chemical reactions because of the ability to assess the effect of the surface conditions on the reaction probability. Investigation of the hydrogen-deuterium exchange reaction on a series of low and high Miller index platinum single crystals has indicated that the steps present on the high index surfaces are necessary for the dissociation and subsequent recombination of hydrogen. We have undertaken a systematic study of a series of small molecule reactions on these stepped surfaces to determine the reaction probability on stepped platinum surfaces. Reactions involving dissociation of Ha₂, D₂, O₂, OH, NH, and CH bonds proceed on the stepped surfaces with much higher reaction probabilities than reactions requiring dissociation of N₂, or CO bonds. All of the reactions studied resulted in cosine product angular distributions except for the formation of CO₂, which exhibited a distribution more peaked at the normal to the surface.     

On the equivalence between magnetic-field-induced phase transitions in the integer quantum Hall effect

Magnetic-field-induced phase transitions in the two-dimensional electron system in a AlGaAs/InGaAs/GaAs heterostructure are studied. Two kinds of magnetic-field-induced phase transitions, plateau-plateau (P-P) and insulator-quantum Hall conductor (I-QH) transitions, are observed in the integer quantum Hall effect regime at high magnetic fields. In the P-P transition, both the semicircle law and the universality of critical conductivities are broken and we do not observe the universal scaling. However, the P-P transition can still be mapped to the I-QH transition by the Landau-level addition transformation, and as the temperature decreases the critical points of these two transitions appear at the same temperature. Our observations indicate that the equivalence between P-P and I-QH transitions can be found by the suitable analysis even when some expected universal properties are invalid.     

Pulse laser induced graphite-to-diamond phase transition:the role of quantum electronic stress

First-principles calculations show that the pulse laser induced graphite-to-diamond phase transition is related to the lattice stress generated by the excited carriers,termed as quantum electronic stress(QES).We found that the excited carriers in graphite generate a large anisotropic QES that increases linearly with the increasing carrier density.Using the QES as a guiding parameter,structural relaxation spontaneously transforms the graphite phase into the diamond phase,as the QES is reduced and minimized.Our results suggest that the concept of QES can be generally applied as a good measure to characterize the pulse laser induced phase transitions,in analogy to pressure induced phase transitions.     

ZnTe结构相变、电子结构和光学性质的研究

运用密度泛函理论体系下的投影缀加波方法, 对闪锌矿和朱砂相结构的ZnTe在高压下的状态方程和结构相变进行了研究, 并分析了相变前后的原胞体积、电子结构和光学性质. 结果表明: 闪锌矿结构转变为朱砂相结构的相变压力为8.6 GPa, 并没有出现类似材料高压导致的金属化现象, 而是表现出间接带隙半导体特性. 相变后, 朱砂相结构Zn和Te原子态密度分布均向低能级方向移动, 带隙变小; 轨道杂化增强, 更有利于Te 5p与Zn 3d间的电子跃迁, 介电常数虚部主峰明显增强, 但宏观介电常数不受压力的影响.     

Structural phase transition and the related electronic and optical properties of MgZnO nanowires

We theoretically investigate the stabilities and electronic structures of MgZnO nanowires with different compositions of Mg. It is found that with increasing composition of Mg, wurzite MgZnO nanowires become less stable, and cubic MgZnO nanowires are energetically favored. The phase transition from wurzite to cubic structure occurs, when the composition of Mg in the Mg _x Zn_{1- x} O nanowire reaches about 0.65. Furthermore, our calculations show that the band gaps can be effectively modulated by composition of Mg for either wurzite or cubic nanowires. This is reflected in the calculated absorption spectra, where the absorption edges shift with variation of the compositions of Mg.     

Pulse laser induced graphite-to-diamond phase transition: the role of quantum electronic stress

Fracking is one of the kernel technologies in the remarkable shale gas revolution. The extended finite element method is used in this paper to numerically investigate the interaction between hydraulic and natural fractures, which is an important issue of the enigmatic fracture network formation in fracking. The criteria which control the opening of natural fracture and crossing of hydraulic fracture are tentatively presented. Influence factors on the interaction process are systematically analyzed, which include the approach angle, anisotropy of in-situ stress and fluid pressure profile.