Low thermal expansion and broad band photoluminescence of Zr_(0.1)Al_(1.9)Mo_(2.9)V_(0.1)O_(12)

A new material of Zr0.1Al1.9Mo2.9V0.1O12 is synthesized by the traditional solid state synthesis method.The phase transition,coefficient of thermal expansion,and luminescence properties of Zr0.1Al1.9Mo2.9V0.1O12 are explored with Raman spectrometer,dilatometer,and x-ray diffraction(XRD)diffractometer.The results show that the Zr0.1Al1.9Mo2.9V0.1O12 possesses the strong broad-band luminescence characteristics almost in the whole visible region.The sample is crystallized in a monoclinic structure group of P21/a(No.14)crystallized at room temperature(RT).The crystal is changed from monoclinic to orthorhombic structure when the temperature increases to 463 K.The material has very low thermal expansion performance in a wide temperature range.Its excellent low thermal expansion and strong pale green light properties in a wide temperature range suggest its potential applications in light-emitting diode(LED)and other optoelectronic devices.     

Magnetoelectric effects in multiferroic Y-type hexaferrites Ba_(0.3)Sr_(1.7)Co_xMg_(2-x)Fe_(12)O_(22)

Y-type hexaferrites with tunable conical magnetic structures are promising single-phase multiferroics that exhibit large magnetoelectric effects. We have investigated the influence of Co substitution on the magnetoelectric properties in the Y-type hexaferrites Ba(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22)(x = 0.0, 0.4, 1.0, 1.6). The spin-induced electric polarization can be reversed by applying a low magnetic field for all the samples. The magnetoelectric phase diagrams of BaBa(0.3)Sr(1.7)CoxMg(2-x)Fe(12)O(22) are obtained based on the measurements of magnetic field dependence of dielectric constant at selected temperatures. It is found that the substitution of Co ions can preserve the ferroelectric phase up to a higher temperature, and thus is beneficial for achieving single-phase multiferroics at room temperature.     

Novel dielectric anomaly in the hole-doped La(2)Cu(1-x)Li(x)O(4) and La(2-x)Sr(x)NiO(4) insulators: signature of an electronic glassy state

The low-frequency dielectric response of hole-doped insulators La(2)Cu(1-x)Li(x)O(4) and La(2-x)Sr(x)NiO(4) shows a large dielectric constant epsilon(') at high temperature and a steplike drop by a factor of 100 at a material-dependent low temperature T(f). T(f) increases with frequency, and the dielectric response shows universal scaling in a Cole-Cole plot, suggesting that a charge-glass state is realized both in the cuprates and in the nickelates.     

Mechanism of Cu deposition on the alpha-Al(2)O(3) (0001) surface

The growth mechanism of the Cu/alpha-Al(2)O(3) (0001) interface is studied by first-principles molecular-dynamics simulations as a function of the transition-metal coverage (theta) and the temperature of the system. On the anhydrous surface growth of Cu(0) 3D clusters is predicted. On the partially hydroxylated surface, a Cu(I) monolayer, relatively stable upon the temperature rising, is first observed (theta<1/3 ML). Increasing Cu loading leads to Cu(I)/Cu(0) mixed phases that when heated aggregate into 3D particles increasing the number of Cu(0) atoms, in agreement with the Auger spectra of Kelber et al.     

New vortex-matter size effect observed in Bi(2)Sr(2)CaCu(2)O(8 + delta)

The vortex-matter 3D to 2D phase transition is studied in micron-sized Bi(2)Sr(2)CaCu(2)O(8 + delta) single crystals using local Hall magnetization measurements. At a given temperature, the second magnetization peak, the signature of a possible 3D--2D vortex phase transition, disappears for samples smaller than a critical length. We suggest that this critical length should be equated with the 2D vortex lattice ab-plane correlation length R(2D)(c). The magnitude and temperature dependence of R(2D)(c) agree well with Larkin-Ovchinnikov collective pinning theory.     

Successive inverse and normal magnetocaloric effects in the Mn-vacancy compound Mn_(0.95)Co_(0.75)Cu_(0.25)Ge

Ferromagnetic-structural transformation has been studied widely in MnCoGe-based materials. However, the magnetostructural transition(MST) from antiferromagnetic(AFM) orthorhombic phase to ferromagnetic(FM) hexagonal phase, which may lead to a large inverse magnetocaloric effect(MCE), has rarely been reported. Here, the introduction of Mn vacancy lowers the structural transition temperature while retains the AFM state in the orthorhombic phase, thus successfully realizing the AFM-FM MST in Mn_(0.95)Co_(0.75)Cu_(0.25)Ge. Moreover, successive inverse and normal MCEs are observed around the first-order AFM-FM MST and the second-order FM-paramagnetic(PM) transition, respectively. A thermostat is proposed based on this special feature, which could release heat above the critical temperature while absorb heat below the critical temperature by simply applying the same magnetization/demagnetization cycles. This thermostat can be very useful in many applications where a constant temperature is required, such as cryostats and incubators.     

Low-temperature thermal fixing of holograms in photorefractive La(3)Ga(5)SiO(14):Pr(3+) crystal

Thermal fixing in a La(3)Ga (5)SiO (14):Pr (3+) photorefractive crystal is demonstrated all the way down to room temperature. This, to our knowledge, is the first report of such an effect in any photorefractive material. From the temperature dependence of the process the activation energy of the carriers involved in the fixing process is measured to be E(A)=0.89 eV . Further, an effective photorefractive charge density of (1.4+/-0.2)x10(16) cm (-3) and Debye screening length of (6.8+/-0.7)x10(-6) cm is measured.     

Signatures of a hybridization gap in magnetic susceptibility of Ce(1-x)La(x)Os(4)Sb(12)

The magnetic susceptibility of Ce(1-x)La(x)Os(4)Sb(12), with x ≈ 0.2, exhibits a pronounced maximum at T(m) = 80 K. This T(m) coincides roughly with the temperature below which a small gap, believed to be a hybridization gap, is observed in spectroscopic measurements in undoped CeOs(4)Sb(12). However, a similar anomaly, at a lower temperature of 50 K, is observed in LaOs(4)Sb(12). Furthermore, there is a monotonic variation of T(m) with x, for x > 0.2, suggesting the same origin of the two anomalies and undermining a simple hybridization gap interpretation of the susceptibility of Ce(1-x)La(x)Os(4)Sb(12) alloys, with x < 1. A possibility of the hybridization gap opening, induced by freezing out of local phonons, strongly coupled with electronic degrees of freedom, is discussed.     

Metal-to-insulator crossover in the low-temperature normal state of Bi(2)Sr(2-x)La(x)CuO(6+delta)

We measure the normal-state in-plane resistivity of Bi(2)Sr(2-x)La(x)CuO(6+delta) single crystals at low temperatures by suppressing superconductivity with 60 T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to an insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates.     

Surface phase transitions and defect-phonon interactions: The Si(111)-(2 × 1) to (7 × 7) to (1 × 1) transitions

A thermodynamic equation is derived, showing the relation between defect-phonon interactions, the free energy of formation of defects and surface disordering or melting. When applied to Si(111), the theory shows that the (2 × 1) to (7 × 7) transition temperature is deduced from the (7 × 7) to (1 × 1) temperature via the relative concentrations of dangling bonds on the (2 × 1) and (7 × 7) surfaces.     

Z(2) vortices and the SU(2) string tension

Topologically determined Z(2) variables in pure SU(2) lattice gauge theory are discussed. They count the number of “vortex souls”. The high temperature expansion for the corresponding Z(2) loops is examined. They obey an area law. The coefficient of the area is shown to be equal to the string tension to all orders of the high temperature expansion. This shows that the string tension is determined by the probability distribution of the vortex souls, at least in the high temperature region. The dependence of the string tension α(β, h) on an external field h that is coupled to the Z(2) field strength is calculated to lowest order of the high temperature expansion. In this approximation, α(β, h) is determined by the free energy of a 2-dimensional Ising model in an external magnetic field $\text{1}\text{2}\text{log}\text{(β/4}\text{tanh}\text{h)}$ at an inverse temperature $\text{1}\text{2}\text{log}\text{3}\text{4}\text{π = 0.429}$.     

Translational heating of D(2) molecules thermally desorbed from Si(100) and Ge(100) surfaces

The translational energies of D(2) molecules thermally desorbed from the Si(100) and Ge(100) surfaces under a heating rate of 6 K/s have been measured. In contrast to the previous laser desorption study, results show a considerable translational heating; the observed translational temperature is about 3 times higher than the desorption temperature for both surfaces. This fact indicates that energy barriers for adsorption are present even in the desorption pathway. Detailed balance is applicable to the adsorption and desorption dynamics of hydrogen on the Si(100) surface.     

Crystal-lattice coupling to the vortex-melting transition in YBa(2)Cu(3)O(7-delta)

Distinct discontinuities in the thermal expansion of the crystal lattice are observed at the melting transition of the vortex lattice in a naturally untwinned reversible YBa(2)Cu3O(7-delta) single crystal using high-resolution dilatometry. This coupling between the vortex transition and the crystal lattice demonstrates that the crystal lattice is more than a mere host for the vortices, and it is attributed to a strong pressure dependence of the superconducting transition temperature and thus to the condensation energy at the vortex-melting temperature.     

Multiple magnetic transitions in single crystals of Ce_(12)Fe_(57.5)As_(41) and La_(12)Fe_(57.5)As_(41)

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce_(12)Fe_(57.5)As_(41)and La_(12)Fe_(57.5)As_(41).The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds.Both compounds exhibited multiple magnetic orders within 2-300 K and metamagnetic transitions at various fields.Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce_(12)Fe_(57.5)As_(41)and La_(12)Fe_(57.5)As_(41),respectively,followed by antiferromagnetic type spin reorientations near Curie temperatures.The magnetic properties underwent complex evolution in the magnetic field for both compounds.An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce_(12)Fe_(57.5)As_(41).The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure.A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce_(12)Fe_(57.5)As_(41).A temperature-field phase diagram was present for these two rare earth systems.In addition,a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150-300 K,which is rarely found in 3D-based compounds.It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.     

La_(0.7-x)Gd_xSr_(0.3)MnO_3(x=0.20,0.30,0.40,0.50)的TCR研究(英文)

研究了La位Gd掺杂对La0.7-xGdxSr0.3MnO3(x=0.20,0.30,0.40,0.50)体系的电阻温度系数(TCR)的影响.实验结果表明:Gd掺杂将引起电阻率曲线的急剧变化,导致出现大的TCR;而且随Gd掺杂的增加,TCR在x=0.30出现峰值,然后随掺杂量增加逐步降低.体系出现大的TCR,来源于Gd掺杂引起的额外磁性耦合.     

The effective potential and symmetry breaking in theO(N) xO(N) model

We compute the finite temperature effective potential in theO(N)xO(N) symmetric model for largeN in spacetime dimensionsd=4, 3, 2 and discuss the spontaneous symmetry breaking patterns. Ford=3 we find that the symmetry, if broken at zero temperature is restored at arbitrarily small nonzero temperature. Ford=4 the model is plagued by an intrinsic instability; in particular, there is no stable ground state at high temperature.     

Electron spin relaxation in pseudo-Jahn-Teller low-symmetry Cu(II) complexes in diaqua(L-aspartate)Zn(II).H(2)O crystals.

Low-temperature (4-55 K) pulsed EPR measurements were performed with the magnetic field directed along the z-axis of the g-factor of the low-symmetry octahedral complex [(63)Cu(L-aspartate)(2)(H2O)2] undergoing dynamic Jahn-Teller effect in diaqua(L-aspartate)Zn(II) hydrate single crystals. Spin-lattice relaxation time T(1) and phase memory time T(M) were determined by the electron spin echo (ESE) method. The relaxation rate 1/T(1) increases strongly over 5 decades in the temperature range 4-55 K. Various processes and mechanisms of T(1)-relaxation are discussed, and it is shown that the relaxation is governed mainly by Raman relaxation processes with the Debye temperature Theta(D)=204 K, with a detectable contribution from disorder in the doped Cu(2+) ions system below 12 K. An analytical approximation of the transport integral I(8) is given in temperature range T=0.025-10Theta(D) and applied for computer fitting procedures. Since the Jahn-Teller distorted configurations differ strongly in energy (delta(12)=240 cm(-1)), there is no influence of the classical vibronic dynamics mechanism on T(1). Dephasing of the ESE (phase relaxation) is governed by instantaneous diffusion and spectral diffusion below 20 K with resulting rigid lattice value 1/T(0)(M)=1.88 MHz. Above this temperature the relaxation rate 1/T(M) increases upon heating due to two mechanisms. The first is the phonon-controlled excitation to the first excited vibronic level of energy Delta=243 cm(-1), with subsequent tunneling to the neighbor potential well. This vibronic-type dynamics also produces a temperature-dependent broadening of lines in the ESEEM spectra. The second mechanism is produced by the spin-lattice relaxation. The increase in T(M) is described in terms of the spin packets forming inhomogeneously broadened EPR lines.     

Dirty superconductivity in the electron-doped cuprate Pr(2-x)Ce(x)CuO(4-delta): tunneling study

We report a tunneling study between Pr(2-x)Ce(x)CuO(4-delta) and lead as a function of doping, temperature, and magnetic field. The temperature dependence of the gap follows the BCS prediction. Our data fit a nonmonotonic d-wave order parameter for the whole doping range studied. From our data we are able to conclude that the electron-doped cuprate Pr(2-x)Ce(x)CuO(4-delta) is a weak-coupling BCS dirty superconductor.     

LaB0.5Mn0.5O3(B=Co,Fe)的磁性及电输运特性的研究

采用固相烧结工艺制备了钙钛矿锰氧化物LaCo0.5Mn0.5 O3和LaFe0.5 Mn0.5O3,系统研究了样品的磁性、电输运特性和磁电阻效应.室温下LaCo0.5M0.5O3和LaFe0.5Mn0.5O3均呈现顺磁行为.电子顺磁共振得到LaFe0.5Mn0.5O3样品的朗德因子g=1.9661,与Fe3+离子(g=...     

Magnetic glass in shape memory alloy: Ni(45)Co(5)Mn(38)Sn(12)

The first order martensitic transition in the ferromagnetic shape memory alloy Ni(45)Co(5)Mn(38)Sn(12) is also a magnetic transition and has a large field induced effect. While cooling in the presence of a field this first order magnetic martensite transition is kinetically arrested. Depending on the cooling field, a fraction of the arrested ferromagnetic austenite phase persists down to the lowest temperature as a magnetic glassy state, similar to the one observed in various intermetallic alloys and in half doped manganites. A detailed investigation of this first order ferromagnetic austenite (FM-A) to low magnetization martensite (LM-M) state transition as a function of temperature and field has been carried out by magnetization measurements. Extensive cooling and heating in unequal field (CHUF) measurements and a novel field cooled protocol for isothermal MH measurements (FC-MH) are utilized to investigate the glass like arrested states and show a reverse martensite transition. Finally, we determine a field-temperature (HT) phase diagram of Ni(45)Co(5)Mn(38)Sn(12) from various magnetization measurements which brings out the regions where thermodynamic and metastable states coexist in the HT space, clearly depicting this system as a 'magnetic glass'.