Total suppression of superconductivity by high magnetic fields in YBa(2)Cu(3)O(6.6)

We have studied the variation of transverse magnetoresistance of underdoped YBCO(6.6) crystals, either pure or with reduced T(c) down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconducting fluctuation contribution to the conductivity is suppressed only above a threshold field H(c)'(T), which is found to vanish at T(c)' > T(c). In the pure YBCO(6.6) sample, H(c)' is already 50 T at T(c). We find that increasing disorder weakly depresses H(c)'(0), T(c)', and T(nu), the onset of the Nernst signal. Thus, these energy scales appear more characteristic of the 2D local pairing than the pseudogap temperature which is not modified by disorder.     

Entropy of vortex cores near the superconductor-insulator transition in an underdoped cuprate

We present a study of Nernst effect in underdoped La(2-x)Sr(x)CuO4 in magnetic fields as high as 28 T. At high fields, a sizable Nernst signal was found to persist in the presence of a field-induced nonmetallic resistivity. By simultaneously measuring resistivity and the Nernst coefficient, we extract the entropy of vortex cores in the vicinity of this field-induced superconductor-insulator transition. Moreover, the temperature dependence of the thermoelectric Hall angle provides strong constraints on the possible origins of the finite Nernst signal above T(c), as recently discovered by Xu et al. [Nature (London) 406, 486 (2000)].     

Unusual Nernst effect suggesting time-reversal violation in the striped cuprate superconductor La(2-x)Ba(x)CuO4

The striped cuprate La(2-x)Ba(x)CuO(4) (x=1/8) undergoes several transitions below the charge-ordering temperature T(co)=54 K. From Nernst experiments, we find that, below T(co), there exists a large, anomalous Nernst signal e(N,even)(H,T) that is symmetric in field H, and remains finite as H→0. The time-reversal violating signal suggests that, below T(co), vortices of one sign are spontaneously created to relieve interlayer phase frustration.     

Ambipolar Nernst effect in NbSe2

The first study of the Nernst effect in NbSe2 reveals a large quasiparticle contribution with a magnitude comparable and a sign opposite to the vortex signal. Comparing the effect of the charge density wave (CDW) transition on Hall and Nernst coefficients, we argue that this large Nernst signal originates from the thermally induced counterflow of electrons and holes and indicates a drastic change in the electron scattering rate in the CDW state. The results provide new input for the debate on the origin of the anomalous Nernst signal in high-T(c) cuprates.     

Staggered flux vortices and the superconducting transition in the layered cuprates

We propose an effective model for the superconducting transition in the high-T(c) cuprates motivated by the SU(2) gauge theory approach. In addition to variations of the superconducting phase we allow for local admixture of staggered flux order. This leads to an unbinding transition of vortices with a staggered flux core that are energetically preferable to conventional vortices. Based on parameter estimates for the two-dimensional t-J model we argue that the staggered flux vortices provide a way to understand a phase with a moderate density of mobile vortices over a large temperature range above T(c) that yet exhibits otherwise normal transport properties. This picture is consistent with the large Nernst signal observed in this region.     

Ferromagnetism and metal-insulator transition in the disordered Hubbard model

A detailed study of the paramagnetic to ferromagnetic phase transition in the one-band Hubbard model in the presence of binary-alloy disorder is presented. The influence of the disorder (with concentrations x and 1-x of the two alloy ions) on the Curie temperature T(c) is found to depend strongly on electron density n. While at high densities, n>x, the disorder always reduces T(c); at low densities, n相似文献   

Band-dependent normal-state coherence in Sr2RuO4: evidence from Nernst effect and thermopower measurements

We present the first measurement on the Nernst effect in the normal state of the odd-parity, spin-triplet superconductor Sr2RuO4. Below 100 K, the Nernst signal was found to be negative, large, and, as a function of magnetic field, nonlinear. Its magnitude increases with the decreasing temperature until reaching a maximum around T* approximately equal to 20-25 K, below which it starts to decrease linearly as a function of temperature. The large value of the Nernst signal appears to be related to the multiband nature of the normal state and the nonlinearity to band-dependent magnetic fluctuation in Sr2RuO4. We argue that the sharp decrease in the Nernst signal below T* is due to the suppression of quasiparticle scattering and the emergence of band-dependent coherence in the normal state.     

Influence of pair breaking and phase fluctuations on disordered high Tc cuprate superconductors

Electron irradiation has been used to introduce point defects in a controlled way in underdoped and optimally doped YBa(2)Cu(3)O(7-delta) crystals. This technique allows us to perform very accurate measurements of T(c) and of the ab plane resistivity in a wide range of defect contents x(d) down to T(c)=0. The variation of T(c) and of the transition width with x(d) do not follow current predictions of pair-breaking theories. The data are rather compatible, at least for the highly damaged regime, with the expected influence of phase fluctuations. These results open new questions about the evolution of the defect induced T(c) depression over the phase diagram of the cuprates.     

Field-enhanced diamagnetism in the pseudogap state of the cuprate Bi2Sr2CaCu2O(8+delta) superconductor in an intense magnetic field

In hole-doped cuprates, Nernst experiments imply that the superconducting state is destroyed by spontaneous creation of vortices which destroy phase coherence. Using torque magnetometry on Bi2Sr2CaCu2O(8+delta), we uncover a field-enhanced diamagnetic signal M above the transition temperature Tc that increases with applied field to 32 Tesla and scales just like the Nernst signal. The magnetization results above Tc distinguish M from conventional amplitude fluctuations and strongly support the vortex scenario for the loss of phase coherence at Tc.     

First-order transition from a Kondo insulator to a ferromagnetic metal in single crystalline FeSi(1-x)Ge(x)

The phase diagram of FeSi(1-x)Ge(x), obtained from magnetic, thermal, and transport measurements on single crystals, shows a discontinuous transition from Kondo insulator to ferromagnetic metal with x at a critical concentration, x(c) approximately 0.25. The gap of the insulating phase strongly decreases with x. The specific heat gamma coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with an insulator-metal transition induced by a reduction of the hybridization with x in conjunction with disorder on the Si/Ge ligand site.     

Influence of disorder on the local density of states in high- T(c) superconducting thin films

Using a low temperature scanning tunneling microscope in the spectroscopic mode, we find that the disorder in a Bi(2)Sr(2)CaCu(2)O(8+delta) thin film modifies dramatically the quasiparticle local density of states. Small, but well-defined superconducting regions, coexisting with dominating semiconducting areas, show well-pronounced gap structures, similar to those observed previously in high-quality single crystals. Surprisingly, between these two regions, the detailed shape of the quasiparticle spectrum is virtually identical to the pseudogap previously observed at temperatures T>T(c), or in the vortex core, at 4.2 K. Thus, the role of the disorder in destroying the superconducting phase is comparable to that of the magnetic field or thermal fluctuations.     

Magnetic-field-induced lattice anomaly inside the superconducting state of CeCoIn5: anisotropic evidence of the possible Fulde-Ferrell-Larkin-Ovchinnikov state

We report high magnetic field linear magnetostriction experiments on CeCoIn5 single crystals. Two features are remarkable: (i) a sharp discontinuity in all the crystallographic axes associated with the upper superconducting critical field B(c2) that becomes less pronounced as the temperature increases and (ii) a distinctive second orderlike feature observed only along the c axis in the high field (10 T < or approximately B< or = B(c2)) low temperature (T < or approximately 0.35 K) region. This second order transition is observed only when the magnetic field lies within 20 degrees of the ab planes and there is no signature of it above B(c2), which raises questions regarding its interpretation as a field induced magnetically ordered phase. Good agreement with previous results suggests that this anomaly is related to the transition to a possible Fulde-Ferrel-Larkin-Ovchinnikov superconducting state.     

Inhomogeneous superconductivity in organic conductors: the role of disorder and magnetic field

Several experimental studies have shown the presence of spatially inhomogeneous phase coexistence of superconducting and non-superconducting domains in low dimensional organic superconductors. The superconducting properties of these systems are found to be strongly dependent on the amount of disorder introduced in the sample regardless of its origin. The suppression of the superconducting transition temperature T(c) shows a clear discrepancy with the result expected from the Abrikosov-Gor'kov law giving the behavior of T(c) with impurities. On the basis of the time dependent Ginzburg-Landau theory, we derive a model to account for this striking feature of T(c) in organic superconductors for different types of disorder by considering the segregated texture of the system. We show that the calculated T(c) quantitatively agrees with experiments. We also focus on the effect of superconducting fluctuations on the upper critical fields H(c2) of layered superconductors showing slab structure where superconducting domains are sandwiched by non-superconducting regions. We found that H(c2) may be strongly enhanced by such fluctuations.     

Spin Nernst effect in the absence of a magnetic field

We study the spin Nernst effect of a mesoscopic four-terminal cross-bar device with the Rashba spin–orbit interaction (SOI) in the absence of a magnetic field. The interplay between the spin Nernst effect and the seebeck coefficient is investigated for a wide range of the Rashba SOI. When no peaks appeared in the seebeck coefficient, an oscillatory spin Nernst effect still occurs. In addition, the disorder effect on the spin Nernst effect is also studied. We find that the spin Nernst effect can be enhanced up to threefold by disorder. Besides, due to the interface effect, the counter propagating of the charge current to the direction of the temperature gradient is possible for a nonuniform system.     

Li1-xNaxCu2O2单晶磁化率的研究

本文报道了自助溶剂法制备的Li1-xNaxCu2O2（x=0,0.05,0.1,0.2）单晶沿c方向的磁化率随温度变化χ（T）的实验结果.随着Na掺杂量的增加,磁化率整体上移,高温区磁化率更加平缓;不同掺杂样品磁化率都在40K附近存在最大值,掺杂没有引起磁化率最大值对应的温度的变化,但引起了更低温下（T〈14K）磁化率...     

Disorder induced transition into a one-dimensional wigner glass

The destruction of quasi-long-range crystalline order as a consequence of strong disorder effects is shown to accompany the strict localization of all classical plasma modes of one-dimensional Wigner crystals at T=0. We construct a phase diagram that relates the structural phase properties of Wigner crystals to a plasmon delocalization transition recently reported. Deep inside the strictly localized phase of the strong disorder regime, we observe glasslike behavior. However, well into the critical phase with a plasmon mobility edge, the system retains its crystalline composition. We predict that a transition between the two phases occurs at a critical value of the relative disorder strength. This transition has an experimental signature in the ac conductivity as a local maximum of the largest spectral amplitude as a function of the relative disorder strength.     

Phase-coherence effects in vortex transport entropy

Nernst and electrical resistivity measurements in superconducting YBa2Cu3O7-delta and Bi2Sr2CaCu2O8+delta with and without columnar defects show a distinctive thermodynamics of the respective liquid vortex matter. At a field-dependent high temperature region in the H-T phase diagram, the Nernst signal is independent of structural defects in both materials. At lower temperatures, in YBa2Cu3O7-delta, defects contribute only to the vortex mobility, and the transport entropy is that of a system of vortex lines. The transition to lower temperatures in Bi2Sr2CaCu2O8+delta has a different origin; the maximum in the Nernst signal when decreasing temperature is not associated with transport properties but with the entropy behavior of pancake vortices in the presence of structural defects.     

Transverse Josephson plasma mode in T* phase SmLa(1-x)Sr(x)CuO(4-delta) single crystals

Far-infrared reflectivity along the c axis in T* phase SmLa(1-x)Sr(x)CuO(4-delta) single crystals is measured down to 8 cm(-1). Below T(c), the conductivity peak is observed at 25 cm(-1) for x = 0.15 ( T(c) = 30 K) along with two reflectivity edges at 13 and 27 cm(-1). The conductivity peak is attributed to the transverse Josephson plasma mode between two longitudinal Josephson plasma modes, while the oscillator strength of the peak is found to be smaller than that calculated using the Josephson-coupled multilayer model. The difference is explained by assuming that only a few junctions at the disordered (La,Sr)(2)O(2-delta) block layer take part in the plasma oscillation with omega(pI(')) = 27 cm(-1).     

Evidence for a new magnetic field scale in CeCoIn5

The Nernst coefficient of displays two distinct anomalies in magnetic field. The feature detected at Hk approximately 23 T is similar to what is observed in CeRu2Si2 at Hm = 7.8 T where a metamagnetic transition occurs. In CeCoIn5, new frequencies are observed in de Haas-van Alphen oscillations when the field exceeds 23 T where the Dingle temperature decreases by about 30%. Based on the Nernst coefficient anomalies, the magnetic phase diagram of CeCoIn5 is revised.     

Field-induced phase transitions in chiral smectic liquid crystals studied by the constant current method

In ferroelectric liquid crystals, phase transitions can be induced by an electric field. The current constant method allows these transition to be quickly localized and thus the(E, T) phase diagram of the studied product can be obtained.In this work, we make a slight modification to the measurement principles based on this method. This modification allows the characteristic parameters of ferroelectric liquid crystal to be quantitatively measured. The use of a current square signal highlights a phenomenon of ferroelectric hysteresis with remnant polarization at null field, which points out an effect of memory in this compound.