Underdoped Crystals Research Articles

STUDY OF LOCAL STRUCTURE IN UNDER-DOPED La2-xSrxCuO4-y BY POLARIZED EXAFS

Temperature dependence of local structure of the CuO 2 plane has been studied by in plane polarized (E//ab) Cu K-edge extended X-ray absorption fine structure (EXAFS) experiments on an under-doped single crystal of superconducting La1.87Sr0.13CuO 4-y system with T c =32 K . The temperature dependence of the Debye–Waller factors for the Cu-O and Cu-La/Sr pairs are determined. We find that the motion of Cu and O atoms in the Cu-O bond is partially correlated at higher temperatures, going over to an uncorrelated motion at lower temperatures. The results are discussed in terms of the local lattice instabilities and stripes as reported in other systems.

Manipulation of single vortices in YBa2Cu3O6.354 with a locally applied magnetic field

We demonstrate the controlled, reversible manipulation of individual vortices in a superconductor with a locally applied magnetic field. The local field is supplied by a field coil on a superconducting quantum interference device (SQUID). The SQUID is used to image the vortices before and after moving. This device can be used both to push individual vortices and to create individual vortex–antivortex pairs. We calculate the force applied on a rigid vortex and find that ∼0.5 pN is necessary to move vortices in underdoped single crystals of YBa2Cu3O6.354 with Tc∼ 12 K.

Gd substitution effect on the normal-state transport properties of Bi 2Sr 2Ca 1− xGd xCu 2O 8+ δ single crystals

Doping dependence of the anisotropic resistivity and in-plane thermopower have been systematically studied for Bi 2Sr 2Ca 1− x Gd x Cu 2O 8+ δ single crystals ( x=0, 0.09, 0.19, 0.32 and 0.41), whose hole concentration changes from overdoped to underdoped level. For the underdoped crystals, the in-plane resistivity ρ ab ( T) exhibits an obvious downturn from the T-linear behavior at characteristic temperature T *, whose value increases with x, which manifests the opening of the spin gap in the normal state. The c-axis resistivity ρ c ( T) and the anisotropy ρ c ( T)/ ρ ab ( T) increase rapidly with increasing Gd-doping concentration. The in-plane thermopower S increases monotonously with decreasing the carrier concentration, and its temperature dependence for the underdoped crystals shows a deviation from the T-linear dependence at high temperature region. By assuming a characteristic temperature T scale where the thermopower S begins to deviate from T-linear behavior, S( T)/ S( T scale) vs. T/ T scale can be well scaled into a universal curve for all the underdoped crystals. The T scale, which is supposed to relate with pseudogap, shows the similar doping dependence to that of T *. However, there are quantitatively difference between T * and T scale.

Preparation and picosecond optical response of Bi system high- Tc superconductors

Bi 2Sr 2Ca 1− x Y x Cu 2O 8+ δ single crystals were grown by traveling solvent floating zone method. The crystallinity and the superconducting property of the single crystals were characterized by X-ray diffraction and in-plane resistivity measurements. To investigate the ultrafast optical response, time-resolved pump–probe reflectivity measurements were carried out on the single crystals. As a result, fairly long relaxation times of about 20 and 6 ps of excited carriers were estimated for the crystals with x=0 and 0.2, respectively, as compared to 1.7 ps for YBa 2Cu 3O 7− δ thin films in previous study. The relaxation time observed on the underdoped crystal with x=0.2 showed a good consistency with that of Bi 2Sr 2CaCu 2O 8+ δ thin film in underdoped regime.

On the growth of underdoped Bi 2Sr 2CaCu 2O 8+ δ single crystals by TSFZ method

The growth of underdoped Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) single crystals has been studied by the traveling solvent floating zone (TSFZ) technique under low oxygen pressures p(O 2)=1, 8×10 −1, 5×10 −1, 1×10 −1, 5×10 −2, 1×10 −3 and 5×10 −4 bar, respectively. A starting composition of Bi 2.1Sr 1.9CaCu 2O 8+ δ and a slow growth rate of 0.2 mm/h were employed for the growth. The effect of oxygen pressure on the growth habit, structure and superconducting properties of underdoped Bi-2212 crystals was investigated. It was observed that the stability of the molten zone and the size of crystals were not sensitively influenced by decreasing p(O 2) from 1 to 5×10 −4 bar, however, the oxygen distribution in as-grown crystals became inhomogeneous as p(O 2) decreased. Underdoped Bi-2212 crystals with the considerable size up to 15×5×1 mm 3 were obtained. X-ray diffraction (XRD) studies indicate that the as-grown crystals have orthorhombic symmetry. The crystals do not show phase decomposition until growth pressure p(O 2)=1×10 −3 bar and the Bi-2212 phase decomposes significantly when p(O 2) decreased to 5×10 −4 bar, according to XRD results. With decreasing p(O 2) from 1 down to 1×10 −3 bar the c-axis extends monotonically from 30.731(0) to 30.912(8) Å while the superconducting transition temperature, T c, reduces from 92 to 79 K and the transition width, Δ T c, increases from 2 to 10 K. A further decrease of p(O 2) to 5×10 −4 bar leads to a broader Δ T c (∼20 K) due to the oxygen inhomogeneity and phase segregation.

Magnetic and magneto-transport properties of NdBaCuO single crystals with varying Nd/Ba substitution and oxygen content

The effects of Nd/Ba substitution and oxygen contents on the peak effect of J c and transport properties in the normal states of Nd 1+ y Ba 2− y Cu 3O 7− x single crystals are studied. We find that the Nd/Ba substitution plays a more important role for the peak effect than the oxygen vacancies in nearly optimum-doped crystals. In heavily underdoped crystals with y∼0, we observe a peculiar charge localization in the out-of-plane and in-plane conductivity followed by superconducting transition at low temperature. Magnetization studies reveal that this behavior can be explained by the existence of spin-glass-like antiferromagnetic ordering above T c in the oxygen deficient region of x=0.4–0.6.

Study of the peak effect in pure, Pb and Pb+Y doped Bi-2212 single crystals

The peak effect (PE) in pure, Pb and Pb+Y doped Bi2212 single crystals with different oxygen doping levels was studied by measuring M– H loops over a wide temperature range. The PE in pure Bi2212 crystals was obtained only for crystals with optimal oxygen doping and over-doping but not observed for oxygen under-doped crystals. For Pb doped Bi2212 crystals, the PE appeared at a higher field than in the pure crystals and persisted up to T c. For (Bi 1.64Pb 0.36)Sr 2Ca 1− x Y x Cu 2O 8+ y ( x=0, 0.05, 0.11, 0.33) single crystals, results show that at low temperatures, the peak field is smaller than in solely Pb doped crystals and decreases as x increases ( x>0.1). However, the peak field at high temperature for the x=0.05 sample is higher than for heavily Pb doped Bi2212 crystals, indicative of strong pinning due to the co-doping. The formation of Bi 5+ rich clusters, which cause the reduction of the c-axis lattice parameter and ρ c , is proposed to be responsible for the appearance of the PE in the undoped crystals. The co-existence of Pb 4+- and Bi 5+-rich clusters causes the strong PE in Pb doped Bi2212 crystals. Y 3+ is proposed to be an effective dopant at low doping levels for flux pinning at high temperatures. The PE for all the crystals was characterised by plotting ( H max− H min)/ H max vs T/ T c, where H min and H max represent the fields at which the magnetisation starts to increase ( H min) and reaches a maximum ( H max) at the peak position. Results showed that the evolution of the peak effect with temperature in the Pb and Pb+Y doped crystals was similar to that seen in Y123.

C-Electron transport in Bi 2Sr 2CaCu 2O 8+ δ and Bi 2Sr 2CuO 6+ δ probed by Josephson plasma resonance

We have measured the Josephson plasma resonance for underdoped Bi 2Sr 2CaCu 2O 8+ δ and Bi 2Sr 2CuO 6+ δ by a novel scanning microwave spectrometer in a wide frequency range (21–147 GHz). The resonance frequency and the line width enable us to determine a superfluid density and quasiparticle conductivity σ qp c in the c-axis direction accurately. In the slightly underdoped crystals, the superfluid density shows very little change at low temperature, which is consistent with a d-wave coherent tunneling model. Below T c, σ qp c falls to low values, then decreases gradually with decreasing T. This behavior is very different from that in the ab plane.

Observation of the Andreev reflection in thec-axis transport ofBi2Sr2CaCu2O8+xsingle crystals nearTcand the search for the preformed-pair state

We observed an enhancement of the c-axis differential conductance around the zero bias in $\mathrm{Au}/{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ (Bi2212) junctions near the superconducting transition temperature ${T}_{c}.$ We attribute the conductance enhancement to the Andreev reflection between the surface Cu-O bilayer with suppressed superconductivity and the neighboring superconducting inner bilayer. The continuous evolution from depression to an enhancement of the zero-bias differential conductance, as the temperature approaches ${T}_{c}$ from below, points to weakening of the barrier strength of the nonsuperconducting layer between adjacent Cu-O bilayers. We observed that the conductance enhancement persisted up to a few degrees above ${T}_{c}$ in junctions prepared on slightly overdoped Bi2212 crystals. However, no conductance enhancement was observed above ${T}_{c}$ in underdoped crystals, although a recently proposed theoretical consideration suggests an even wider temperature range of enhanced zero-bias conductance. This seems to provide a negative perspective to the existence of the phase-incoherent preformed pairs in the pseudogap state.

Tunneling spectroscopy of various underdoped Bi 2Sr 2CaCu 2O 8+ δ

Tunneling spectra of vacuum annealed and Y-substituted underdoped Bi2212 single crystals have been measured using break junctions. The observed gap value is indeed larger than that of the as-grown (slightly overdoped) Bi2212 with T c=87 K, but the peak structure disappears at the lower T c≈40–70 K depending on the growth condition. This thermal behavior is in contrast to the as-grown sample showing the gap-closing temperature as high as ≈250 K. Further, the gap value of the vacuum-annealed Bi2212 is larger than that of the Y-substituted Bi2212 for the same T c, suggesting the existence of another control parameter than T c.

Simple microcalorimeter for measuring microgram samples at low temperatures

An innovative microcalorimeter has been developed for measuring specific heat of very small microgram samples in the temperature range from 1.5 to 50 K and in magnetic fields up to 11 T. The device is built from a commercial sapphire temperature chip (Cernox), which is modified by means of standard microfabrication techniques and which is used as a sample holder, temperature sensor, and sample heater. Compared to existing microcalorimeters the simple design of our instrument allows a fabrication of the device in a few process steps by using facilities present in a standard laboratory clean room. As an illustrative example for the performance of our device, the specific heat of an underdoped (La1−xSrx)2CuO4 and CaRuO3 single crystal has been measured by means of the relaxation time method as well as the ac method.

Thermopower of Bi2Sr2Ca1-xRxCu2Oy (R = Gd, Pr) single crystals

The temperature and carrier concentration dependence of the thermopower (TEP)S in Bi2Sr2Ca1-xRxCu2Oy (R = Gd, Pr) single crystals were carefully studied. Upon Gd or Prdoping, the superconductivity changes systematically from overdoping tounderdoping with the TEP increasing monotonically. S(T)/S(Tscale)against T/Tscale for all the underdoped crystals can be well scaledinto a universal curve, where Tscale is the temperature at which Sbegins to deviate from T-linear behaviour. This scaling behaviour is assumedto correlate with the opening of the pseudogap. Furthermore, the temperaturedependence of the TEP is analysed and discussed in terms of two differenttwo-band models and the Nagaosa-Lee model.

The detailed transport property of the underdoped Bi-2212 system in the pseudogap state

The detailed character of transport properties in underdoped Bi-2212 systems are analysed in the pseudogap state. In a certain underdoped regime, in-plane resistivity ρ ab ( T) shows a typical S-shaped temperature dependence and a perfect ρ ab(T)=ρ 0 *+β exp (−Δ/T) fit is observed over a wide temperature range from slight above T c up to T * where the pseudogap begins opening. The underdoped crystal, in contrast to the optimum crystal where no pseudogap is observed, in-plane transverse magnetoresistance shows a slower temperature decreasing rate with temperature increasing in the pseudogap state, and a larger value around T *. About the temperature dependence of thermopower S( T), all underdoped samples completely fall into a scaling curve S/ S * vs. T/ T * above T *. Whereas, S( T) below T * deviates from this scaling law. The above transport properties should be intimately related with the detailed prospects of Fermi surface destruction in the pseudogap state below T *.

Tunneling spectroscopy of heavily underdoped crystals of Bi 2Sr 2CaCu 2O 8+δ

Crystals of Bi 2Sr 2CaCu 2O 8+δ with optimal T c =95 K have been underdoped using two different methods and the superconducting gaps have been obtained by tunneling. In some cases, three different tunneling geometries have been utilized: point contact, STM and break junctions. The first doping method involves control of the oxygen content by annealing in various partial pressures of oxygen. These crystals exhibit a narrow spread of gap values over a wide doping range from overdoped (T c =56 K) to underdoped with T c =70 K. However, for underdoped crystals with T c midpoints in the range 25 K − 63 K, there is a dramatic increase in the spread of gap values which may signal the development of static phase separation of either chemical or electronic origin. To avoid possible chemical phase separation, we have explored another doping procedure which incorporates Dy substitution on the Ca site. These crystals exhibit a relatively narrow superconducting transition width and some preliminary tunneling spectra will be presented.

Polarized electronic Raman scattering in high-T c superconductors

We study the polarized Electronic Raman Scattering (ERS) of several classes of high-T c cuprates as functions of temperature, doping, and pressure with an emphasis on the effect of superconductivity. The doping dependence of the single-layer HgBa 2CuO 4+ δ (Hg1201) confirms our earlier observations from two-layer Bi 2Sr 2CaCu 2O 8+ δ (Bi2212) that the superconducting energy gap is highly anisotropic at optimal doping, but that 2Δ peaks associated with superconductivity are not present in underdoped crystals. Low temperature ERS measurements of the n-type (T′) Pr 1.85Ce 0.15CuO 4 at optimal doping (T c = 20K) show negligible gap anisotropy, consistent with other spectroscopic results which suggested an isotropic s-wave order parameter. Normal state Raman scattering measurements of underdoped Bi2212 at pressures up to 7.6 GPa reveal anomalies in the lattice dynamics which offer insight into local oxygen ordering and may influence the observed large hysteresis in T c (P).

Magnetoresistivity in strongly underdoped YBa2Cu3Ox single crystals

We report magnetoresistivity measurements on YBa 2 Cu 3 O 6.36 single crystals in applied magnetic fields H∥ c up to 14 T. We observe several unusual features: the out-of-plane magnetoresistivity Δρ c /ρ c is negative at high temperatures T and becomes positive for T < 150 K. The in-plane magnetoresistivity Δρ ab /ρ ab is positive and displays a maximum at T ≈ 180 K. There is a direct correlation between the sign and T-dependence of magnetoresistivities and the corresponding temperature coefficients of resistivities; i.e. Δρ c, ab ∼ dρ c,ab / dT over the measured T range for the in-plane component and for T > 150 K for the out-of-plane component.

Low field flux dynamics of Pr-doped Bi-2212 single crystals

Magnetic hysteresis M(H,T) and magnetic relaxation M(t) results on various Pr-doped single crystals Bi 2Sr 2Ca 1−xPr xCu 2O 8+δ are reported. Superconducting H-T phase diagram with applied field parallel to single crystal c-axis indicates that with progressive Pr doping from optimum composition (x = 0.11, T c = 88 K) to underdoped single crystal (x > 0.11), all characteristic field lines in H-T diagram shift to lower reduced temperature T/T c and lower field region with narrower second peak (H 2p) window. Low field magnetic relaxation data near H 2p indicates a distinct change of flux creep behavior from collective creep model for field smaller than second peak onset field (H on) to vortex glass model for H > H on. Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-lattice to entangled vortex glass.

Temperature induced normal state redistribution of B 1g spectral weight in underdoped La 2-xSr xCuO 4

In electronic Raman Scattering experiments carried out on underdoped crystals of La 2-xSr xCuO 4 (La214) there is a depletion of low energy B 1g spectral weight relative to that measured in optimally doped (x ≈ 0.16) samples. In lightly underdoped (x ≈ 0.13) samples the depletion is independent of temperature for 20K < T < 300K. As the doping level is decreased further, additional spectral weight is lost and, furthermore, in the more heavily underdoped (x ≈ 0.08) samples the depletion at low energies also increases with decreasing temperature. Estimates are obtained for the magnitude of the pseudogap, E g, and the onset temperature T*.

POSSIBLE EVIDENCE OF CHARGE-STRIPE ORDERING IN THE ab-PLANE RESISTIVITY OF STRONGLY UNDERDOPED La2-xSrxCuO4 SINGLE CRYSTALS

We measured the ab-plane resistivity of La 2-x Sr x CuO 4 underdoped single crystals (x = 0.052 and x=0.06) between 4.2 and 300 K by using an AC version of the Van der Pauw technique. We suggest that a possible scenario of charge-stripe ordering at the lowering of the temperature can be delineated by starting from the main features of the ρ ab (T) curves of these samples. In particular, the deviation from the linearity of the ab-plane resistivity, occurring at a temperature T ch , could be related to the beginning of the charge localization, and the upturn of ρ ab to the progressive pinning of the resulting charge stripes. By starting from an analysis of our ab-plane resistivity curves, we determined the temperature of charge ordering T ch for our samples, thus extending in consistent way the T ch vs. x curve reported in literature to the very low-doping region of the phase diagram.

Same superconducting criticality for underdoped and overdoped La2-xSrxCuO4 single crystals

By measuring the superconducting diamagnetic moments for an underdoped and an overdoped La2-xSrxCuO4 single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to H(c2), but the overdoped sample has two transitions with the higher one at H(c2). Further investigation reveals the same upper-critical field H(c2) for both samples although the overall charge densities are very different, indicating the possibility of a very direct and detailed equivalence of the superconducting condensation process in the two doping limits. The second transition for the overdoped sample can be understood as the bulk coupling between the superconducting clusters produced by macroscopic phase separation.