arXiv subjects

Th. Wolf

Publications and source records attributed to Th. Wolf.

62 records · Page 4Linked to original sources

arXiv2001

Pressure Effect and Specific Heat of RBa2Cu3Ox at Distinct Charge Carrier Concentrations: Possible Influence of Stripes

In YBa2Cu3Ox, distinct features are found in the pressure dependence of the transition temperature, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal, whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh) behavior, the features coalesce for all samples at nh=0.11 and nh=0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO2 planes. Analyzing our results we obtain different mechanisms in three doping regions: Tc changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dTc/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content below nh=0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials.

cond-mat.supr-con↗

arXiv1999

The Effects of Chemical Doping and Hydrostatic Pressure on Tc of Y(1-y)CayBa2Cu3Ox and NdBa2Cu3Ox Single Crystals

We studied Tc of YBa2Cu3Ox (Y123), Y0.89Ca0.11Ba2Cu3Ox (YCa123) and NdBa2Cu3Ox (Nd123) single crystals with various oxygen contents x. Compared to Tc(x) of Y123 the Tc(x) curves of YCa123 are shifted to lower oxygen contents and the maximum transition temperature Tc,max decreases with increasing Ca content whereas in Nd123 Tc(x) is shifted to higher oxygen contents and Tc,max is increased. According to the universal parabolic Tc(nh) behavior the differences in Tc(x)of Y123, YCa123 and Nd123 can be ascribed to different hole concentrations nh in the CuO2 planes caused by doping via changes in chemistry or structure. In order to study the influence of structural changes on Tc we examined the hydrostatic pressure effect dTc/dp (p up to 0.6 GPa). In the underdoped region, at nh=0.11, the examined compounds show a peak in dTc/dp which is very pronounced for systems with well ordered CuO chains. As this peak occurs at the same nh in all investigated systems it is not caused by oxygen ordering, but its origins might be found in a strong influence of lattice deformations on the electronic structure.

cond-mat.supr-con↗

arXiv1999

Calorimetric Investigation of NdBa2Cu3Ox Single Crystals

The specific heat of Nd(Y)Ba2Cu3Ox single crystals without substitution of Nd for Ba has been measured for various oxygen contents. In comparison with YBa2Cu3Ox, the maximum attainable transition temperature of the crystals is about 4 K higher and shifted from x = 6.92 to x = 7.0, the highest possible oxygen content. In spite of the difference in the Tc(x) dependences, the jump in the specific heat DeltaC/Tc in both systems increases continuously being maximum at x = 7.0. Bond valence sum calculations based on neutron diffraction data point to a retarded generation of holes with increasing oxygen content in the Nd system and a different hole distribution between the chains and the planes in comparison to YBa2Cu3Ox accounting for the different behaviour of both systems.

cond-mat.supr-con↗

arXiv1999

The Effect of Chemical Doping and Hydrostatic Pressure on Tc of Y1-yCayBa2Cu3Ox Single Crystals

We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals under high He pressure. For each Ca content various samples with different oxygen contents have been prepared to probe the influence of Ca on Tc(x), dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated nh values from Tc and Tc,max for each sample. It is shown that in the overdoped region dTc/dp can be described by a pressure induced charge transfer with dnh/dp = 3.7E-3 [1/GPa] and a dTc,max/dp value of 0.8 K/GPa, irrespective of the Ca content. In the underdoped region additional pressure effects lead to a peak in dTc/dp at approximately 0.11 holes/CuO2 plane. However, with increasing Ca content this peak is strongly depressed. This is explained in terms of an increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp at very low nh values can be assigned to the ortho II ordering in the CuO chain system.

cond-mat.supr-con↗

arXiv1999

In-Plane and Out-of-Plane Optical Properties of NdBa2Cu3Ox Single Crystals close to x = 7.0

We present results of reflectivity measurements with E parallel to c and E perpendicular to c on NdBa2Cu3Ox (Nd123) single crystals close to full oxygen doping. Along the c-axis the optical conductivity shows a well developed absorption band around 450 cm^-1 at all temperatures. The in-plane optical properties are dominated by crystal-field excitations at low energies, a prominent step at 400 cm^-1 and a weaker feature between 500 - 550 cm^-1. A comparison of the c-axis optical conductivity and the in-plane scattering rate with neutron scattering derived spin susceptibility spectra suggests that the in-plane and out-of-plane anomalies are caused by the same mechanism, probably electron--spin scattering.

cond-mat.supr-con↗

arXiv1999

Oxygen Ordering Superstructures and Structural Phase Diagram of YBa_2Cu_3O_{6+x} Studied by Hard X-ray Diffraction

High energy x-ray diffraction is used to investigate the bulk oxygen ordering properties of YBa_2Cu_3O_{6+x}. Superstructures of Cu-O chains aligned along the b axis and ordered with periodicity ma, along the a axis have been observed. For $x < 0.62$ the only observed superstructure is ortho-II with m=2. At room temperature we find ortho-III (m=3) for $0.72\le x\le 0.82$, ortho-V (m=5) in a mixed state with ortho-II at x=0.62, and ortho-VIII (m=8) at x=0.67. Ortho-II is a 3D ordered structural phase, the remaining ones are essentially 2D. None of the superstructures develops long range ordering. Studies of the ortho-II ordering properties in samples prepared with x=0.5 but by different methods show that finite size domains with internal thermodynamic equilibrium are formed. The crystal perfection as well as the thermal annealing history determine the domain size. The temperature dependence of the observed superstructure ordering is investigated explicitly and a structural phase diagram is presented.

cond-mat.supr-con↗

arXiv1999

The superconducting gap Δ_{c}, the pseudogap Δ_{p} and pair fluctuations above Tc in overdoped YCaBaCuO from femtosecond time-domain spectroscopy

The low-energy electronic excitation spectrum and gap structure in optimally doped and overdoped Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-δ} single crystals are investigated by real-time measurements of the quasiparticle relaxation dynamics with femtosecond optical spectroscopy. From the amplitude of the photoinduced reflectivity as a function of time, temperature and doping x we find clear evidence for the coexistence of two distinct gaps in the entire overdoped phase. One is a temperature-independent pseudogap $Δ_{p}$ and the other is a T-dependent collective gap $Δ_{c}(T)$ which has a BCS-like T-dependence closing at Tc. From quasiparticle relaxation time measurements above Tc we ascertain that fluctuations associated with the collective gap $Δ_{c}(T)$ are limited to a few K, consistent with time-dependent Ginzburg-Landau theory and are distinct from the pseudogap whose presence is apparent well above Tc for all x.

cond-mat.supr-con↗

arXiv1999

Mid-infrared absorption in YBa2Cu3O6: Failure of spin-wave theory in undoped cuprates?

The optical conductivity sigma (omega) of undoped YBa2Cu3O6 is studied in detail in the mid-infrared range. Substitutions on all but the Ba site are used to identify the prominent absorption processes at 2800 and 3800 cm^-1. Experimental evidence for bimagnon-plus-phonon absorption is collected. A more critical analysis of the lineshape and the spectral weight reveals the limits of this approach. Although phonon-2-magnon multiple scattering seems to reproduce the lineshape, the necessary coupling is unrealistically large. The strong increase of high frequency spectral weight with increasing temperature makes the failure of spin-wave theory even more evident.

cond-mat.str-el↗