arXiv subjects

J. Karpinski

Publications and source records attributed to J. Karpinski.

171 records · Page 10Linked to original sources

arXiv2001

Localisation of doped holes in edge-shared CuO_2 chain cuprates: consequences for dynamic spectral weight transfer

We present a joint experimental and theoretical study of the electronic structure of the cuprate chain systems A_(1-x)CuO_2 (A=Ca,Sr,Ba), as measured using O-K and Cu-L_3 x-ray absorption spectroscopy. The doping-dependent behaviour in these systems is radically different to that in conventional 2D cuprate networks formed from corner sharing CuO_4 plaquettes, and follows from the strongly suppressed inter-plaquette hybridisation resulting from the 90 degree Cu-O-Cu interaction pathway in the chain systems. Spectroscopically, this results in (a) a classical mixed-valent scenario whereby the different final states in the Cu-L_3 spectra can be used directly to 'read-off' the Cu valency and (b) a drastic reduction in the dynamic spectral weight transfer from the upper Hubbard band to the low energy scale in the O-K spectra. The final picture emerges of localisation of the doped holes, with the chain then comprised of a mixture of pure Cu(II)O_4 and Cu(III)O_4 plaquettes.

cond-mat.str-el↗

arXiv2000

Quasiparticle dynamics and gap structure in Hg1223 investigated with femtosecond spectroscopy

Measurements of the temperature dependence of the quasiparticle (QP) dynamics in Hg1223 with femtosecond time-resolved optical spectroscopy are reported. From the temperature dependence of the amplitude of the photoinduced reflection, the existence of two gaps is deduced, one temperature dependent Dc that closes at Tc, and another temperature independent ''pseudogap'' Dp. The zero-temperature magnitudes of the two gaps are Dc/kTc = 6 +/- 0.5 and Dp/kTc = 6.4 +/- 0.5 respectively. The quasiparticle lifetime is found to exhibit a divergence as T -> Tc from below, which is attributed to the existence of a superconducting gap which closes at Tc. Above Tc the relaxation time is longer than expected for metallic relaxation, which is attributed to the presence of the ''pseudogap''. The QP relaxation time is found to increase significantly at low temperatures. This behavior is explained assuming that at low temperatures the relaxation of photoexcited quasiparticles is governed by a bi-particle recombination process.

cond-mat.supr-con↗

arXiv2000

Large oxygen-isotope effect in Sr_{0.4}K_{0.6}BiO_{3}: Evidence for phonon-mediated superconductivity

Oxygen-isotope effect has been investigated in a recently discovered superconductor Sr_{0.4}K_{0.6}BiO_{3}. This compound has a distorted perovskite structure and becomes superconducting at about 12 K. Upon replacing ^{16}O with ^{18}O by 60-80%, the T_c of the sample is shifted down by 0.32-0.50 K, corresponding to an isotope exponent of alpha_{O} = 0.40(5). This isotope exponent is very close to that for a similar bismuthate superconductor Ba_{1-x}K_{x}BiO_{3} with T_c = 30 K. The very distinctive doping and T_c dependencies of alpha_{O} observed in bismuthates and cuprates suggest that bismuthates should belong to conventional phonon-mediated superconductors while cuprates might be unconventional supercondutors.

cond-mat.supr-con↗

arXiv2000

Quasiparticle relaxation dynamics in Hg-1223 studied by femtosecond time-resolved optical spectroscopy

Following recent progress in understanding the relaxation dynamics of photoexcited carriers in materials exhibiting a small gap in the low-energy excitation spectrum we have performed pump-probe measurements on near optimally doped Hg-1223. We show that the behavior is very similar as in optimally-doped YBCO, where the data can be interpreted with the coexisting presence of two energy gaps: normal state T-independent pseudogap and a mean-field-like collective gap, associated with intrinsic spatially inhomogeneous ground state. An important difference between the two compounds is found in the low temperature relaxation time, which in Hg-1223 is found to be strongly temperature and photoexcitation intensity dependent.

cond-mat.supr-con↗

arXiv2000

Quasiparticle relaxation dynamics in cuprates and lifetimes of low-energy states: Femtosecond data from underdoped to overdoped YBCO and mercury compounds

We show that low-energy spectral features in the cuprates can be separated into different components by the measurement of the recombination dynamics of different low-energy excitations in real-time using femtosecond laser spectroscopy. Quasiparticle (QP) recombination across the gap and intra-gap localized state relaxation processes exhibit qualitatively different time- and temperature-dependences. The relaxation measurements also show the existence of two distinct coexisting energy gaps near optimum doping and in the overdoped region, one more or less temperature independent (which exists above and below Tc) and one which closes at Tc in a mean-field like fashion. Thus systematic studies of QP recombination as a function of doping and temperature suggest that the ground state of the cuprates is a mixed Boson-Fermion system with localised states present over the entire region of the phase diagram.

cond-mat.supr-con↗

arXiv2000

Vortex melting and decoupling transitions in YBa$_{2}$Cu$_{4}$O$_{8}$ single crystals

The vortex correlation along the c-axis in high quality single crystals of YBa$_{2}$Cu$_{4}$O$_{8}$ has been investigated as a function of temperature T in different magnetic fields, using the quasi-flux transformer configuration. A simultaneous sharp drop associated with the vortex lattice melting is observed in both the primary and secondary voltages(V$_{top}$ and V$_{bot}$). Just above the melting temperature, the vortices form three-dimensional line liquid with the correlation length along the c direction $L_{c}\leq $ t, the sample thickness. The temperature where a resistive peak in R$_{bot}$ develops corresponds to the decoupling temperature T$_{d}$ at which the vortices loose their correlation along the c-direction and they dissolve into the two dimensional pancake vortices. The H-T phase diagram for the YBa$_{2}$Cu$_{4}$O$_{8}$ single crystal is obtained.

cond-mat.supr-con↗

arXiv1999

Torque magnetometry on single-crystal high temperature superconductors near the critical temperature: a scaling approach

Angular-dependent magnetic torque measurements performed near the critical temperature on single crystals of HgBa_{2}CuO_{4+y}, La_{2-x}Sr{x}CuO_{4}, and YBa_{2}Cu_{3}O_{6.93} are scaled, following the 3D XY model, in order to determine the scaling function dG^{\pm}(z)/dz which describes the universal critical properties near T_{c}. A systematic shift of the scaling function with increasing effective mass anisotropy γ= (m_{ab}*/m_{c}*)^{1/2} is observed, which may be understood in terms of a 3D-2D crossover. Further evidence for a 3D-2D crossover is found from temperature-dependent torque measurements carried out in different magnetic fields at different field orientations δ, which show a quasi 2D "crossing region'' (M*,T*). The occurrence of this "crossing phenomenon'' is explained in a phenomenological way from the weak z dependence of the scaling function around a value z = z*. The "crossing'' temperature T* is found to be angular-dependent. Torque measurements above T_{c} reveal that fluctuations are strongly enhanced in the underdoped regime where the anisotropy is large, whereas they are less important in the overdoped regime.

cond-mat.supr-con↗

arXiv1998

Anisotropic in-plane thermal conductivity of single-crystal YBa_2Cu_4O_8

We report measurements of the in-plane thermal conductivity ($κ$) of YBa_2Cu_4O_8 (Y-124) single crystals in the temperature range $4K\leq T\leq 300K$ and compare them with previous results on YBa_2Cu_3O_{6+x} (Y-123). For transport perpendicular to the CuO chains, $κ_a(300K)\simeq 10$W/mK, and along the chains, $κ_b(300K)\simeq 40$W/mK. The temperature dependence of $κ$ for both transport directions is much stronger than in YBa_2Cu_3O_{6+x} (Y-123), indicative of substantially superior lattice heat conduction in Y-124, and resulting in maximum values for $κ$ at $T\sim 20$K exceeding 200W/mK. The data imply a surprisingly large anisotropy in the lattice conduction. $κ_a$ and $κ_b$ are enhanced in the superconducting state as in other cuprates. The magnitude and anisotropy of the enhancement are discussed and compared to those of Y-123.

cond-mat.supr-con↗

arXiv1998

Vortex Lattice Melting into Disentangled Liquid Followed by the 3D-2D Decoupling Transition in YBa_2Cu_4O_8 Single Crystals

A sharp resistance drop associated with vortex lattice melting was observed in high quality YBa_2Cu_4O_8 single crystals. The melting line is well described well by the anisotropic GL theory. Two thermally activated flux flow regions, which were separated by a crossover line B_cr=1406.5(1-T/T_c)/T (T_c=79.0 K, B_cr in T), were observed in the vortex liquid phase. Activation energy for each region was obtained and the corresponding dissipation mechanism was discussed. Our results suggest that the vortex lattice in YBa_2Cu_4O_8 single crystal melts into disentangled liquid, which then undergoes a 3D-2D decoupling transition.

cond-mat.supr-con↗