arXiv subjects

C. Panagopoulos

Publications and source records attributed to C. Panagopoulos.

At least 73 records · Page 4Linked to original sources

arXiv2006

Magnetic order in the normal state of the archetypal high-Tc superconductor La2-xSrxCuO4

We report detailed bulk magnetization measurements of the normal state in the high transition temperature (high-Tc) superconductor La2-xSrxCuO4. A magnetic order in the form of hysteresis in the low field magnetization was observed at temperatures well above Tc but below the pseudogap temperature. The order arises from the interaction of magnetic domains, and the doping (x) dependence of its onset and strength broadly follows that of Tc(x).

cond-mat.supr-con↗

arXiv2005

Thermo-magnetic hysteretic properties resembling superconductivity in the normal state of La1.85Sr0.15CuO4

We have performed detailed magnetic and thermal hysteresis experiments in the normal-state magnetization of La1.85Sr0.15CuO4 single crystal. Using a combination of in-field and in-zero-magnetic-field measurements at different stages of thermal history of the sample, we identified subtle effects associated with the presence of magnetic signatures which resemble those below the superconducting transition temperature (Tc=36 K) but survive up to 250 K.

cond-mat.supr-con↗

arXiv2004

Self-generated electronic heterogeneity and quantum glassiness in the high temperature superconductors

We present a systematic study of the spin and charge dynamics of copper oxide superconductors as a function of carrier concentration $x$. Our results portray a coherent physical picture, which reveals a quantum critical point at optimum doping ($x=x_{opt}$), and the formation of an inhomogeneous glassy state at $x < x_{opt}$. This mechanism is argued to arise as an intrinsic property of doped Mott insulators, and therefore to be largely independent of material quality and level of disorder.

cond-mat.str-el↗

arXiv2003

Exposing the spin glass ground state of the non-superconducting La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_{y}$O$_4$ high-$T_c$ oxide

We have studied the spin glass behaviour of non-superconducting La$_{2-x}$Sr$_x$Cu$_{0.95}$Zn$_{0.05}$O$_4$ ($x=0.10-0.22$). As in the superconducting analogues of these samples the spin glass transition temperature $T_g$ decreases with increasing $x$, and vanishes at $x=0.19$. A local enhancement in $T_g$ at $x=0.12$ is also observed and attributed to stripe ordering. The disappearance of $T_g$ for $x\geq0.19$ is discussed in terms of a quantum phase transition.

cond-mat.supr-con↗

arXiv2003

Superfluid response in monolayer high-Tc cuprates

We have studied the doping dependence of the in-plane and out-of-plane superfluid density, ρ^s(0), of two monolayer high-Tc superconductors, HgBa_2CuO_{4+δ} and La_{2-x}Sr_xCuO_4, using the low frequency ac-susceptibility and the muon spin relaxation techniques. For both superconductors, ρ^s(0) increases rapidly with doping in the under- and optimally doped regime and becomes nearly doping independent above a critical doping, p_c = 0.20.

cond-mat.supr-con↗

arXiv2002

Pseudogap and quantum-transition phenomenology in HTS cuprates

The low-energy excitation spectrum of HTS cuprates is examined in the light of thermodynamic, transport, quasiparticle and spin properties. Changes in the thermodynamic spectrum associated with the normal-state pseudogap disappear abruptly at a critical doping state, $p_{crit}$ = 0.19 holes per Cu. Moreover, ARPES data at 100K show that heavily damped quasiparticles (QP) near ($π$,0) suddenly recover long lifetimes at $p_{crit}$, reflecting an abrupt loss of scattering from AF spin fluctuations. This picture is confirmed by $μ$SR zero-field relaxation measurements which indicate the presence of a novel quantum glass transition at $p_{crit}$. Consistent with this picture resistivity studies on thin films of Y$_{0.7}$Ca$_{0.3}$Ba$_2$Cu$_3$O$_{7-δ}$ reveal linear behavior confined to a V-shaped domain focussed on $p_{crit}$ at $T$=0. The generic phase behavior of the cuprates may be governed by quantum critical fluctuations above $p_{crit}$ and the pseudogap appears to be caused by short-range AF correlations.

cond-mat.supr-con↗

arXiv2002

The superfluid density in cuprate high-Tc superconductors - a new paradigm

The doping dependence of the superfluid density, r_s, of high-Tc superconductors is usually considered in the context of the Uemura relation, namely Tc proportional to rs, which is generally assumed to apply in the underdoped regime. We show that a modified plot of Tc/Do versus rs, where Do is the maximum d-wave gap at T=0, exhibits universal features that point to an alternative interpretation of the underlying physics. In the underdoped region this plot exhibits the canonical negative curvature expected when a ground-state correlation competes with superconductivity (SC) by opening up a gap in the normal-state DOS. In particular rs is suppressed much faster than Tc/Do or indeed Tc. The pseudogap is found to strongly modify the SC ground state.

cond-mat.supr-con↗

arXiv2002

Low-frequency spins and the ground state in high-Tc cuprates

We study the spin fluctuations of pure and Zn-substituted high temperature superconductors (HTS) using the muon spin relaxation (muSR) technique. Superconductivity is found to coexist with low frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by muSR decreases with increasing doping and vanishes above a critical doping pc=0.19 where the normal state pseudogap vanishes. Furthermore, it is at pc that our penetration depth studies show an abrupt change in the doping dependence of the superfluid density. For p>pc the absolute value of the superfluid density is large and nearly independent of carrier concentration whereas for p<pc it drops rapidly, signalling a crossover to weak superconductivity. These results are discussed in terms of a quantum transition involving fluctuating short-range magnetic order that separates the superconducting phase diagram of HTS into two distinct ground states.

cond-mat.supr-con↗

arXiv2002

Temperature dependence of electrical resistivity of high-Tc cuprates - from pseudogap to overdoped regions

The effects of planar hole concentration, p, and in-plane disorder, Zn (y), on the DC resistivity, r(T), of sintered samples of Y_{1-x}Ca_xBa_2(Cu_{1-y}Zn_y)_3O_{7-d} were investigated over a wide doping range by changing both the oxygen deficiency (d) and Ca content (x). From the r(T,p) data we extracted characteristic crossover temperatures on the underdoped and overdoped sides, T* and T_m respectively, above which r(T) is linear. We compare our results with a number of other polycrystalline, thin film and single crystal cuprate superconductors and find similar behavior in the p-dependence of T*(p), T_m(p), and the resistivity exponent, m(p), in fits to r(T) = r_0 + aT^m on the overdoped side. Our findings point towards the possible existence of a quantum critical point (QCP) at the doping p=0.19 +/- 0.01.

cond-mat.supr-con↗

arXiv2002

Microscopic Hamiltonian for Zn or Ni substituted high temperature cuprate superconductors

We have derived the effective low energy Hamiltonian for Zn or Ni substituted high-T_c cuprates from microscopic three-band models consisting of the most relevant Cu or impurity 3d and O 2p orbitals. We find that both scattering potential and hopping integral induced by impurities have a finite range but decay very fast with distance from the impurity. The Zn scattering potential is very strong and attractive for electrons. The Ni scattering potential is much weaker than the Zn case, resulting from the hybridization between Ni ions and O holes. This profound difference is due to neither the electric charge nor d-level location, but rather because of the interplay between the valence state of the impurity and the strong correlation background. It gives a natural account for the unusual effect of Ni and Zn on the reduction of superconducting transition temperature. The interlayer hopping of electrons is highly anisotropic and nonlocal, determined by the in-plane electronic structure. This leads to a quantum interference of states from different sites and affects strongly the scanning tunneling spectrum perpendicular to CuO_2 planes.

cond-mat.supr-con↗

arXiv2002

Evidence for a generic novel quantum transition in high-Tc cuprates

We study the low-energy spin fluctuations and superfluid density of a series of pure and Zn-substituted high-$T_c$ superconductors (HTS) using the muon spin relaxation and $ac$-susceptibility techniques. At a critical doping state, $p_c$, we find (i) simultaneous abrupt changes in the magnetic spectrum in the superconducting ground state and (ii) that the slowing down of spin fluctuations becomes singular at T=0. These results provide experimental evidence for a novel quantum transition that separates the superconducting phase diagram of HTS into two distinct ground states.

cond-mat.supr-con↗

arXiv2001

Critical Behaviour in the Spin Fluctuations and Superfluid Density of La$_{2-x}$Sr$_x$CuO$_4$

We study the doping dependence of low frequency spin fluctuations and the zero-temperature superfluid density of La$_{2-x}$Sr$_x$CuO$_4$ using the muon spin relaxation ($μ$SR) and ac-susceptibility techniques. Superconductivity is found to coexist with low frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by $μ$SR decreases with increasing $x$ and vanishes above a critical doping $x_c\sim 0.19$. This value of $x_c$ coincides with the doping at which the normal state pseudogap extrapolates to zero. The superfluid density behaves in the opposite way to the low frequency spin fluctuations. It increases with $x$ and becomes nearly doping-independent for $x>x_c$. These results are consistent with predictions involving quantum criticality at $x_c$.

cond-mat.supr-con↗

arXiv2001

Penetration Depth Measurements in MgB_2: Evidence for Unconventional Superconductivity

We have measured the magnetic penetration depth of the recently discovered binary superconductor MgB_2 using muon spin rotation and low field $ac$-susceptibility. From the damping of the muon precession signal we find the penetration depth at zero temperature is about 85nm. The low temperature penetration depth shows a quadratic temperature dependence, indicating the presence of nodes in the superconducting energy gap.

cond-mat.supr-con↗

arXiv2000

Antiferromagnetic Correlations versus Superfluid Density in La_{2-x}Sr_xCuO_4

We have performed muon spin relaxation and low field ac-susceptibility measurements in a series of high quality samples of La_{2-x}Sr_xCuO_4 (x=0.08-0.24) as a function of temperature. Superconductivity is found to coexist with low temperature spin glass order up to the optimally doped region where the normal state pseudogap also closes. The systematic depletion of the superfluid density with the enhancement of aniferromagnetic correlations with underdoping indicates a competition between antiferromagnetic correlations and superconductivity.

cond-mat.supr-con↗

arXiv1999

Effects of doping on thermally excited quasiparticles in the high-$T_c$ superconducting state

The physical properties of low energy superconducting quasiparticles in high- $T_c$ superconductors are examined using magnetic penetration depth and specific heat experimental data. We find that the low energy density of states of quasiparticles of La$_{2-x}$Sr$_x$CuO$_4$ scales with $(x-x_c)/T_c$ to the leading order approximation, where $x_c $ is the critical doping concentration below which $T_c=0$. The linear temperature term of the superfluid density is renormalized by quasiparticle interactions and the renormalization factor times the Fermi velocity is found to be doping independent.

cond-mat.supr-con↗

arXiv1999

c-Axis Superfluid Response and Pseudogap in High-Tc Superconductors

To gain insight into the out-of-plane electrodynamics of high-$T_c$ superconduct ors we have measured the absolute values and temperature dependence of the c-a xis magnetic penetration depth $λ_c(T)$ for two typical single layer high -$T_c$ cuprates, La$_{2-x}$Sr$_x$CuO$_4$ and HgBa$_2$CuO$_{4+x}$ as a function o f doping. A distinct change in the behaviour of $λ_c$ is observed near 0. 20 holes/Cu, which is related to the opening of the normal state pseudogap. The variation of $λ_c$ with doping is qualitatively similar to that of the in -plane component, $λ_{ab}$, which suggests that the c-axis superfluid r esponse is mainly governed by the superconducting condensation energy. The stron g doping dependence of $λ_c(0)$ for p<0.20 provides an explanation for the discrepancies in the literature.

cond-mat.supr-con↗

arXiv1999

Effects of carrier concentration on the superfluid density of high-T_c cuprates

The absolute values and temperature, T, dependence of the in-plane magnetic penetration depth, $λ_{ab}$, of La_{2-x}Sr_xCuO_4 and HgBa_2CuO_{4+δ} have been measured as a function of carrier concentration. We find that the superfluid density, $ρ_s$, changes substantially and systematically with doping. The values of $ρ_{s}(0)$ are closely linked to the available low energy spectral weight as determined by the electronic entropy just above T_c and the initial slope of $ρ_{s}(T)/ρ_{s}(0)$ increases rapidly with carrier concentration. The results are discussed in the context of a possible relationship between $ρ_s$ and the normal-state (or pseudo) energy gap.

cond-mat.supr-con↗

arXiv1998

Low Temperature Superfluid Response of High-Tc Superconductors

We have reviewed our theoretical and experimental results of the low temperature superfluid response function of high temperature superconductors (HTSC). In clean high-Tc materials the in-plane superfluid density rho_s^{ab} varies linearly with temperature. The slope of this linear T term is found to scale approximately with 1/Tc which, according to the weak coupling BCS theory for a d-wave superconductor, implies that the gap amplitude scales approximately with Tc. A T^5 behavior of the out-of-plane superfluid density rho_s^c for clean tetragonal HTSC was predicted and observed experimentally in the single layer Hg-compound HgBa_2CuO_{4+delta}. In other tetragonal high-Tc compounds with relatively high anisotropy, such as Hg_2Ba_2Ca_2Cu_3O_{8+delta}, rho_s^c varies as T^2 due to disorder effects. In optimally doped YBa_2Cu_3O_{7-delta}, rho_s^c varies linearly with temperature at low temperatures, but in underdoped YBa_2Cu_3O_{7-delta}, rho_s^c varies as T^2 at low temperatures; these results are consistent with our theoretical calculations.

cond-mat.supr-con↗