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Origin of the pseudogap in high temperature superconductors

Description: Underdoped high temperature superconductors (HTS) exhibit a normal state for energies E > E{sub g} and/or temperatures T > T{sub 0}, and a pseudogap in their electronic spectrum for E < E{sub g} and/or T{sub 0} > T > {Tc}. Strikingly similar behavior occurs in the transition metal dichalcogenides (TMD) 2H-MX{sub 2}, where M = Ta, Nb, and X = S, Se, both in the normal (T > T{sub 0}) and in the incommensurate charge-density wave (T{sub ICDW} > T > T{sub c}) states. Such strikingly similar behavior has also been seen in the organic layered superconductors (OLS) {kappa}-(ET){sub 2}X, where ET is bis(ethylenedithio)tetrathiafulvalene, and X = Cu[N(CN){sub 2}]Br, and Cu(SCN){sub 2}, both in the normal region T > T{sub SDW} > {Tc} and in the spin-density wave region T{sub SDW} > T > T{sub c}. In all three materials classes, the anomalous transport and thermodynamic properties associated with the pseudogap or density-wave regime are completely independent of the applied magnetic field strength, whereas the same properties below {Tc} are all strongly field-dependent. Hence, the authors propose that the pseudogap in the HTS arises from charge- and/or spin-density waves, and not from either superconducting fluctuations or preformed charged quasiparticle pairs.
Date: February 8, 2000
Creator: Klemm, R. A.
Partner: UNT Libraries Government Documents Department

Angular dependence of the Josephson critical current across c-axis twist junctions of layered superconductors.

Description: The authors studied theoretically the c-axis Josephson critical current for bicrystals of high temperature superconductors twisted an angle {phi}{sub 0} about the c-axis with respect to each other. They used the effective Lawrence-Doniach models appropriate for the cases of pure s-wave or d{sub x{sup 2}-y{sup 2}}-wave order parameters, and of a dominant d{sub x{sup 2}-y{sup 2}}-wave order parameter combined with a subdominant one of either s-wave or d{sub xy}-wave form, as a function of the temperature and {phi}{sub 0}. The results demonstrate that this new phase-sensitive experiment can serve as a very useful test of order parameter symmetry. In particular, the recent zero-field experiments in the vicinity of T{sub c} by li et al. on c-axis twist junctions of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} are very difficult to explain with a dominant d{sub x{sup 2}-y{sup 2}}-wave order parameter.
Date: July 31, 1998
Creator: Klemm, R. A.
Partner: UNT Libraries Government Documents Department

What is the symmetry of the high T{sub c} order parameter?

Description: In recent years, there has been a raging controversy regarding the orbital symmetry of the superconducting order parameter (OP) in the high temperature superconductors. Many experiments were interpreted in terms of a d{sub x{sup 2}-y{sup 2}}-wave OP, but many others were interpreted in terms of a more conventional s-wave OP. We review the problems of both intrinsic and extrinsic natures with the phase-sensitive experiments on YBCO. The authors further show that the photoemission experiments of the purported superconducting gap in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub g+{delta}} are entirely consistent with charge- and/or spin-density wave formation in that material. The presence of such density waves greatly complicates the analysis of most experiments. Hence, we conclude that the orbital symmetry of the superconducting OP is still unknown in any of the high temperature superconductors.
Date: April 16, 1998
Creator: Klemm, R. A.
Partner: UNT Libraries Government Documents Department

Generic properties of layered superconductors

Description: A wide variety of layered materials have been found to be superconducting. Among these are graphite intercalation donor compounds, transition metal dichalcogenides, intercalated transition metal dichalcogenides, organic layered superconductors, artificial superlattices, and the cuprates. The normal state properties of all of these compounds are very similar, due to the fact that they all have well-defined Fermi surfaces, and quasi-linear in-plane resistivities at high temperatures. At lower temperatures, charge- or spin-density waves are common in these systems. With the exception of the graphite intercalation compounds, all of the fayered superconductors are type-II, and generally highly anisotropic in their superconducting properties. Dimensional crossover from anisotropic bulk to thin film behavior is common to many of these materials.
Date: October 1, 1992
Creator: Klemm, R.A.
Partner: UNT Libraries Government Documents Department

Fluctuation phenomena in layered superconductors

Description: Gaussian fluctuations in layered superconductors have been the subject of study for many years. Although the FD was studied in detail long ago, the FC (fluctuation conductivity) was studied only recently, since the MT and DOS diagrams were previously neglected. Recent comparisons with experiment on YBCO have shown that the DOS diagrams are important and can lead to qualitatively different behaviors for the FC parallel and perpendicular to the layers. In both cases, Gaussian fluctuations fit the data above {Tc} very well, even for YBCO. To date, nearly all calculations of fluctuation quantities were for B{parallel}{cflx c}. Nevertheless, it should be possible to treat an arbitrary B, but the evaluation of the required matrix elements for the fluctuation quantities will be more complicated.
Date: October 1, 1996
Creator: Klemm, R.A.
Partner: UNT Libraries Government Documents Department

Simple model of the anisotropic penetration depth in high T{sub c} superconductors

Description: We present a simple model of some high {Tc} cuprates based upon superconducting (S) and normal (N) layers, which quantitatively fits the data of Bonn et al. for the low-temperature T dependence of the penetration depths {lambda}{sub a,b,c} in untwinned YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}, assuming s-wave intralyer pairing. This SN model also leads to anisotropic surface states, which complicate analysis of photoemission and tunneling measurements.
Date: April 1, 1996
Creator: Klemm, R.A. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Energy gap structure and tunneling characteristics of layered superconductors

Description: The authors have analyzed the energy gaps and density-of-states (DOS) of layered superconductors with two inequivalent layers in a unit cell along the c-axis. In the physically interesting parameter range where the interlayer hopping strengths of the quasiparticles are comparable to the critical temperature, the peaks in the DOS curve do not correspond to the order parameters (OP`s) of each layer, but depend on the OP`s and the interlayer hopping strengths in a complex manner. In contrast to a BCS superconductor, the DOS of layered systems have logarithmic singularities. The simulated tunneling characteristics bear close resemblance to experimental results.
Date: June 1, 1993
Creator: Liu, S.H. & Klemm, R.A.
Partner: UNT Libraries Government Documents Department

Why we still don`t know the symmetry of the order parameter in high temperature superconductors

Description: One of the most elusive topics in the study of high temperature superconductors (HTCS) is the orbital symmetry of the superconducting order parameter {Delta}. The low temperature T behavior observed in many experiments suggests that {Delta} might exhibit line nodes, as expected for d{sub x}{sub {minus}y}2 orbital symmetry. However, such behavior also arises from intrinsic proximity coupling and surface states, etc. The paramagnetic Meissner effect, once cited as indirect evidence for a d-wave {Delta}, was recently observed in Nb. Some phase-sensitive Josephson junction experiments gave strong evidence for a substantial isotropic, or s-wave, component to {Delta}, whereas others supported the above d-wave form. However, such experiments are intrinsically unreliable in experimental geometries with sample corners, or with meandering grain boundary junctions and stoichiometry inhomogeneities. Finally, the pseudogap observed recently in underdoped HTCS could arise from a charge-density wave.
Date: March 1, 1997
Creator: Klemm, R.A.
Partner: UNT Libraries Government Documents Department

Dependence of the Josephson coupling of unconventional superconductors on the properties of the tunneling barrier

Description: The Josephson coupling between a conventional and an unconventional superconductor is investigated as a function of the properties of the tunneling barrier. A simple model is adopted for the tunneling probability and it is shown that its variation dramatically affects the I{sub c}R{sub n} product of an s-d, as opposed to an s-s junction. Based on these conclusions, experiments are proposed to probe the symmetry of the order parameter in high temperature superconductors.
Date: May 30, 1994
Creator: Ledvij, M. & Klemm, R. A.
Partner: UNT Libraries Government Documents Department

Surface state and normal layer effects in high T{sub c} superconductors

Description: In addition to the conducting CuO{sub 2} (S) layers, most high-{Tc}, superconductors also contain other conducting (N) layers, which are only superconducting due to the proximity effect. The combination of S and N layers can give rise to complicated electronic densities of states, leading to quasilinear penetration depth and NMR relaxation rate behavior at low temperatures. Surface states can also complicate the analysis of tunneling and photoemission measurements. Moreover, geometrical considerations and inhomogeneously trapped flux are possible explanations of the paramagnetic Meissner effect and of corner and ring SQUID experiments. Hence, all of the above experiments could be consistent with isotropic s-wave superconductive within the S layers.
Date: January 1, 1996
Creator: Klemm, R.A.; Ledvij, M. & Liu, Samuel H.
Partner: UNT Libraries Government Documents Department

Surface state and normal layer effects

Description: In addition to the conducting CuO{sub 2} (S) layers, most high-T{sub c} superconductors also contain other conducting (N) layers, which are only superconducting due to the proximity effect. The combination of S and N layers can give rise to complicated electronic densities of states, leading to quasilinear penetration depth and NMR relaxation rate behavior at low temperatures. Surface states can also complicate the analysis of tunneling and, photoemission measurements. Moreover, geometrical considerations and in homogeneously trapped flux axe possible explanations of the paramagnetic Meissner effect and of corner and ring SQUID experiments. Hence, all of the above experiments could be consistent with isotropic s-wave superconductivity within the S layers.
Date: August 1, 1995
Creator: Klemm, R.A.; Ledvij, M. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Comment on {open_quotes}superfluid anisotropy in YBCO: Evidence for pair tunneling superconductivity{close_quotes}

Description: Recently, Xiang and Wheatley extended our proximity coupling SN model of the (S) CuO{sub 2} plane and (N) CuO chain layers in YBCO compounds, using the single particle bands {epsilon}{sup 0}{sub 1k} = -t(cos k{sub a} + cos k{sub b}) - {mu}{sub 1} and {epsilon}{sup 0}{sub 2k} = -t{sub 2y}cos k{sub b} - {mu}{sub 2}, respectively. Although we showed that it was possible to fit the penetration depth {lambda}{sub ab}(T) in twinned YBCO with either s- or d-wave pairing and both bands free-particle-like, they claimed that: (1) It is impossible to fit the quasi-linear {lambda}{sub a}(T) in untwinned YBCO without order parameter line nodes, such as for d{sub x2-y2} pairing. (2) Even d-wave SN fits to the {lambda}{sub a,b,c} data are inferior to those obtained from a model of d-wave interlayer pair tunneling (DIPT). We remark that their claim is false, and ignores other data favoring proximity coupling.
Date: January 17, 1996
Creator: Klemm, R.A.; Ledvij, M. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+delta} c-axis twist Josephson junctions : a new phase-sensitive test of order parameter symmetry.

Description: Li et al. found that the critical current density J{sub c}{sup J} across atomically clean c-axis twist junctions of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} is the same as that of the constituent single crystal, J{sub c}{sup S}, independent of the twist angle {phi}{sub 0}, even at and below T{sub c}. They investigated theoretically if a d{sub x{sup 2}{minus}y{sup 2}} wave order parameter might twist by mixing in d{sub xy}-wave components, but found that such mixing cannot possibly explain the data near to T{sub c}. Combined with group theoretical arguments, they then conclude that the order parameter contains at least a substantial s-wave component, but does not contain any purported d{sub x{sup 2}{minus}y{sup 2}}-wave component, except possibly below a second, unobserved phase transition. By studying tunneling models, they further conclude that the intrinsic c-axis Josephson tunneling in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} is likely to be mostly incoherent.
Date: September 20, 1999
Creator: Klemm, R. A.; Arnold, G.; Bille, A.; Rieck, C. T. & Scharnberg, K.
Partner: UNT Libraries Government Documents Department

c-axis twist Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} josephson junctions : a new phase-sensitive test of order parameter symmetry.

Description: Li et al. found that the critical current density J{sub c}{sup J} across atomically clean c-axis twist junctions of Bi{sub 2}Sr{sub 2}Ca Cu{sub 2}O{sub 8+{delta}} is the same as that of the constituent single crystal, J{sub c}{sup S}, independent of the twist angle {phi}{sub 0}, even at T{sub c}. They investigated theoretically if a d{sub x{sup 2}-y{sup 2}}-wave order parameter might twist by mixing in d{sub xy} components, but find that such twisting cannot possibly explain the data near to T{sub c}. Hence, the order parameter contains an s-wave component, but not any d{sub x{sup 2}-y{sup 2}}-wave component. In addition, the c-axis Josephson tunneling is completely incoherent. They also propose a c-axis junction tricrystal experiment which does not rely upon expensive substrates.
Date: January 22, 1999
Creator: Klemm, R. A.; Rieck, C. T. & Scharnberg, K.
Partner: UNT Libraries Government Documents Department

Intra-versus interlayer pairing in the copper oxide superdonductors: Response to a magnetic field

Description: We have investigated the response of layered- superconductors to an external magnetic field using the semiclassical phase approximation. The linearized-gap equations have been derived, and solved numerically to calculate the upper critical field H{sub c2}(T) for layered superconductors with one or two layers per unit cell for both s-wave and interlayer BCS-like pairing mechanisms. In the weak-hopping limit the equations reduce to the Lawrence-Doniach form, and for general hopping the appropriate gap equations are derived and numerically analyzed. One encounters the familiar dimensional crossover in the H{sub c2}{parallel}(T) curve for weak hopping. A different type of dimensional crossover can occur in the two-layer case with unequal intralayer or interlayer coupling strengths, such that at the dimensional crossover temperature, the magnetic field suppresses the superconductivity in the weakly coupled layers while leaving the strongly coupled layers superconducting. The effect is enhanced by unequal hopping strengths. The flux lattice consists of alternating superconducting and normal layers.
Date: January 1, 1994
Creator: Klemm, R. A. & Liu, Samuel H.
Partner: UNT Libraries Government Documents Department

Energy gap structure of layered superconductors

Description: We report the energy gap structure and density-of-states (DOS) of a model layered superconductor with one superconducting layer and one normal layer in a unit cell along the c-axis. In the physically interesting parameter range where the interlayer hopping strengths of the quasiparticles are comparable to the critical temperature, the peaks in the DOS curve do not correspond to the order parameter (OP) of the superconducting layer, but depend on the OP and the band dispersion in the c-direction in a complex manner. In contrast to a BCS superconductor, the DOS of layered systems have logarithmic singularities. Our simulated tunneling characteristics bear close resemblance to experimental results.
Date: November 1, 1993
Creator: Liu, S. H. & Klemm, R. A.
Partner: UNT Libraries Government Documents Department