Charge density waves as the origin of dip-hump structures in differential tunneling conductance of cuprates: the case of d-wave superconductivity (original) (raw)
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Physica C, 2014
Quasiparticle differential current–voltage characteristics (CVCs) GðVÞ of non-symmetric tunnel junctions between d-wave superconductors with charge-density waves (CDWs) and normal metals were calculated. The dependences GðVÞ were shown to have aV-like form at small voltagesV and low temperatures, and to be asymmetric at larger V owing to the presence of CDW peak in either of theV-branches. The spatial scatter of the dielectric (CDW) order parameter smears the CDW peak into a hump and induces a peak-dip-hump structure (PDHS) typical of CVCs observed for such junctions. At temperatures larger than the superconducting critical one, the PDHS evolves into a pseudogap depression. The results agree well with the scanning tunneling microscopy data for i2Sr2CaCu2O8+d and YBa2Cu3O7d. The results differ substantially from those obtained earlier for CDWs-wave superconductors.
Physica C-superconductivity and Its Applications, 2008
We have calculated quasiparticle current–voltage characteristics J(V) for non-symmetric CDWS–I–N tunnel junctions between a partially gapped charge-density wave (CDW) s-wave superconductor and a normal metal (I stands for an insulator), as well as for symmetric CDWS–I–CDWS junctions. Relevant parameters of CDWS are considered spatially inhomogeneous in accordance with experimental data for various cuprates, especially Bi2Sr2CaCu2O8+δ (BSCCO). The calculated dependences G(V) = dJ(V)/dV demonstrate conspicuous dip-hump structures (DHSs) at low temperatures, T, and pseudogap shallow well at high T > Tc above the critical temperature. In CDWS–I–N junctions, DHSs were shown to be observed for either one or both voltage polarities, depending on the CDW order parameter phase. Similar symmetric DHSs were found for CDWS–I–CDWS junctions. J(V) for break-junctions made of BSCCO were measured. Qualitative agreement was reached between our theoretically calculated and experimental G(V) dependences.
Charge-density-wave origin of the dip-hump structure in tunnel spectra of the BSCCO superconductor
Physical Review B, 2007
Differential conductance G as the function of the bias voltage V across the tunnel junction between a normal metal and an inhomogeneous superconductor with charge density waves ͑CDW's͒ has been calculated by spatial averaging over random domains with varying superconducting-and normal-state properties. For these materials, irregularly distorted CDW patterns with spatially scattered values of various parameters were earlier shown to manifest themselves in a great body of experimental data. The results of the calculations were applied to explain the well-known dip-hump structure in the G͑V͒ dependence for Bi 2 Sr 2 CaCu 2 O 8+␦ and other high-T c cuprates.
The state of the art concerning tunnel measurements of energy gaps in cuprate oxides has been analyzed. A detailed review of the relevant literature is made, and original results calculated for the quasiparticle tunnel current J(V) between a metallic tip and a disordered d-wave superconductor partially gapped by charge density waves (CDWs) are reported, because it is this model of high-temperature superconductors that becomes popular owing to recent experiments in which CDWs were observed directly. The current was calculated suggesting the scatter of both the superconducting and CDW order parameters due to the samples' intrinsic inhomogeneity. It was shown that peculiarities in the current-voltage characteristics inherent to the case of homogeneous super-conducting material are severely smeared, and the CDW-related features transform into experimentally observed peak-dip-hump structures. Theoretical results were used to fit data measured for YBa 2 Cu 3 O 7–δ and Bi 2 Sr 2 CaCu 2 O 8+δ. The fitting demonstrated a good qualitative agreement between the experiment and model calculations. The analysis of the energy gaps in high-T c superconductors is important both per se and as a tool to uncover the nature of superconductivity in cuprates not elucidated so far despite of much theoretical effort and experimental progress. PACS: 71.45.Lr Charge-density-wave systems; 74.55.+v Tunneling phenomena: single particle tunneling and STM; 74.81.–g Inhomogeneous superconductors and superconducting systems, including electronic inhomogeneities.
Charge-Density-Wave Origin of Dip-Hump Structures in the Tunnel Spectra of Bi2Sr2CaCu2O8+delta
Acta Physica Polonica A, 2007
We calculated the differential conductance G as the function of the bias voltage V across the tunnel junction between a normal metal or a conventional superconductor and an inhomogeneous superconductor with charge density waves. Spatial averaging over random domains with varying superconducting and normal state properties was carried out. For high-Tc oxides, irregularly distorted charge density wave patterns with spatially scattered values of various parameters were earlier shown to manifest themselves in a great body of experimental data. The results of calculation were applied to explain the well-known dip-hump structure in the G(V) dependence for Bi2Sr2CaCu2O8+δ and other cuprates.
Journal of Physics-condensed Matter, 2008
Temperature, T , variations of the tunnel conductance G(V ) were calculated for junctions between a normal metal and a spatially inhomogeneous superconductor with a dielectric gap on the nested sections of the Fermi surface or between two such superconductors. The dielectric gapping was considered to be a consequence of the charge density wave (CDW) appearance due to the electron-phonon (for a Peierls insulator) or a Coulomb (for an excitonic insulator) interactions. Spatial averaging was carried out over random domains with varying parameters of the CDW superconductor (CDWS). The calculated tunnel spectra demonstrate a smooth transformation from asymmetric patterns with a pronounced dip-hump structure at low T into those with a pseudogap depletion of the electron densities of states at higher T in the vicinity or above the actual critical temperatures of the superconducting transition for any of the CDWS domains. Thus, it is demonstrated that both the dip-hump structure and pseudogapping are manifestations of the same phenomenon. A possible CDW-induced asymmetry of the background contribution to G(V ) is also touched upon. The results explain the peculiar features of G(V ) for Bi 2 Sr 2 CaCu 2 O 8+δ and other related high-T c cuprates.
Effect of charge density waves on the tunnel spectra of the Bi 2 Sr 2 CaCu 2 O 8+δ superconductor
Physics of The Solid State, 2007
The differential tunnel conductance G S of the junction between a normal metal and a superconductor with a charge density wave (CDW) is calculated as a function of the voltage V across the junction. The results are averaged over the spread of superconducting and CDW energy gaps in the nanoscale-inhomogeneous superconductor. It is shown that, if both order parameters are nonzero, a dip-hump structure is formed beyond the superconducting gap of G S ( V ). If the phase of the CDW order parameter is not equal to π /2, a dip-hump structure will appear solely or mainly for one sign of the bias polarity. The results agree with the experimental data for Bi 2 Sr 2 CaCu 2 O 8 + δ and other high-temperature oxides
Charge-density-wave features in tunnel spectra of high-Tc superconductors
2009
Abstract. Tunnel conductances G (V)= dJ/dV (V), where J (V) is the quasiparticle current and V is the voltage, have been calculated for nonsymmetric (CDWS-IN) and symmetric (CDWS-ICDWS) tunnel junctions (N, I, and CDWS stand for a normal metal, insulator, and electronically inhomogeneous CDW superconductor, respectively). The CDWS inhomogeneity was shown to be responsible for the appearance of smooth but conspicuous dip-hump structures (DHSs) in G (V) at T≪ Tc.
Low Temperature Physics, 2008
Tunnel conductance G V () for break-junctions made of as-grown single-crystal Bi 2 Sr 2 CaCu 2 O 8+d samples with T c »-86 89 K was measured and clear-cut dip-hump structures (DHSs) were found in the range 80 120-mV of the bias voltage V. The theory of tunneling in symmetric junctions between inhomogeneous charge-density-wave (CDW) superconductors, considered in the framework of the s-pairing model, has been developed. CDWs have been shown to be responsible for the appearance of the DHS in the tunnel current-voltage characteristics and properly describes experimental results.
Physical Review B, v. 92, N 5, 054512, 2015
Quasiparticle tunnel current either between identical d-wave superconductors partially gapped by charge density waves (SCDWs) or between an SCDW and a normal metal was calculated. The cases of unidirectional and checkerboard CDWs were considered. The tunnel conductance was found in both cases to possess a number of peculiarities, which cannot be described by introducing a single combined gap. The results are in qualitative agreement with experimental data obtained for a number of cuprates by the scanning tunnel spectroscopy, intrinsic tunneling, and break-junction measurements. The difference between the experiment and the theory seems to stem from the spread of gap values occurring due to the intrinsic spatial inhomogeneity of nonstoichiometric oxides and reflected in the cuprate tunnel spectra.