Electronic Raman scattering in YBa2Cu4O8

We report on the electronic Raman scattering in YBa₂Cu₄O₈ and find that, In contrast to the other superconducting cuprates, the continuum in the normal state is strongly temperature pendent in all polarizations. This temperature dependence is found to follow a Bose-Einstein-like form. We conclude that the absence of the Bose-Einstein factor in the electronic continuum is not essential to high-T_c superconductivity, as is sometimes implied. Additionally, we report on the rearrangement of the continuum which occurs belowT _c.     

Electronic Raman scattering in YBa2Cu4O8

We report on the electronic Raman scattering in YBa₂Cu₄O₈ and find that, In contrast to the other superconducting cuprates, the continuum in the normal state is strongly temperature ?pendent in all polarizations. This temperature dependence is found to follow a Bose-Einstein-like form. We conclude that the absence of the Bose-Einstein factor in the electronic continuum is not essential to high-T_c superconductivity, as is sometimes implied. Additionally, we report on the rearrangement of the continuum which occurs belowT _c.     

Experimental Verifications for Microscopic Superconductivity in Pseudogap State

Based on the prepairing theory on pseudogap, this paper concludes that all the until now experimental data on temperature-dependent relations of resistivity of high-T _c cuprates indicate the existence of microscopic superconductivity in the pseudogap state, and gives some interesting predictions.     

Pseudogap Analysis of Normal State Transport Behavior of 11 and 1111 Fe-Based Superconductors

In this paper we analyze transport data of different families of Fe-based superconductors, within a pseudogap framework. The Fe(Te, Se) samples exhibit s-shaped resistivity ρ(T) curves, while SmFeAs(O, F) ones present a departure from their high-temperature linear behavior. In either case, a characteristic temperature can be identified. These temperatures correspond to those at which abrupt changes in the temperature behavior of Hall resistance and of Seebeck coefficient occur, suggesting that they are signatures of pseudogaps opening in the density of states. The direct correlation between these characteristic temperatures and the superconducting transition temperatures suggests that the pseudogap and the superconducting state originate from the same mechanism. Scaling procedures of resistivity curves confirm such proportionality. On the other hand, excess Fe content in Fe(Te, Se) samples affects the pseudogap temperature much more strongly than the superconducting T _c . Finally, we find out that the pseudogap in the 1111 family is almost insensitive to disorder, as in high-T _c cuprates.     

Charge bag model of superconductivity in intercalated layered compounds

In certain layered compounds the superconducting transition temperature (T _c ) is enhanced on intercalation. The superconductivity in these materials arises over a charge density wave background. To explain this enhancement inT _c we propose the charge bag model analogous to the spin bag model proposed earlier by Schrieffer as a mechanism for highT _c superconductivity.     

Polaron Coherence as Origin of the Pseudogap Phase in High Temperature Superconducting Cuprates

Within a two-component approach to high T _c copper oxides including polaronic couplings, we identify the pseudogap phase as the onset of polaron ordering. This ordering persists in the superconducting phase. A huge isotope effect on the pseudogap onset temperature T ^* is predicted and in agreement with experimental data. The anomalous temperature dependence of the mean square copper–oxygen ion displacement observed above, at and below T _c , stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded.     

On the Pseudogap State in Y- and La-Type High-Temperature Superconductors: Doping Dependence, Isotope Effects, and Electronic Properties Studied by Neutron Spectroscopy

The doping dependence as well as the oxygen and copper isotope effects on the pseudogap temperature T* were investigated by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in rare-earth based Y- and La-type high-temperature superconductors. We found three essential results from our truly bulk-sensitive experiments: (1) For all doping levels we had T* > T _c, even in the optimally doped and overdoped regimes. (2) In bilayer high-T _c compounds we observed huge oxygen and copper isotope effects on T*, which give evidence that the pseudogap formation is governed by lattice modes involving both the oxygen and copper ions. In single layer high-T _c compounds, on the other hand, no shift of T* was found for the copper isotope substitution. (3) Our results obtained for all doping levels support the unusual temperature dependence of the gap function in the pseudogap region reported for underdoped Bi-type compounds, i.e., a breakup of the Fermi surface into disconnected arcs in the temperature range T _c < T < T*.     

Inelastic Scattering on the Thermal Conductivity of Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

The thermal conductivity of Ba_1−x K _x Fe₂As₂ shows a large peak at a temperature somewhat below half the value of its superconducting state critical temperature T _c. A corresponding peak was observed in the microwave conductivity. In this case, the peak was understood as the composite effect of a very anisotropic superconducting gap on at least one of the superconducting bands with nodes, possibly on the electron pocket at the M point of the Brillouin zone, combined with a rapid decrease in the inelastic scattering rate with decreasing temperature T. In this paper, we show that the thermal conductivity peak also follows from the same model. These results point to a commonality with the cuprates and, as in that instance, is evidence for an electronic mechanism. In such a mechanism, the fluctuation spectrum responsible for the scattering and pairing glue becomes gapped with the onset of superconductivity, and consequently, the low temperature inelastic scattering rapidly tends to zero.     

Impurity Effects on Energy Gap in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8+<Emphasis Type="Italic">y</Emphasis></Subscript> Investigated by Electronic Raman Scattering

The Raman scattering experiments were carried out in Zn-doped and Zn-free Bi₂Sr₂CaCu₂O_8+y with optimal hole concentration below and above T _c . The energy of pair-breaking peak in the B _1g Raman spectrum, corresponding to the magnitude of superconducting gap 2Δ ₀, is suppressed by 1% Zn-doping. In the normal state, the B _1g Raman spectrum for Zn-doped sample shows no pseudogap behaviour, suggesting that the pseudogap is strongly smeared by Zn-doping.     

T c amplication and pseudogap at a shape resonance in a superlattice of quantum stripes

The shape resonance of the superconducting gap for a 2D electron gas in a superlattice of quantum stripes with a finite 1 D periodic potential barrier is studied when the Fermi level is tuned near the bottom of the third superlattice subband. The maximumT _c . appears at a critical charge densityT _c . Forp > p _c (in the underdoped regime) the chemical potential is tuned in a pseudogap region characterized by a decrease of the density of states and an opening of the partial gap at the Fermi surface. The pseudogap is closed forp< p _c (in the overdoped regime).     

Two Pseudogap Behavior in La2?xSrxCuO4: Thermoelectric Power at High Temperature

Thermoelectric power (TEP) of high-T _C superconductors has been investigated in a wide range of temperature (T _C < T < 700 K) for La_2–x Sr _x CuO₄. TEP of La_2–x Sr _x CuO₄ shows different temperature dependences in three temperature regions. In the low-temperature region, a positive broad TEP peak is observed near T _p, which shifts to lower temperature upon doping. As temperature increases, TEP decreases linearly at intermediate temperature. In the high-temperature region, TEP deviates from the linear temperature dependence at a certain temperature, T _h showing a saturation behavior. As the doping concentration increases, the characteristic temperatures, T _C, T _p, and T _h, show systematic changes. In comparison with pseudogap temperature estimated from other experiments, the large pseudogap behavior in TEP at high temperature has been discussed and distinguished from the small pseudogap observed at lower temperature. A possibility of bound pairs formation in the normal state opening the pseudogap at high temperature is discussed briefly. The coexistence of bound pairs and the normal independent carriers for T _C < T < T_h could be the origin of the intrinsic inhomogeneity.     

Quantized Vortices and Diamagnetic Precursor to the Meissner State in High-T c Superconductors

We report the magnetic imaging for underdoped and optimally-dopedLa_2–x Sr _x CuO₄ (LSCO) thin films on single substrates and nearly optimallydoped YBa₂Cu₃O_7–x (YBCO) thin films on tricrystal substrates in the temperature range both below and above T _c using scanning SQUID microscopy. Below T _c, clear integer- and half-integer quantized vortices were observable. Above T _c, however, the inhomogeneous diamagnetic domains appeared. The local diamagnetic domains that led to the Meissner state were found in the broad temperature range for underdoped samples and in the narrow limited temperature range for optimally-doped samples. The results provide evidence that local diamagnetic domains are closely related to the pseudogap state. The continuous connection of the domain state above T _c with the state of a half-integer vortex at the tricritical point in the YBCO film below T _c also indicates that the diamagnetic domains are also closely related to the occurrence of d_x ^{2-y 2}-wave superconductivity.     

Doping and Isotope Dependence of the Pseudogap in High-Tc Cuprates Observed by Neutron Crystal-Field Spectroscopy: A Fast Local Probe

The temperature dependence of the relaxation rate of crystal-field excitations in the high-T _c superconductors HoBa₂Cu₄O₈ and HoBa₂Cu₃O _x (6.4 < x < 7) was investigated by inelastic neutron scattering techniques. The data show clear evidence for the opening of an electronic gap in the normal state at T* > T _c in the underdoped regime as well as for slightly overdoped samples. For HoBa₂Cu₄O₈ T* increases from 170 to 220 K upon oxygen isotope substitution (¹⁶O vs ¹⁸O). This huge isotope shift (which is absent in NMR and NQR experiments) suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range 10^–8 > 10^–13 s.     

Physical Picture of High-T c Superconductivity in Cuprates

A simple physical picture of high-T _c superconductivity of CuO₂ planes is proposed. It possesses all characteristic features of HTS, such as a high superconducting transition temperature, the $d_{x^{2}-y^{2}}$ symmetry of order parameter, and the coexistence of a single-electron Fermi surface and a pseudogap in the normal state. Values of pseudogap are calculated for different doping levels.     

Advantages of High-T c Cuprates over Semiconductors for Making Room Temperature Electronic Devices

This paper demonstrates that the high-T _c cuprates in the pseudogap states, which is called pseudogapbody, have many advantages over the semiconductors for making room temperature electronic devices. This paper predicts that the pseudogapbody will replace the semiconductors in many applications, and pseudogapbody electronics will appear as a new subject.     

Neutron and Raman Scattering Studies of Hgba2CuO4+δ

The HgBa₂CuO_4+δ sample was characterized by Neutron diffraction and magnetic measurements. Both of the measurements indicate a high purity of the sample. Raman measurement was performed on a HgBa₂CuO_4+δ compound of T_c = 96 K. The apical oxygen vibration at 592 cm^?1 was found to show (a) an above T_c anomaly, and (b) frequency hardening and linewidth broadening below the superconducting phase transition. The latter is attributed to the coupling of the phonon to the electronic excitation and related to the opening of the superconducting gap below the phonon frequency.     

Raman scattering from single crystal YBa2Cu3O7-δ in a magnetic field

A magneto-Raman study of a YBa₂Cu₃O_7-δ single crystal(T _c = 92 K) was carried out at a resolution of 2 cm^-1 over the temperature range 5–125 K. At temperatures belowT _c we observed a slight narrowing of the 340 cm^-1 Raman mode in a magnetic field of 5.15 T directed parallel to thec-axisof the crystal. The observed magneto-temperature dependences of the Raman continuum intensity measured in the low-frequency region, at ca. 40 cm^-1, indicate contributions of electronic excitations arising from a pair-breaking process. Within the experimental uncertainty, the Raman intensity of the high-frequency continuum was observed to be independent of the magnetic field strength.     

Investigation of Ultrasound Attenuation in Impurity-Helium Solids Containing Liquid Helium

The attenuation of ultrasound in Impurity-Helium (Im-He) solids created upon introduction of impurity atoms or molecules (D ₂ , N ₂ , Kr) into a volume of superfluid helium has been investigated. The observed features of attenuation show that a porous substance consisting of a loosely interconnected continuous network is created in superfluid helium. This network is formed by impurity particles encapsulated in solidified helium. Analysis of attenuation allows us to conclude that Im-He solid samples have a wide distribution of pores from 8 nm to 800 nm. It was established that the character of attenuation in D ₂ -He samples is considerably different from that in heavier Im-He solids, for which two maxima of attenuation were sometimes observed. A sharp peak was observed at T_c0 very close to the bulk helium lambda transition temperature and a second broad peak occurred at T_cc0 . This behavior is similar to that predicted theoretically for liquid helium in restricted fractal porous media.     

Low Temperature Scanning Tunneling Spectroscopy Studies of High Jc NdBa2Cu3O7 ? δ Single Crystals

We report low temperature scanning tunneling spectroscopy (STS) studies on NdBa₂Cu₃O_{7 –} (Nd123) single crystals. Two characteristic spectra with a nearly ohmic background spectrum and a V-shaped background are observed on the cleaved surfaces. The former spectum shows an inhomogeneous distribution of a superconducting energy gap (2) for STS differential conductance map. The latter shows a homogeneous distribution. It is probable that those differences are attributed to a tunneling current from different surface layers. The temperature dependence of tunneling conductance spectrum with a V-shaped background reveals that the superconducting gap disappears around T _c, and no pseudogap behavior exists above T _c.     

Superconductivity of Ba1 ? xKxBiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 – x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.