On field effect studies and superconductor-insulator transition in high-Tc cuprates

We summarize previous field effect studies in high-T _c cuprates and then discuss our method to smoothly tune the carrier concentration of a cuprate film over a wide range using an applied electric field. We synthesized epitaxial one-unit-cell thick films of La_2?x Sr _x CuO₄ and from them fabricated electric double layer transistor devices utilizing various gate electrolytes. We were able to vary the carrier density by about 0.08 carriers per Cu atom, with the resulting change in T _c of 30 K. The superconductor-insulator transition occurred at the critical resistance very close to the quantum resistance for pairs, R _Q = h/(2e)² = 6.5 kΩ. This is suggestive of a quantum phase transition, possibly driven by quantum phase fluctuations, between a “Bose insulator” and a high-T _c superconductor state.     

Entropy of vortex cores near the superconductor-insulator transition in an underdoped cuprate

We present a study of Nernst effect in underdoped La(2-x)Sr(x)CuO4 in magnetic fields as high as 28 T. At high fields, a sizable Nernst signal was found to persist in the presence of a field-induced nonmetallic resistivity. By simultaneously measuring resistivity and the Nernst coefficient, we extract the entropy of vortex cores in the vicinity of this field-induced superconductor-insulator transition. Moreover, the temperature dependence of the thermoelectric Hall angle provides strong constraints on the possible origins of the finite Nernst signal above T(c), as recently discovered by Xu et al. [Nature (London) 406, 486 (2000)].     

Short-range spin correlations and pseudogap in underdoped cuprates

In this paper we show that local spin-singlet amplitude with d-wave symmetry can be induced by short-range spin correlations even in the absence of pairing interactions. In the present scenario for the pseudogap, the normal state pseudogap is caused by the induced local spin-singlet amplitude due to short-range spin correlations, which compete in the low energy sector with superconducting correlations to make T_c go to zero near half-filling.     

Raman active collective mode in underdoped cuprates

We report experimental observation of Bardasis–Schrieffer-type s-symmetry collective mode in the d-wave high T_c superconductors. The mode persists above T_c in the pseudogap phase of the underdoped superconductors — direct spectroscopic evidence of the correlations in the particle–particle channel above T_c.     

Possible field-tuned superconductor-insulator transition in high-Tc superconductors: implications for pairing at high magnetic fields

The behavior of some high temperature superconductors (HTSC), such as La(2-x)Sr(x)CuO(4) and Bi(2)Sr(2-x)La(x)CuO(6 + delta), at very high magnetic fields, is similar to that of thin films of amorphous InOx near the magnetic-field-tuned superconductor-insulator transition. Analyzing the InOx data at high fields in terms of persisting local pairing amplitude, we argue by analogy that the local pairing amplitude also persists well into the dissipative state of the HTSCs, the regime commonly denoted as the "normal state" in very high magnetic field experiments.     

Memory-function conductivity formula and transport coefficients in underdoped cuprates

The two-band memory-function conductivity formula is derived from the quantum kinetic equation in the pseudogap state of underdoped cuprates. The conduction electrons are described by using the adiabatic version of the nested Fermi liquid model, and the effects of Mott correlations are taken into account phenomenologically. The linear dependence of the low-temperature effective number of conduction electrons on the doping level δ (for not too large δ) is found to be in agreement with experimental observation. The momentum distribution function turns out to play an important role in describing temperature effects. The closing of the antiferromagnetic pseudogap at temperatures of the order of room temperature is shown to be a direct consequence of a relatively large width of the quasiparticle peak in this distribution function. The coupling of conduction electrons to external magnetic fields is included in the two-band transport equations in the usual semiclassical way. It is shown that the low-temperature Hall number is proportional to δ as well (again for not too large δ) and that it exhibits singular behaviour when the Fermi surface changes from the hole-like shape into the electron-like shape.     

Incommensurate valence modulation in high-Tc cuprates

An incommensurate modulation has been observed in a Cu-rich La2CuO4.003 crystal. It is shown that the modulation results from a periodically distributed holes lying on the CuO2 planes, and that the hole modulation may be regarded to be a kind of valence modulation. It is shown that appreciable valence modulation contrast may be generated by the mechanisms of hole scattering alone when the period of the modulation is of the order of 2 nm.     

Enhanced superconductivity in superlattices of high-Tc cuprates

The electronic properties of multilayers of strongly correlated models for cuprate superconductors are investigated using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system.     

Competition between antiferromagnetism and superconductivity in high-Tc cuprates

Using variational cluster perturbation theory we study the competition between d-wave superconductivity (dSC) and antiferromagnetism (AF) in the t-t(')-t(')-U Hubbard model. Large scale computer calculations reproduce the overall ground-state phase diagram of the high-temperature superconductors as well as the one-particle excitation spectra for both hole and electron doping. We identify clear signatures of the Mott gap as well as of AF and of dSC that should be observable in photoemission experiments.     

Reflections on the superconductor-insulator transition

The anomalously long superconducting coherence lengths observed in YBCO/PBCO multilayers are interpreted in terms of a boson localization picture for the superconductor-insulator transition in YBCO as Pr is substituted for Y. It is suggested that the combination of finite vortex mass and reduced superfluid density for the proximity-induced order parameter in PBCO should lead to enhanced quantum creep in these multilayer samples.     

Unusual Pr antiferromagnetic ordering in high-Tc cuprates

The anomalous Pr antiferromagnetic order with high Néel temperature T_N(Pr) are reported throughout the whole Pr_1+xBa_2−xCu₃O_7−y or 1212-type Cu(Ba_2−xPr_x)PrCu₂O_7−y system (−0.2<x<1; −0.4<y<1), where three distinct crystal structures were observed: orthorhombic 1212-chain O(I) (space group Pmmm), tetragonal T (P4/mmm), and orthorhombic O(II) (Cmmm). Systematic variation of T_N(Pr) in this system as well as in other 1212 and 2212 cuprates M_nA₂PrCu₂O₇ (n=1, 2, M=Hg, Tl, Pb/Cu, Bi, Nb, Cu; A=Sr, Ba, Ba/Pr) was discussed through the correlation of T_N(Pr) with Pr–O bond length. The importance of quasi-two-dimensional Pr–O–Pr superexchange magnetic coupling through strong wave function overlap between the overextended Pr-4f orbital with eight O-2p_π orbitals in the adjacent CuO₂ bi-layer is discussed. No superconductivity was observed in the present study.     

Detection and implications of a time-reversal breaking state in underdoped cuprates

We present general symmetry considerations on how a time-reversal breaking state may be detected by angle-resolved photoemission using circularly polarized photons as has been proposed earlier. Results of recent experiments utilizing the proposal in underdoped cuprates are analyzed and found to be consistent in their symmetry and magnitude with a theory of the copper oxides. These experiments if correct, together with evidence of a quantum critical point and marginal Fermi-liquid properties near optimum doping, suggest that the essentials of a valid microscopic theory of the phenomena in the cuprates may have been found.     

Staggered local density of states around the vortex in underdoped cuprates

We have studied a single vortex with the staggered flux (SF) core based on the SU(2) slave-boson theory of high T(c) superconductors. We find that, whereas the center in the vortex core is a SF state, as one moves away from the core center a correlated staggered modulation of the hopping amplitude chi and pairing amplitude Delta becomes predominant. We predict that in this region the local density of states exhibits staggered modulation when measured on the bonds, which may be directly detected by STM experiments.     

Temperature dependence of the superconducting gap in high-Tc cuprates

It is proposed that the temperature dependence of the superconducting gap Delta(T) in high-T(c) cuprates can be predicted just from the knowledge of Delta(0) and the critical temperature T(c), and, in particular, Delta(0)/T(c)>4 implies that Delta(T(c)) not equal 0, while Delta(0)/T(c)相似文献