We have studied the Nernst coefficient, v(T), of epitaxial thin films of the superconductor Y1-xCaxBa2Cu3Oy with x =0.05 and x = 0.1. The x = 0.1 sample has been measured at three different values of y, in the over- (OV), optimally- (OP), and under-doped (UD) states. As the doping level is reduced, v(T) starts to fall linearly with T at the temperature (T*) where we expect to see the influence of the pseudogap. The onset temperature of the superconducting fluctuation contribution to v(T) was found to vary slowly with the hole concentration (p) between p = 0.118 and 0.197. For the OP and UD samples, v(T>T*) is unusually large, being comparable with |S tan(Theta)| where S is the Seebeck coefficient and Theta the Hall angle.
The observation of a large Nernst signal in cuprates above the superconducting transition temperature has attracted much attention. A potential explanation is that it originates from superconducting fluctuations. Although the Nernst signal is indeed consistent with gaussian fluctuations for overdoped cuprates, gaussian theory fails to describe the temperature dependence seen for underdoped cuprates. Here, we consider the vertex correction to gaussian theory resulting from the pseudogap. This yields a Nernst signal in good agreement with the data.
We consider the Nernst effect in the underdoped regime of the cuprate high temperature superconductors within the d-density wave (DDW) model of the pseudogap phase. By a combination of analytical and numerical arguments, we show that there is a robust low-temperature positive peak (i.e., maximum) in the temperature dependence of the Nernst coefficient when the DDW state is ambipolar, i.e., when the broken symmetry supports the coexistence of both electron- and hole-like quasiparticles in the excitation spectrum, and the electron pocket dominates at the low temperatures. In contrast, the Nernst…
We use the Nernst effect to delineate the boundary of the pseudogap phase in the temperature-doping phase diagram of cuprate superconductors. New data for the Nernst coefficient $ν(T)$ of YBa$_{2}$Cu$_{3}$O$_{y}$ (YBCO), La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ (Eu-LSCO) and La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ (Nd-LSCO) are presented and compared with previous data including La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). The temperature $T_ν$ at which $ν/T$ deviates from its high-temperature behaviour is found to coincide with the temperature at which the resistivity deviates from its linear-$T$ dependence, which…
Recently, different transport coefficients have been measured in High-Tc superconductors to pinpoint the nature of the pseudogap phase. In particular, the thermoelectric coefficients received a considerable attention both theoretically and experimentally. We numerically simulate the Nernst effect in extreme type-II superconductors using the time-dependent Ginzburg-Landau equations. We report the sign reversal of the thermoelectric coefficient, alpha_xy, at temperatures close to the mean-field transition temperature, Tc^{MF}(H), which qualitatively agrees with recent experiments on high-Tc mate…
It is often claimed that among the strongest evidence for preformed-pair physics in the cuprates are the experimentally observed large values for the diamagnetic susceptibility and Nernst coefficient. These findings are most apparent in the underdoped regime, where a pseudogap is also evident. While the conventional (Gaussian) fluctuation picture has been applied to address these results, this preformed-pair approach omits the crucial effects of a pseudogap. In this paper we remedy this omission by computing the diamagnetic susceptibility and Nernst coefficient in the presence of a normal stat…
The onset temperature of superconducting fluctuation Tonset of Bi2Sr2-xRxCuOy (R=La and Eu) was studied by measuring the Nernst effect. We found that Tonset has a x and R dependence that is quite different from both the pseudogap temperature T* and the critical temperature Tc. Our results support the picture that the incoherent superconductivity, which has been observed below Tonset, is qualitatively different from the pseudogap phenomenon that is characterized by T*. The experimentally obtained phase diagram indicates that the pseudogap state suppresses Tc and enhances superconducting fluctua…
The temperature dependences of g factors of underdoped YBa2Cu3O7-x (Tc approximates 86.5K) indicate that the pseudogap onset temperature T*( approximates 190 K) coincides with the phase boundary established by resistivity, Nernst effect, neutron diffraction and ultrasound measurements in YBCO phase diagram. The obtained empirical relationship between g factors and pseudogap is appropriate for the entire charactering temperature range of Ortho-III YBCO. The EPR signals are ascribed to the localized Cu2+(1) sites based on the quantitative calculations of the g factors by involving the local stru…
The Nernst coefficient of the cuprate superconductor YBa2Cu3Oy was recently shown to become strongly anisotropic within the basal plane when cooled below the pseudogap temperature T*, revealing that the pseudogap phase breaks the four-fold rotational symmetry of the CuO2 planes. Here we report on the evolution of this Nernst anisotropy at low temperature, once superconductivity is suppressed by a magnetic field. We find that the anisotropy drops rapidly below 80 K, to vanish in the T=0 limit. We show that this loss of anisotropy is due to the emergence of a small high-mobility electron-like po…
Influence of the pseudogap on the superconductivity-induced phonon renormalization in high-T$_c$ superconductors
研究了d密度波（DDW）带隙对超导诱导的声子频率和线宽重整化的影响。拉曼和非弹性中子散射实验的结果进行了讨论。结果表明，DDW带隙可以增强$q=0$声子软化的频率范围，这取决于底层的能带结构。此外，我们还发现各向异性的“d波”赝能隙对YBa $_2$ Cu $_3$ O $_7$中340 cm $^{-1}$声子的q依赖线宽展宽也有贡献.
Nernst Effect and Superconducting Fluctuations in Zn-doped YBa$_{2}$Cu$_{3}$O$_{7-δ}$
报道了用脉冲激光沉积技术生长的Zn掺杂$_{2}$ O $_{3}$外延薄膜的面内电阻率、霍尔效应和能斯特效应的测量结果。的赝能隙温度，$T^*$，确定从电阻率的温度依赖性，不改变与Zn掺杂显着。同时，在$T_{c0}$以上的反常能斯特信号的起始温度（$T^ν$）随着超导转变温度$T_{c0}$的降低而急剧降低，这被解释为类涡旋激发的证据。在混合态下，涡旋能斯特信号的最大值也显著降低，这与Zn杂质导致超流密度降低从而抑制超导电性的假设相一致.给出并讨论了$T^*$、$T^ν$和$T_{c0}$与Zn含量的相图。
Employing {\it state of the art} high resolution photoemission spectroscopy, we studied the temperature evolution of the electronic structure of EuFe$_2$As$_2$, an unique pnictide, where antiferromagnetism of Eu layer survives within the superconducting phase due to `FeAs' layers achieved via substitution and/or pressure. High energy and angle resolution helped to reveal pseudogap-quasiparticle features having primarily As 4$p$ character and spin density wave transition induced band folding in the electronic structure. A weakly dispersing feature of dominant As 4$p$ character is discovered aro…
We report transport measurements using pulsed magnetic fields to suppress the superconducting fluctuations (SCF) conductivity in a series of YBa_2Cu_3O_(6+x) samples. These experiments allow us altogether to measure the temperature T'c at which SCF disappear, and the pseudogap temperature T*. While the latter are consistent with previous determinations of T*, we find that T'c is slightly larger than similar data taken by Nernst measurements. A careful investigation near optimal doping shows that T* becomes smaller than T'c, which is an unambiguous evidence that the pseudogap cannot be assigned…
In this paper, we try to understand the pseudogap phenomenon observed in the cuprate superconductor through a model study. Specifically, we explore the so-called low-temperature pseudogap state by turning off the superconducting off diagonal long range order in an ansatz state for the $t$-$J$ model [New Journal of Physics {\bf 13}, 103039 (2011)]. Besides strong non-Gaussian superconducting fluctuations, the resulting state also exhibits a systematic pseudogap behavior in both spin and charge degrees of freedom, manifested in the uniform spin susceptibility, specific heat, non-Drude resistivit…
A theory of a pseudogap phase of high-temperature superconductors where current carriers are translation invariant bipolarons is developed. A temperature T* of a transition from a pseudogap phase to a normal one is calculated. For the temperature of a transition to the pseudogap phase, the isotope coefficient is found. It is shown that the results obtained, in particular, the possibility of negative values of the isotope coefficient are consistent with the experiment. New experiments on the influence of the magnetic field on the isotope coefficient are proposed.
The physical properties of hole-doped cuprate high-temperature superconductors are heavily influenced by an energy gap known as the pseudogap whose origin remains a mystery second only to that of superconductivity itself. A key question is whether the pseudogap closes at a temperature T*. The absence of a specific heat anomaly, together with persistent entropy losses up to 300K, have long suggested that the pseudogap does not vanish at T*. However, amid a growing body of evidence from other techniques pointing to the contrary we revisit this question. Here we investigate if, by adding a temper…
The nature of the pseudogap phase is a central problem in the quest to understand high-Tc cuprate superconductors. A fundamental question is what symmetries are broken when that phase sets in below a temperature T*. There is evidence from both polarized neutron diffraction and polar Kerr effect measurements that time- reversal symmetry is broken, but at temperatures that differ significantly. Broken rotational symmetry was detected by both resistivity and inelastic neutron scattering at low doping and by scanning tunnelling spectroscopy at low temperature, but with no clear connection to T*. H…
We study the Hall and Nernst effects across the antiferromagnetic transition that reconstructs the quasi-2D Fermi surface of the metallic local moment antiferromagnet PdCrO$_2$. We show that non- monotonic temperature dependence in the Hall effect and a sign change in the Nernst effect above the ordering temperature cannot be understood within a simple single-band transport model. The inclusion of coherent scattering by critical antiferromagnetic fluctuations can qualitatively account for these features in the transport coefficients. We discuss the implications of this for the pseudogap phase …
The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low energy electronic excitations. Polarized neutron diffraction, Nernst effect, THz polarimetery and ultrasound measurements on YBa$_2$Cu$_3$O$_y$ suggest that the pseudogap onset below a temperature T* coincides with a bona fide thermodynamic phase transition that breaks time-reversal, four-fold rotation and mirror symmetries respectively. However, the full point group above and below T* has not been resolved and the fate of this transition as…
The effect of electron irradiation with the energy of 2.5 MeV on the temperature dependences of the resistivity of an optimally doped $YBa_2Cu_3O_{7-δ}$ single crystal has been studied. The temperature dependences of both fluctuation conductivity $σ^\prime(T)$ and the pseudogap $Δ^*(T)$ on irradiation dose $ϕ$ have been calculated within the local pair model. Here we show that with an increase in $ϕ$, the value of $ρ(300 K)$ increases linearly, while $T_c$ decreases linearly. Concurrently, the value of $ρ{100 K}$ increases nonlinearly, demonstrating a feature for $ϕ_3$ = 4.3 10$^{18}$ e/$cm^2$…
We investigated several FeSe single crystals grown by two different methods by utilizing experimental techniques namely, resistivity, magnetoresistance, specific heat, scanning tunneling microscopy, and spectroscopy. The residual resistivity ratio (RRR) shows systematic differences between samples grown by chemical vapor transport and flux vapor transport, indicating variance in the amount of scattering centers. Although the superconducting transition temperature $T_c$ is not directly related to RRR, our study evidences subtle differences in the features of an incipient ordering mode related t…
The phase diagram associated with the high Tc superconductors is complicated by an array of different ground states. The parent material represents an antiferromagnetic insulator but with doping superconductivity becomes possible with transition temperatures previously thought unattainable. The underdoped region of the phase diagram is dominated by the so-called pseudogap phenomena whereby in the normal state the system mimics superconductivity in its spectra response but does not show the complete loss of resistivity associated with the superconducting state. An understanding of this regime p…
Ni impurity induced enhancement of the pseudogap in cuprate high T_c superconductors
用椭圆偏振光谱法研究了磁性Ni和非磁性Zn杂质对NdBaPDMATHABMM\{$% _{1-y}$（Ni，Zn）$_{y}\}_{3}$ O $_{7-δ}$}晶体c轴光学电导率中正常态赝能隙（PG）的影响.我们发现，这些强烈抑制超导性的杂质有一个深刻的不同的影响PG。锌引起了一个渐进的和不均匀的PG抑制，而镍强烈增强PG。我们的研究结果挑战的理论，涉及PG无论是前体超导性或其他阶段与外来的序参量，如通量相或d-密度波状态，这应该被抑制的潜在散射。磁性和非磁性杂质之间的明显差异反而指向PG状态中磁相关性的重要作用。
The influence of hydrostatic pressure up to P=1.05 GPa on resistivity R, excess conductivity $σ'(T)$ and pseudogap $Δ*(T)$ is studied in slightly doped single crystals of YBa2Cu3O7-$δ (Tc=49.2 K, δ~ 0.5)$. The critical temperature Tc is found to increase with increasing pressure at a rate dTc/dP = 5.1 K/GPa, while R(300)K decreases at a rate d(ln R)/dP = 0.19% 1/GPa. Near Tc, independently on pressure, the $σ'(T)$ is well described by the Aslamasov-Larkin and Hikami-Larkin fluctuation theories, demonstrating a 3D-2D crossover with increase of temperature. The crossover temperature To determine…
We study heat transport and thermoelectric effects in two-dimensional superconductors in a magnetic field. These are modeled as granular Josephson-junction arrays, forming either regular or random lattices. We employ two different models for the dynamics, relaxational model-A dynamics or resistively and capacitively shunted Josephson junction (RCSJ) dynamics. We derive expressions for the heat current in these models, which are then used in numerical simulations to calculate the heat conductivity and the Nernst coefficient for different temperatures and magnetic fields. At low temperatures and…
The paper analyzes the influence of the hole density, the out-of-plane or in-plane disorder, and the isotopic oxygen mass on the zero temperature energy gap ($2Δ\left(0\right)$) for $\rm{Y}_{1-x}\rm{Ca}_{x}\rm{Ba}_2\left(\rm{Cu}_{1-y}\rm{Zn}_{y}\right)_{3}\rm{O}_{7-δ}$ (YCBCZO) and $\rm{La}_{1.96-x}\rm{Sr}_{x}\rm{Ho}_{0.04}\rm{CuO}_{4}$ (LSHCO) superconductors. It has been found that the energy gap is visibly correlated with the value of the pseudogap temperature ($T^{\star}$). On the other hand, no correlation between $2Δ\left(0\right)$ and the critical temperature ($T_{C}$) has been found. T…
The effect of annealing at room temperature on the fluctuation conductivity (FLC) $σ^\prime(T)$ and pseudogap (PG) $Δ^*(T)$ in the basal ab plane of $ReBa_2Cu_3O_{7-δ}$ (Re = Ho) single crystals with a lack of oxygen has been studied. It is shown that at all stages of annealing, the FLC near $T_c$ can be described by the Aslamazov$-$Larkin and Maki$-$Thompson fluctuation theories, demonstrating a 3D$-$2D crossover with increasing temperature. The crossover temperature $T_0$ was used to determine the coherence length along the $c$-axis, $ξ_c(0)$=2.82 A. At the inter-mediate stage of annealing, …
Earlier we have proposed a new approach to the analysis of superconducting phase diagrams for cuprates and pnictides and have shown that the positions of superconducting domes on the diagrams can be predicted with high accuracy proceeding from only the crystal structure of a particular compound. The proposed approach uses the concept of the self-localization of doped carriers due to their formation of trion complexes that represent a bound state of the doped carrier and charge transfer excitons emerging under its influence. Here, as exemplified by cuprates, we show that the use of the proposed…
The pseudogap (PG) derived from the analysis of the excess conductivity $σ'(T)$ in superlattices and double-layer films of $YBa_2Cu_3O_{7-δ}$-$PrBa_2Cu_3O_{7-δ}$(YBCO-PrBCO), prepared by pulsed laser deposition, is studied for the first time. The $σ'(T)$ analysis has been performed within the local-pair (LP) model based on the assumption of the paired fermion (LPs) formation in the cuprate high-$T_c$ superconductors (cuprates) below the representative temperature $T^*\gg T_c$ resulting in the PG opening. Within the model, the temperature dependencies of the PG, $Δ^*(T)$, for the samples with d…
The influence of the hydrostatic pressure P up to 0.95 GPa on the excess conductivity and the pseudogap Δ*(T) in optimally doped YBCO single crystals (T_c = 91.1K at ambient pressure) is investigated by electrical resistivity measurements. A pronounced enhancement of the pseudogap under pressure of dlnΔ*/dP= 0.32, which is only a factor of 1.12 smaller than in slightly doped single crystals, is revealed for the first time. This implies a somewhat more moderate increase of the coupling strength in optimally doped cuprates with increasing pressure. Simultaneously, the ratio 2Δ*(T_c)/k_BT_c=5 at …
The cuprate superconductors distinguish themselves from the conventional superconductors in that a small variation in the carrier doping can significantly change the superconducting transition temperature (T_c), giving rise to a superconducting dome where a pseudogap (ref. 1,2) emerges in the underdoped region and a Fermi liquid appears in the overdoped region. Thus a systematic study of the properties over a wide doping range is critical for understanding the superconducting mechanism. Here, we report a new technique to continuously dope the surface of Bi2Sr2CaCu2O8+x through an ozone/vacuum …
A new NMR anvil cell design is used for measuring the influence of high pressure on the electronic properties of the high-temperature superconductor YBa$_2$Cu$_4$O$_8$ above the superconducting transition temperature $T_{\rm c}$. It is found that pressure increases the spin shift at all temperatures in such a way that the pseudo-gap feature has almost disappeared at 63 kbar. This change of the temperature dependent spin susceptibility can be explained by a pressure induced proportional decrease (factor of two) of a temperature dependent component, and an increase (factor of 9) of a temperature…
The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity. In particular, whether the pseudogap is associated with a crossover or phase transition has been a long-standing controversial issue. The tetragonal cuprate HgBa$_2$CuO$_{4+δ}$, with only one CuO$_2$ layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the anisotropy of magnetic susceptibility within the CuO$_2$ plane with exceptionally high-precision magnetic torque experiments. Our key finding is that a distinct two-fold in-plane anisotropy…
A study of the ferromagnetic transition of $SrRuO_3$ in nanometer thick bilayers with $YBa_2Cu_3O_y$, $La_{1.88}Sr_{0.12}CuO_{4-y}$, Au and Cr: Signature of injected carriers in the pseudogap regime
High-temperature superconductivity emerges in the CuO$_2$ plane upon doping a Mott insulator. To ascertain the influence of Mott physics plus short-range correlations, we solve a three-band copper-oxide model in the charge-transfer regime using cellular dynamical mean-field theory with continuous-time quantum Monte Carlo as an impurity solver. We report the normal and superconducting phase diagram of this model as a function of doping, interaction strength and temperature. Upon hole doping of the charge-transfer insulator, the phase boundary between pseudogap and correlated metal consists of a…
