We examine the possibility that the experimentally observed enhancement of superconducting (SC) fluctuations above the SC transition temperature in the underdoped cuprates is caused by stripes -- an intrinsic electronic inhomogeneity, common to hole-doped cuprates. By evaluating the strengths of the diamagnetic response and the Nernst effect within the striped SC model, we find results that are qualitatively consistent with the experimental observations. We make a prediction for anisotropic thermopower in detwinned samples that can be used to further test the proposed scenario.
We have precisely measured the Nernst effect in Nd-doped La$_{2-x}$Sr$_x$CuO$_4$ single crystals with controlling the strength (stability) of the stripe order. We found that the onset temperature $T_{onset}$, where the Nernst signal starts increasing, does not change conspicuously in spite of Nd-doping. At low temperatures, on the other hand, the absolute value of the Nernst signal is strongly suppressed in accordance with the strength of the stripe order. These results imply that the fluctuation of (charge) stripe order enhances the Nernst signal below $T_{onset}$ at high temperatures, and th…
Experiments on underdoped cuprate superconductors suggest an intricate relation between the normal-state Nernst effect and stripe order: The Nernst signal appears enhanced near 1/8 hole doping and its onset temperature scales with the stripe-ordering temperature over some range of doping. Here, we employ a phenomenological quasiparticle model to calculate the normal state Nernst signal in the presence of stripe order. We find that Fermi pockets caused by translational symmetry breaking lead to a strongly enhanced Nernst signal with a sign depending on the modulation period of the ordered state…
The striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ ($x=\frac18)$ undergoes several transitions below the charge-ordering temperature $T_{co}$ = 54 K. From Nernst experiments, we find that, below $T_{co}$, there exists a large, anomalous Nernst signal $e_{N,even}(H,T)$ that is symmetric in field $H$, and remains finite as $H\to 0$. The time-reversal violating signal suggests that, below $T_{co}$, vortices of one sign are spontaneously created to relieve interlayer phase frustration.
We investigate the transport properties of La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ ($x=0.04$, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For $x=0.125$ and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except $x=0.2$ exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe orde…
Linear diamagnetism is predicted in the vortex-liquid phase of layered superconductors at temperatures just below the mean-field phase transition on the basis of a high-temperature analysis of the corresponding frustrated XY model. The diamagnetic susceptibility, and the Nernst signal by implication, is found to vanish with temperature as (T_c0 - T)^3 in the vicinity of the meanfield transition at T_c0. Quantitative agreement with recent experimental observations of a diamagnetic signal in the vortex-liquid phase of high-temperature superconductors is obtained.
The Nernst effect in metals is highly sensitive to two kinds of phase transition: superconductivity and density-wave order. The large positive Nernst signal observed in hole-doped high-Tc superconductors above their transition temperature Tc has so far been attributed to fluctuating superconductivity. Here we show that in some of these materials the large Nernst signal is in fact caused by stripe order, a form of spin / charge modulation which causes a reconstruction of the Fermi surface. In LSCO doped with Nd or Eu, the onset of stripe order causes the Nernst signal to go from small and negat…
We report on a thermoelectric investigation of the stripe and superconducting phases of the cuprate La$_{2-x}$Ba$_{x}$CuO$_{4}$ near the $x=1/8$ doping known to host stable stripes. We use the doping and magnetic field dependence of field-symmetric Nernst effect features to delineate the phenomenology of these phases. Our measurements are consistent with prior reports of time-reversal symmetry breaking signatures above the superconducting $T_{\rm c}$, and crucially detect a sharp, robust, field-invariant peak at the stripe charge order temperature, $T_{\rm {\scriptscriptstyle CO}}$. Our observ…
After a brief review of current ideas on stripe order in cuprate high-temperature superconductors, we discuss the quasiparticle Nernst effect in the cuprates, with focus on its evolution in non-superconducting stripe and related nematic states. In general, we find the Nernst signal to be strongly enhanced by nearby van-Hove singularities and Lifshitz transitions in the band structure, implying that phases with translation symmetry breaking often lead to a large quasiparticle Nernst effect due to the presence of multiple small Fermi pockets. Open orbits may contribute to the Nernst signal as we…
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…
We report magnetic force microscopy (MFM) measurements on underdoped $BaFe_2(As_{1-x}P_x)_2$ ($x=0.26$) that show enhanced superconductivity along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel vortices when we cool in a finite magnetic field and act as barriers when we drag vortices with the magnetic MFM tip. The stripes disappear when we warm the sample towards the superconducting transition temperature. We show that stripes can move when we warm the sample and comment on the relationship between the stripes and similar stripes observed previously i…
The Nernst effect has recently proven a sensitive probe for detecting unusual normal state properties of unconventional superconductors. In particular, it may sensitively detect Fermi surface reconstructions which are connected to a charge or spin density wave (SDW) ordered state, and even fluctuating forms of such a state. Here we summarize recent results for the Nernst effect of the iron pnictide superconductor $\rm LaO_{1-x}F_xFeAs$, whose ground state evolves upon doping from an itinerant SDW to a superconducting state, and the cuprate superconductor $\rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ which…
We study a model for the transverse thermoelectric response due to quantum superconducting fluctuations in a two-leg Josephson ladder, subject to a perpendicular magnetic field B and a transverse temperature gradient. The off-diagonal Peltier coefficient (α_{xy}) and the Nernst effect are evaluated as functions of B and the temperature T. The Nernst effect is found to exhibit a prominent peak close to the superconductor-insulator transition (SIT), which becomes progressively enhanced at low T. In addition, we derive a relation to diamagnetic response: α_{xy}= -M/T_0, where M is the equilibrium…
A series of recent experiments on the Nernst effect, upper critical field $H_{c2}$ and enhanced diamagnetic magnetization $M$ by Wang et al (PRB 73, 024510), were interpreted as to provide evidence for the pseudogap scenario which long-range phase coherence is destroyed by thermally-created vortices. We present here calculations in good agreement with these measurements but based on the local formation of conventional Bogoliubov-deGennes (BdG) superconducting regions in doped disordered cuprate superconductors. Our results imply in an alternate scenario which the disorder plays a fundamental r…
We present a new method to study the Nernst effect and diamagetism of an extreme type-II superconductor dominated by phase fluctuations. We work directly with vortex variables and our method allows us to tune vortex parameters (e.g., core energy and number of vortex species). We find that diamagnetic response and transverse thermoelectric conductivity ($α_{xy}$) persist well above the Kosterlitz-Thouless transition temperature, and become more pronounced as the vortex core energy is increased. However, they \textit{weaken} as the number of internal vortex states are increased. We find that $α_…
When a superconductor is warmed above its critical temperature $T_c$, long range order is destroyed by fluctuations in the order parameter. These fluctuations can be probed by measurements of conductivity, diamagnetism, and of the Nernst effect. Here, we study a regime where superconductivity is destroyed by phase fluctuations arising from a dilute liquid of mobile vortices. We find that the Nernst effect and diamagnetic response differ significantly from Gaussian fluctuations -- in particular, a much sharper decay with temperature is obtained. We predict a rapid onset of Nernst signal at a te…
Superconducting quantum interference device (SQUID) microscopy shows stripes of increased diamagnetic susceptibility in underdoped, but not overdoped, single crystals of Ba(Fe1-xCox)2As2. These stripes of increased diamagnetic susceptibility are consistent with enhanced superfluid density on twin boundaries. Individual vortices avoid pinning on or crossing the stripes, and prefer to travel parallel to them. These results indicate a relationship between superfluid density, local strain, and frustrated magnetism, and demonstrate two mechanisms for enhancing critical currents.
We address those puzzling experiments in underdoped high $T_c$ superconductors which have been associated with normal state "vortices" and show these data can be understood as deriving from preformed pairs with onset temperature $T^* > T_c$. For uncorrelated bosons in small magnetic fields, and arbitrary $T^*/T_c$, we present the exact contribution to \textit{all} transport coefficients. In the overdoped regime our results reduce to those of standard fluctuation theories ($T^*\approx T_c$). Semi-quantitative agreement with Nernst, ac conductivity and diamagnetic measurements is quite reasonabl…
We present an analytical study of diamagnetism and transport in a film with superconducting phase fluctuations, formulated in terms of vortex dynamics within the Debye-Hückle approximation. We find that the diamagnetic and Nernst signals decay strongly with temperature in a manner which is dictated by the vortex core energy. Using the theory to interpret Nernst measurements of underdoped La$_{2-x}$Sr$_x$CuO$_4$ above the critical temperature regime we obtain a considerably better fit to the data than a fit based on Gaussian order-parameter fluctuations. Our results indicate that the core energ…
In hole-doped cuprates, Nernst experiments imply that the superconducting state is destroyed by spontaneous creation of vortices which destroy phase coherence. Using torque magnetometry on $\rm Bi_2Sr_2CaCu_2O_{8+δ}$, we uncover a field-enhanced diamagnetic signal $M$ above the transition temperature $T_c$ that increases with applied field to 32 Tesla and scales just like the Nernst signal. The magnetization results above $T_c$ distinguish $M$ from conventional amplitude fluctuations, and strongly support the vortex scenario for the loss of phase coherence at $T_c$.
Quantum oscillations have revealed the presence of a small pocket in the Fermi surface of the cuprate superconductor YBCO, whose nature and origin are the subject of much debate. Interpretations include electron and hole pockets; scenarios include Fermi-surface reconstruction by antiferromagnetism, d-density-wave order, and stripe order. Here we report quantum oscillations in the Seebeck and Nernst coefficients of YBCO and show, from the magnitude and sign of the Seebeck coefficient, that they come from an electron pocket. Using measurements of the Seebeck coefficient as a function of hole dop…
High-Tc cuprate, A3C60 and other unconventional superconductors (SCs) have very high Tc with respect to the energy scale of superconducting (SC) charges inferred from the superfluid density (SFD). The observed linear dependence of Tc on the SFD can hardly be expected in BCS SCs while being reminiscent of Bose Einstein Condensation of pre-formed pairs. As additional non-BCS like behaviors, responses similar to those in the bulk SC states have been observed at temperatures (T) well above Tc in the vortex-like Nernst effect, diamagnetic susceptibility, and transient optical conductivity in photo-…
Transport measurements provide important characterizations of the nature of stripe order in the cuprates. Initial studies of systems such as La(1.6-x)Nd(0.4)Sr(x)CuO(4) demonstrated the strong anisotropy between in-plane and c-axis resistivities, but also suggested that stripe order results in a tendency towards insulating behavior within the planes at low temperature. More recent work on La(2-x)Ba(x)CuO(4) with x=1/8 has revealed the occurrence of quasi-two-dimensional superconductivity that onsets with spin-stripe order. The suppression of three-dimensional superconductivity indicates a frus…
In the cuprate superconductors, Nernst and torque magnetization experiments have provided evidence that the disappearance of the Meissner effect at $T_c$ is caused by the loss of long-range phase coherence, rather than the vanishing of the pair condensate. Here we report a series of torque magnetization measurements on single crystals of $\mathrm{La_{2-x}Sr_xCuO_4}$ (LSCO), $\mathrm{Bi_2Sr_{2-y}La_yCuO_6}$ (Bi 2201), $\mathrm{Bi_2Sr_2CaCu_2O_{8+δ}}$ (Bi 2212) and optimal $\mathrm{YBa_2Cu_3O_7}$. Some of the measurements were taken to fields as high as 45 T. Focusing on the magnetization above …
In this brief report an attempt is made for a 'mise-a-point' of the subject of the phase fluctuations of the superconducting order parameter above Tc in cuprates, particularly as they appear in underdoped compounds. Measurements of torque magnetometry, Nernst effect and isothermal diamagnetic magnetization curves published in the last years are taken into consideration. Although by different experimental approaches and in different magnetic field ranges it can be stated that vortex-antivortex excitations and phase fluctuations among islands of local non-zero order parameter lacking of long ran…
The Seebeck and Nernst coefficients $S$ and $ν$ of the cuprate superconductor YBa$_2$Cu$_3$O$_y$ (YBCO) were measured in a single crystal with doping $p = 0.12$ in magnetic fields up to H = 28 T. Down to T=9 K, $ν$ becomes independent of field by $H \simeq 30$ T, showing that superconducting fluctuations have become negligible. In this field-induced normal state, $S/T$ and $ν/T$ are both large and negative in the $T \to 0$ limit, with the magnitude and sign of $S/T$ consistent with the small electron-like Fermi surface pocket detected previously by quantum oscillations and the Hall effect. The…
The transition temperature Tc of cuprate superconductors falls when the doping p is reduced below a certain optimal value. It is unclear whether this fall is due to strong phase fluctuations or to a decrease in the pairing gap. Different interpretations of photoemission data disagree on the evolution of the pairing gap and different estimates of the upper critical field Hc2 are in sharp contradiction. Here we resolve this contradiction by showing that superconducting fluctuations in the underdoped cuprate Eu-LSCO, measured via the Nernst effect, have a characteristic field scale that falls wit…
The underdoped cuprates have a number of interesting and unusual properties that often seem hard to reconcile with one another. In this paper we show how many of these diverse phenomena can be synthesized into a single coherent theoretical picture. Specifically we present a description where a pseudogap and gapless Fermi arcs exist in the normal state above the superconducting transition temperature ($T_c$), but give way to the observed quantum oscillations and other phenomena at low temperature when the superconductivity is suppressed by a magnetic field. We show the consistency between these…
