Nernst Effect In Normal State Research Articles

Quasiparticle Nernst effect in stripe-ordered cuprates

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 and other details of the Fermi surface. This implies differences between antiferromagnetic and charge-only stripes. We also analyze the anisotropy of the Nernst signal and compare our findings with recent data from ${\text{La}}_{1.6\ensuremath{-}x}{\text{Nd}}_{0.4}{\text{Sr}}_{x}{\text{CuO}}_{4}$ and ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{y}$.

Nernst effect in the electron-doped cuprate superconductors

We calculate the normal-state Nernst signal in the cuprates resulting from a reconstruction of the Fermi surface due to spin-density wave order. An order parameter consistent with the reconstruction of the Fermi surface detected in electron-doped materials is shown to sharply enhance the Nernst signal close to optimal doping. Within a semiclassical treatment, the obtained magnitude and position of the enhanced Nernst signal agrees with Nernst measurements in electron-doped cuprates. Our result is mainly caused by the role of Fermi-surface geometry under influence of a spin-density wave gap. We discuss also possible roles of short-ranged magnetic order in the normal-state Nernst effect and the Fermi-surface reconstruction observed by photoemission spectroscopy.

Nernst effect of a new iron-based superconductor LaO1−xFxFeAs

We report the first Nernst effect measurement on a new iron-based superconductor LaO1−xFxFeAs (x=0.1). In the normal state, the Nernst signal is negative and very small. Below Tc, a large positive peak caused by vortex motion is observed. The flux flowing regime is quite large compared to the conventional type-II superconductors. The sharp decrease in the Nernst signal at the depairing magnetic field Hc2 is not obvious even for temperatures close to Tc. Furthermore, a clear deviation of the Nernst signal from normal state background and an anomalous suppression of off-diagonal thermoelectric current in the normal state between Tc and 50 K are observed. We argue that this anomaly in the normal state Nernst effect could correlate with the spin-density wave (SDW) fluctuations although the contribution of superconducting fluctuations cannot be excluded.

Normal-state Nernst effect in electron-dopedPr2−xCexCuO4−δ: Superconducting fluctuations and two-band transport

We report a systematic study of normal state Nernst effect in the electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ over a wide range of doping (0.05$\leq x \leq$0.21) and temperature. At low temperatures, we observed a notable vortex Nernst signal above T$_c$ in the underdoped films, but no such normal state vortex Nernst signal is found in the overdoped region. The superconducting fluctuations in the underdoped region are most likely incoherent phase fluctuations as found in hole-doped cuprates. At high temperatures, a large normal state Nernst signal is found at dopings from slightly underdoped to highly overdoped. Combined with normal state thermoelectric power, Hall effect and magnetoresistance measurements, the large Nernst effect is compatible with two-band model. For the highly overdoped films, the large Nernst effect is anomalous and not explainable with a simple hole-like Fermi surface seen in photoemission experiments.

Bose–Einstein Condensation in the Pseudogap Phase of Cuprate Superconductors

We have identified the unscreened Fröhlich electron–phonon interaction (EPI) as the most essential for pairing in cuprate superconductors as now confirmed by isotope substitution, recent angle-resolved photoemission (ARPES), and some other experiments. Low-energy physics is that of mobile lattice polarons and bipolarons in the strong EPI regime. Many experimental observations have been predicted or explained in the framework of our “Coulomb–Fröhlich” model, which fully takes into account the long-range Coulomb repulsion and the Fröhlich EPI. They include pseudo-gaps, unusual isotope effects and upper critical fields, the normal state Nernst effect, diamagnetism, the Hall–Lorenz numbers, and a giant proximity effect (GPE). These experiments along with the parameter-free estimates of the Fermi energy and the critical temperature support a genuine Bose–Einstein condensation of real-space lattice bipolarons in the pseudogap phase of cuprates. On the contrary, the phase fluctuation (or vortex) scenario is incompatible with the insulating-like in-plane resistivity and the magnetic-field dependence of orbital magnetization in the resistive state of underdoped cuprates.

Bose–Einstein condensation of strongly correlated electrons and phonons in cupratesuperconductors

The long-range Fröhlich electron–phonon interaction has been identified as the mostessential for pairing in high-temperature superconductors owing to poor screening, as isnow confirmed by optical, isotope substitution, recent photoemission and some othermeasurements. I argue that low-energy physics in cuprate superconductors is that ofsuperlight small bipolarons, which are real-space hole pairs dressed by phonons in dopedcharge-transfer Mott insulators. They are itinerant quasiparticles existing in the Blochstates at low temperatures as also confirmed by the continuous-time quantumMonte–Carlo algorithm (CTQMC) fully taking into account realistic Coulomb andlong-range Fröhlich interactions. Here I suggest that a parameter-free evaluation ofTc, unusual upper critical fields, the normal state Nernst effect, diamagnetism,the Hall–Lorenz numbers and giant proximity effects strongly support thethree-dimensional (3D) Bose–Einstein condensation (BEC) of mobile small bipolaronswith zero off-diagonal order parameter above the resistive critical temperatureTc at variance with phase fluctuation scenarios of cuprates.