Giant Nernst Effect Research Articles

Weyl Semimetal States Generated Extraordinary Quasi‐Linear Magnetoresistance and Nernst Thermoelectric Power Factor in Polycrystalline NbP

AbstractUnsaturated and extremely large magnetoresistance (MR), as well as the giant Nernst effect, are intriguing transport phenomena in Weyl semimetals, which are technically appealing for potential applications in magneto‐electric sensors and transverse thermoelectric conversion. The prominent properties are originated from Weyl semimetal states, i.e., the coexistence of electron and hole pockets combined with linear band dispersion. However, previous studies have been focused on small‐sized single crystals, rendering the practical applications of Weyl semimetals. Here, it is reported an unsaturated, quasi‐linear MR as well as a very large Nernst power factor PFxy in the prepared centimeter‐sized and polycrystalline Weyl semimetal NbP. An extraordinary MR of ≈2 × 104% is observed below 60 K with a magnetic field up to 55 T and persists to elevated temperatures. The unusual quasi‐linear MR behavior is explained by the theory of classical linear MR arising from structural disorder. The polycrystalline NbP exhibits state‐of‐the‐art PFxy that reaches a maximum value of 74.81 μW cm–1 K–2 at 9 T and 220 K, which is 1.5 times larger than its longitudinal thermoelectric power factor PFxx. Given that polycrystalline Weyl semimetal, NbP is suitable for large‐scale production, the results pave the way for its practical applications in magneto‐electric sensors and transverse thermoelectric conversion.

Unusual thermoelectric properties of BaFe2As2 in high magnetic fields

Electric and thermoelectric transport properties are mutually intertangled in diffusive transport equations. In particular, in high mobility multiband systems an anomalous behavior may occur, which can be tracked down to the properties of the individual bands. Here, we present magnetoelectric and magnetothermoelectric transport properties of a ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ high-quality single crystal, for different magnetic field directions up to 30 T. We detect a giant Nernst effect and an anomalous field dependence of the Seebeck coefficient. The extraction of the Peltier tensor coefficients ${\ensuremath{\alpha}}_{xx}, {\ensuremath{\alpha}}_{xy}$, and ${\ensuremath{\alpha}}_{xz}$ allows us to disentangle the main transport mechanisms in play. The large ${\ensuremath{\alpha}}_{xy}$ and ${\ensuremath{\alpha}}_{xz}$ values and their field dependence provide evidence of the presence of a high mobility band, compatible with a Dirac dispersion band, crossing the Fermi level, and suggest a possible three-dimensional nature of the Dirac fermions.

Scalable Nernst thermoelectric power using a coiled galfenol wire

The Nernst thermopower usually is considered far too weak in most metals for waste heat recovery. However, its transverse orientation gives it an advantage over the Seebeck effect on non-flat surfaces. Here, we experimentally demonstrate the scalable generation of a Nernst voltage in an air-cooled metal wire coiled around a hot cylinder. In this geometry, a radial temperature gradient generates an azimuthal electric field in the coil. A Galfenol (Fe0.85Ga0.15) wire is wrapped around a cartridge heater, and the voltage drop across the wire is measured as a function of axial magnetic field. As expected, the Nernst voltage scales linearly with the length of the wire. Based on heat conduction and fluid dynamic equations, finite-element method is used to calculate the temperature gradient across the Galfenol wire and determine the Nernst coefficient. A giant Nernst coefficient of -2.6 μV/KT at room temperature is estimated, in agreement with measurements on bulk Galfenol. We expect that the giant Nernst effect in Galfenol arises from its magnetostriction, presumably through enhanced magnon-phonon coupling. Our results demonstrate the feasibility of a transverse thermoelectric generator capable of scalable output power from non-flat heat sources.

Serbynet al.Reply:

Original Article: A. Sergeev, M. Reizer, and V. Mitin, Comment on “Giant Nernst Effect due to Fluctuating Cooper Pairs in Superconductors”, Phys. Rev. Lett. 106, 139701 (2011).

Comment on “Giant Nernst Effect due to Fluctuating Cooper Pairs in Superconductors”

In a recent Letter, Serbyn et al. [A] investigated thermomagnetic effects above the superconducting transition and generalized previous works for arbitrary magnetic fields and temperatures. While the results of [A] have been confirmed in [B], we have strong objections: (i) According to our results [C], the linear response calculation does not require any correction from the magnetization currents; (ii) The result of [A,B] is giant, because unlike the normal Fermi liquid, it is of zero order in the particle-hole asymmetry. Changing the interaction constant in the Cooper channel leads to ridiculously large results even for nonsuperconducting metals; (iii)Derived in [A] the Einstein-type relation for thermomagnetic coefficient contradicts to text-book results. [A] M.N. Serbyn, M.A. Skvortsov, A.A. Varlamov, V. Galitski, Phys. Rev. Lett. 102, 067001 (2009). [B] K. Michaeli and A.M. Finkel'stein, EPL 86, 27007 (2009). [C] A. Sergeev et al., Phys. Rev. B 77, 064501 (2008).

Giant Nernst Effect due to Fluctuating Cooper Pairs in Superconductors

A theory of the fluctuation-induced Nernst effect is developed for a two-dimensional superconductor in a perpendicular magnetic field. First, we derive a simple phenomenological formula for the Nernst coefficient, which naturally explains the giant Nernst signal due to fluctuating Cooper pairs. The latter signal is shown to be large even far from the transition and may exceed by orders of magnitude the Fermi liquid terms. We also present a complete microscopic calculation of the Nernst coefficient for arbitrary magnetic fields and temperatures, which is based on the Matsubara-Kubo formalism. It is shown that the magnitude and the behavior of the Nernst signal observed experimentally in disordered superconducting films can be well understood on the basis of superconducting fluctuation theory.

Possible nature of the pseudogap anomalies in HTSC

An HTSC model, in which the interaction of valence-band electrons with diatomic negative U centers is assumed to be responsible for the anomalous properties of HTSC compounds, is proposed and used to explain the nature of the pseudogap and pseudogap anomalies (including the giant Nernst effect, the anomalous diamagnetism above Tc, the “transfer” of the optical spectral weight). For YBa2Cu3O6 + δ, the pseudogap opening temperature T* and Tc are calculated as functions of the degree of doping δ. The calculated dependences agree quantitatively with the experimental dependences without using scale fitting parameters. The good agreement between the calculated and experimental results can serve as an argument for the proposed HTSC model.

Magnetothermal evidence of a partial gap at the Fermi surface ofUPt2Si2

Motivated by the observation of a giant Nernst effect in ${\text{URu}}_{2}{\text{Si}}_{2}$, the thermoelectric response of the related system ${\text{UPt}}_{2}{\text{Si}}_{2}$ was investigated using thermal and electric transport properties such as the Nernst and Seebeck effects, thermal conductivity, Hall effect, and electrical resistivity. Unlike ${\text{URu}}_{2}{\text{Si}}_{2}$, ${\text{UPt}}_{2}{\text{Si}}_{2}$ is neither superconducting nor exhibits a ``hidden-order'' state. Nevertheless a pronounced Nernst effect anomaly is found to coincide with the onset of the antiferromagnetic order in ${\text{UPt}}_{2}{\text{Si}}_{2}$. Although the absolute values are substantially lower, its appearance and characteristics can favorably be compared to the giant Nernst effect in ${\text{URu}}_{2}{\text{Si}}_{2}$, indicating the common feature of a partial Fermi surface gap.

D-Wave density waves in high Tc cuprates and CeCoIn5

Unconventional density waves (UDW) have a long history starting with the speculation of Halperin and Rice in 1968. However, a more realistic approach started around 1999 in order to clarify the nature of the pseudogap in the underdoped region of hole-doped high Tc cuprates. Also d-wave density waves (dDW) evolved from early unrealistic 2D model with Z2 symmetry to more realistic 3D mean-field condensate with U(1) gauge symmetry. More recently, the giant Nernst effect and the angle dependent magnetoresistance in LSCO, YBCO, Bi2212 and CeCoIn5 are successfully described in terms of dDW, where the Landau quantization of the quasiparticle spectrum in dDW in a magnetic field (the Nersesyan effect) plays the crucial role.

Giant Nernst effect in heavy-electron metals

Recent studies of the Nernst effect in a number of heavy-fermion systems have led to a previously unsuspected result. In some circumstances, the Nernst signal of quasi-particles becomes very large and can easily overwhelm the well-known Nernst effect produced by the movement of the superconducting vortices under the influence of a thermal gradient. In particular, the Nernst coefficient attains an exceptionally large magnitude in the ordered states of URu 2 Si 2 and PrFe 4 P 12 . In all these cases, the order of magnitude of the Nernst signal appears compatible with the Boltzmann picture which links the Nernst coefficient to the energy-dependence of the Hall angle.

Gossamer superconductivity, new paradigm?

AbstractWe shall review our recent works on d‐wave density wave (dDW) and gossamer superconductivity (i.e. d‐wave superconductivity in the presence of dDW) in high‐T c cuprates and CeCoIn5. a) We show that both the giant Nernst effect and the angle dependent magnetoresistance (ADMR) in the pseudogap phases of the cuprates and CeCoIn5 are manifestations of dDW. b) The phase diagram of high‐T c cuprates is understood in terms of mean field theory, which includes two order parameters Δ 1 and Δ 2, where one order paremeter is from dDW and the other from d‐wave superconductivity. c) In the optimally to the overdoped region we find the spatially periodic dDW, an analogue of the Fulde‐Ferrell‐Larkin‐Ovchinnikov (FFLO) state, becomes more stable. d) In the underdoped region where Δ 2/Δ 1 ≪ 1 the Uemera relation is obtained within the present model. We speculate that the gossamer superconductivity is at the heart of high‐T c cuprate superconductors, the heavy‐fermion superconductor CeCoIn5 and the organic superconductors κ ‐(ET)2Cu(NCS)2 and (TMTSF)2PF6. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

D-wave density waves in CeCoIn5and highTccuprates

It is well known that there are many parallels between the heavy fermion superconductor CeCoIn 5 and high T c cuprates: a) the layered structure and quasi 2D Fermi surface, b) proximity to the antiferromagnetic state, c) d-wave superconductivity. Recently many properties of the pseudogap phases in these systems have been well described by d-wave density wave (d-DW). We have interpreted the giant Nernst effect found in the pseudogap phase by Wang et al. in LSCO, YBCO and Bi2212 in terms of d-DW. More recently the Nemst effect and the scaling behavior in the angle dependent magnetoresistance in CeCoIn 5 are reported. We have explained these experimental data in term of d-wave density wave.

Giant Nernst effect in the pseudogap phase of high Tc superconductors

Abstract We have investigated theoretically the Nernst effect in unconventional (d-wave) charge and spin density waves (UDW). In the presence of magnetic field, Landau levels are formed, and the gapless behaviour of the low energy excitations change into gapped behaviour. When additional electric field is applied, the quasiparticles drift with a velocity of E × B/B2, and carry entropy. From this, the Nernst coefficient can be calculated using the Kelvin relation. The present results account very nicely for the measured Nernst signal in the pseudogap phase of high Tc superconductor La2−xSrxCuO4 and Bi2Sr2−yLayCuO6. This indicates that the large Nernst effect is a clear signiture of UDW.

RECENT ADVANCES IN UNCONVENTIONAL DENSITY WAVES

The unconventional density wave (UDW) was speculated on as a possible electronic ground state in the excitonic insulator in 1968. The recent surge of interest in UDW's is partly due to the proposal that the pseudogap phase in high Tc cuprate superconductors is a d-wave density wave (d-DW). Here we review our recent works on UDW's within the framework of mean field theory. In particular, we have shown that many properties of the low temperature phase (LTP) in α-( BEDT-TTF )2 MHg ( SCN )4, with M = K , Rb and Tl , are well characterized in terms of the unconventional charge density wave (UCDW). In this identification the Landau quantization of the quasiparticle motion in a magnetic field (the Nersesyan effect) plays the crucial role. Indeed, the angle-dependent magnetoresistance and the negative giant Nernst effect are two hallmarks of UDW's.

On the angular dependences of the superconducting and normal state properties of the Bechgaard Salts: Triplet Superconductivity, Enhanced H 2 near the S-I boundary, Giant Nernst Effect at Lebed Magic Angles

Our recent experiments on (TMTSF) 2 PF 6 indicate that: 1) the Knight shift along both the a and b axes remains unchanged on going through the superconducting transition (suggesting triplet superconductivity with d vector along c), 2) Hc2 is enhanced near the critical pressure for the superconducting-insulating transition by the formation of slabs parallel to the magnetic field and the anisotropy in Hc2 is determined by penetration depth rather than coherence length anisotropies, and 3) in the normal state, for fields near the Lebed magic angles the current is locked in of the direction of the magic angles and there is an anomalous Nernst effect which near the magic angles is -200 times larger than what would be expected from Boltzmann transport.