Nernst-effect anisotropy as a sensitive probe of Fermi-surface distortions from electron-nematic order

Abstract

We analyze the thermoelectric response in layered metals with spontaneously broken rotation symmetry. We identify the anisotropy of the quasiparticle Nernst signal as an extremely sensitive probe of Fermi surface distortions characteristic of the ordered state. This is due to a subtle interplay of different transport anisotropies which become additionally enhanced near van-Hove singularities. Applied to recent experiments, our results reinforce the proposal that the underdoped cuprate superconductor YBa2Cu3O 6+δ displays such "electron-nematic" order in the pseudogap regime.Spontaneous breaking of lattice rotation symmetry due to electronic correlations is currently in the focus of intense interest, most prominently in cuprate hightemperature superconductors such as YBa 2 Cu 3 O 6+δ (Refs. 1-3) and in the metamagnetic metal Sr 3 Ru 2 O 7 (Ref. 4). In analogy to liquid crystals, a phase with broken rotation (but preserved translation) symmetry has been dubbed electron nematic. 5In cuprates, electron-nematic order has been discussed early on as intermediate phase which occurs upon melting of a uni-directional charge-density-wave ("stripe") phase. 5-7 Microscopically, it is one of the known instabilities of the two-dimensional (2d) Hubbard model. [8][9][10] The first clear-cut signature of electron-nematic order in cuprates was found in neutron scattering experiments on YBa 2 Cu 3 O 6.45 , 1 where the spin-fluctuation spectrum was found to develop a distinct anisotropic incommensurability below a temperature T of about 150 K. Earlier transport measurements on YBa 2 Cu 3 O 6+δ (Ref. 11) also detected resistivity anisotropies ρ a /ρ b of order 2 in the underdoped regime, but remained less conclusive. The reason is that the crystal structure of YBa 2 Cu 3 O 6+δ contains CuO chains which break the otherwise tetragonal symmetry of the CuO 2 planes. In an orderparameter language, this implies a small field which couples linearly to the nematic order parameter. This has two main effects: (i) potentially existing nematic order will be aligned and (ii) a nematic ordering transition will be smeared out. While (i) enables observables to show a macroscopic anisotropy, which might otherwise be masked by domain formation, (ii) implies that electronic nematic order and purely structural effects cannot be sharply distinguished. Remarkably, locally broken rotation symmetry has been found on the surface of Bi 2 Sr 2 CaCu 2 O 8+δ and Ca 2−x Na x CuO 2 Cl 2 using scanning tunneling microscopy. 12 In Sr 3 Ru 2 O 7 , nematic order is a candidate explanation for the low-T phase which masks the metamagnetic critical endpoint at around 8 T. 4,13 Resistivity anisotropies have been detected here, but a full picture has not yet emerged, because the rather unusual thermodynamics near the low-T phase boundaries is not understood.Very recent measurements 2,3 of the Nernst effect, 14 that is the transverse voltage induced by a thermal gradient in the presence of a magnetic field, have uncovered a surprisingly large anisotropy in YBa 2 Cu 3 O 6+δ ...

Pseudogap temperature T* of cuprate superconductors from the Nernst effect

“…This is indeed the case. (A large enhancement of ν, from small and negative to large and positive, is also found in calculations of Fermi-surface reconstruction by commensurate [47] and incommensurate [48] antiferrromagnetic order.) In Fig.…”

Spin nematic fluctuations near a spin-density-wave phase

“…Although cuprate superconductors exhibit nematicity in the magnetic excitation spectra [44] and thus might be possible systems for spin nematic order, the line of onset temperatures of nematicity is nearly parallel to that of the incommensurate magnetic order as a function of doping in YBa Cu O y 2 3 6+ [45], which is at variance with figure 8. Instead, the nematicity in cuprates was discussed in terms of a feedback effect from charge nematicity [46,47].…”

Strong particle-hole asymmetry of charge instabilities in doped Mott insulators

“…There is growing evidence that the PG is related to some charge order or its fluctuations in h-cuprates [8-11, 73, 74]. In particular, the dCDW [15,[17][18][19] and dPI [22][23][24] are candidates. If the charge order is indeed responsible for the PG, the current theory suggests that the property of the PG should be different between hole doping and electron doping because of the strong particle-hole asymmetry of charge-order instabilities.…”

“…The so-called YRZ model [12] exploits the concept of the resonating-valence-bond theory [13,14] and successfully captures some features of the PG. On the other hand, various experimental observations in the PG state are also well captured in terms of charge instabilities such as d-wave charge density wave (dCDW) [15][16][17][18][19], a loop current order [20,21], d-wave Pomeranchuk instability (dPI) [22][23][24], conventional charge density wave (CDW) [25][26][27][28] including stripes [29,30], and phase separation (PS) [25,26,31].Quite recently a charge-order instability was observed by X-rays in two different h-cuprates, Y-based [32][33][34] and Bi-based [35,36] cuprates. This charge order is not accompanied by a magnetic order, in sharp contrast with the spin-charge stripes [37] discussed extensively in La-based cuprates [29].…”