Selection rules for Cooper pairing in two-dimensional interfaces and sheets

Abstract

Thin sheets deposited on a substrate and interfaces of correlated materials offer a plethora of routes towards the realization of exotic phases of matter. In these systems, inversion symmetry is broken which strongly affects the properties of possible instabilities -in particular in the superconducting channel. By combining symmetry and energetic arguments, we derive general and experimentally accessible selection rules for Cooper instabilities in noncentrosymmetric systems, which yield necessary and sufficient conditions for spontaneous time-reversal-symmetry breaking at the superconducting transition and constrain the orientation of the triplet vector. We discuss in detail the implications for various different materials. For instance, we conclude that the pairing state in thin layers of Sr 2 RuO 4 must, as opposed to its bulk superconducting state, preserve time-reversal symmetry with its triplet vector being parallel to the plane of the system. All triplet states of this system allowed by the selection rules are predicted to display topological Majorana modes at dislocations or at the edge of the system. Applying our results to the LaAlO 3 /SrTiO 3 heterostructures, we find that while the condensates of the (001) and (110) oriented interfaces must be time-reversal symmetric, spontaneous time-reversal-symmetry breaking can only occur for the less studied (111) interface. We also discuss the consequences for thin layers of URu 2 Si 2 and UPt 3 as well as for single-layer FeSe. On a more general level, our considerations might serve as a design principle in the search for time-reversal-symmetry-breaking superconductivity in the absence of external magnetic fields.npj Quantum Materials (2017) 2:9 ;

Two-dimensional superconductivity at the (111) LaAlO3/SrTiO3 interface

“…The 2DES at the (111)LAO/STO interface [11] is an interesting subject of investigation, combining a polar discontinuity at the interface with such a hexagonal lattice. Signatures of the sixfold symmetry related to the (111)STO orientation have recently been observed by angle-resolved photoemission spectroscopy (ARPES) [12,13] and magnetoresistance [14,15] measurements, making the system potentially suitable for exotic time-reversal symmetry breaking superconductivity [16]. Moreover, ARPES measurements at the surface of (111)STO have confirmed a distinct orbital ordering of the t 2g manifold [12], where all the bands are degenerate at the point.…”

Signatures of electronic nematicity in (111) LaAlO3/SrTiO3 interfaces

“…This configuration has been likened to a strongly correlated analogue of graphene, and has been predicted to exhibit topological properties, unconventional superconductivity, as well as nematic phases. [14][15][16] Electric transport measurements have shown that the (111) LAO/STO interface exhibits many of the properties already seen in (001) LAO/STO, including a coexistence of superconductivity and magnetism. [17][18][19][20] However, the feature that distinguishes the (111) interface from the (001) interface is the strong anisotropy with respect to surface crystal direc-arXiv:1708.04809v2 [cond-mat.str-el] 28 Aug 2017 tion seen in almost all properties, including conductivity, Hall effect, superconductivity, quantum capacitance and longitudinal magnetoresistance.…”

“…This configuration has been likened to a strongly correlated analogue of graphene, and has been predicted to exhibit topological properties, unconventional superconductivity, as well as nematic phases. [14][15][16] Electric transport measurements have shown that the (111) LAO/STO interface exhibits many of the properties already seen in (001) LAO/STO, including a coexistence of superconductivity R⊡ (kΩ) 0 5 10 15 20 Vg (V) 40 50 60 70 80 90 100 [112] [110] Ti 1 Ti 2 Ti3 O [110] [112] [ 1 1 1 ] [110] (a) (b) I VL VH VL VH [112] [110] I T=4.4K [112] FIG. 1. a) Schematic representation of the first three monolayers at the LAO/STO interface with the [112] and [110] labeled.…”

Pairing in two dimensions and possible consequences for superconductivity: extension of Cooper's approach

“…It is thought that carriers flow in the two-dimensional (Fe 2+ As 3-) layers, while impurity doping to the (La 3+ O 2-) layer transfers the carriers generated in the (La 3+ O 2-) layer to the (Fe 2+ As 3-) layers etc [3]. Till now the major theoretical effort has been focused on the search for mechanism responsible for the pair formation in two-dimensions followed by inter-layer pair tunneling [2][3][4][5][6][7][8][9][10][11][12][13][14]. Sachdev et al [2] used large-N expansion to study finite temperature pairing fluctuations in 2D.…”

“…Sommer et al [12] described the evolution of fermion pairing in the dimensional crossover from threedimensional to two-dimensional as a strongly interacting Fermi gas of 6 Li atoms becomes confined to a stack of two-dimensional layers formed by a one-dimensional optical lattice. Recently, Scheurer et al [13] describes selection rules for Cooper pairing in two-dimensional interfaces and sheets of Sr 2 RuO4, LaAlO 3 /SrTiO 3 heterostructures, URu 2 Si 2 and UPt 3 as well as for single-layer FeSe. In these systems, time-reversal-symmetry is broken which strongly affects the properties of possible instabilities in particular in the superconducting channel.…”