Superconductors with Staggered Non-centrosymmetricity

Sigrist, Manfred; Agterberg, D. F.; Fischer, Mark H.; Goryo, Jun; Loder, Florian; Rhim, S. H.; Maruyama, Daisuke; Yanase, Youichi; Yoshida, Tomohiro; Youn, S. J.

doi:10.7566/jpsj.83.061014

Cited by 68 publications

(64 citation statements)

References 33 publications

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“…sgn (∆ out ) = −sgn (∆ in ). This kind of sign change can be understood by analogy to layered superconductors, where non-centrosymmetricity can induce Rashba SOC with signs that alternate between layers, driving a similar sign change of the order parameter [107][108][109]. While our model does not explicitly contain a Rashba-like term, spin-orbit physics is present in the form of DM and Γ interactions.…”

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confidence: 99%

Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films

Laurell

Fiete

2017

Phys. Rev. Lett.

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We theoretically study the magnetic properties of pyrochlore iridate bilayer and trilayer thin films grown along the [111] direction using a strong coupling approach. We find the ground state magnetic configurations on a mean field level and carry out a spin-wave analysis about them. In the trilayer case the ground state is found to be the all-in/all-out (AIAO) state, whereas the bilayer has a deformed AIAO state. For all parameters of the spin-orbit coupled Hamiltonian we study, the lowest magnon band in the trilayer case has a non-zero Chern number. In the bilayer case we also find a parameter range with non-zero Chern numbers. We calculate the magnon Hall response for both geometries, finding a striking sign change as function of temperature. Using a slave-boson mean-field theory we study the doping of the trilayer system and discover an unconventional timereversal symmetry broken d +id superconducting state. Our study complements prior work in the weak coupling limit and suggests that the [111] grown thin film pyrochlore iridates are a promising candidate for topological properties and unconventional orders. 75.30.Ds,71.27.+a Introduction -One of the main focal points of quantum materials research is topological states of matter, where the essential physics is generally due to spin-orbit coupling (SOC) [1][2][3][4]. Another is correlated electron systems, in which electron-electron interactions dominate, leading to interaction-driven insulators, magnetism, and unconventional superconductivity [5]. Even more possibilities open up when both SOC and electron-electron interactions are present, such as fractionalized topological insulators (TI) [6][7][8][9][10][11][12][13][14][15][16][17], interaction-induced TI [18][19][20][21][22][23][24], and unconventional magnetic states [25][26][27]. A promising place to look for both types of physics is in 5d transition-metal oxides, which tend to have comparable electron-electron interaction and SOC strengths [28][29][30].

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Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films

Laurell

Fiete

2017

Phys. Rev. Lett.

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“…We notice that the FFLO phase has been proposed in noncentrosymmetric SCs under a magnetic field [6,23], and emphasize two essential differences between our case and noncentrosymmetric SCs. (1) In non-centrosymmetric SCs, the FFLO phase is induced by a linear gradient termK i j ∆ * B i q j ∆ (K i j is a parameter) that breaks inversion symmetry.…”

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confidence: 99%

“…The absence of local inversion symmetry can lead to the "hidden" spin polarization [18,19], the spinlayer locking [20,21] and other exotic physical phenomena [22]. The superconductivity for these materials has been studied in the CeCoIn 5 /YbCoIn 5 hybrid system [10,23], SrPtAs [23][24][25][26] and other bilayer Rashba systems [27]. Inhomogeneous Fulde-Ferrell-Larkin-Ovchinikov (FFLO) states were…”

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Unconventional Superconductivity in Bilayer Transition Metal Dichalcogenides

Liu

2017

Phys. Rev. Lett.

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Bilayer transition metal dichalcogenides (TMDs) belong to a class of materials with two unique features, the coupled spin-valley-layer degrees of freedom and the crystal structure that is globally centrosymmetric but locally non-centrosymmetric. In this work, we will show that the combination of these two features can lead to a rich phase diagram for unconventional superconductivity, including intra-layer and inter-layer singlet pairings and inter-layer triplet pairings, in bilayer superconducting TMDs. In particular, we predict that the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov state can exist in bilayer TMDs under an in-plane magnetic field. We also discuss the experimental relevance of our results and possible experimental signatures. 74.25.Dw Introduction.-Unconventional superconductivity [1][2][3], which is beyond the simple s-wave spin-singlet superconductivity in the Bardeen-Cooper-Schrieffer theory, can emerge in two dimensional (2D) systems, such as surfaces [4][5][6] or interfaces [7], superconducting heterostructures [8] and 2D or quasi-2D superconducting materials [9][10][11][12][13][14]. Recently, it was demonstrated that "Ising" superconductivity can exist in monolayer transition metal dichalcogenides (TMDs), such as MoS 2 [11,13] and NbSe 2 [12], based on experimental observation that in-plane upper critical field H c2, is far beyond the paramagnetic limit. The space symmetry group of the monolayer TMD is the D 3h group without inversion symmetry. Thus, the monolayer superconducting TMDs belong to the so-called non-centrosymmetric superconductors (SCs) [3], for which spin-up and spin-down Fermi surfaces are split by strong spin-orbit coupling (SOC), leading to a mixing of spin singlet and triplet pairings [15,16]. The existence of triplet components can enhance H c2, in non-centrosymmetric SCs [17]. In monolayer TMDs, Ising SOC fixes spin axis along the out-of-plane direction and greatly reduces the Zeeman effect of in-plane magnetic fields, thus explaining the experimental observations of high H c2, . A high H c2, was also observed in bilayer TMDs (e.g. NbSe 2 ) [12]. The crystal structure of bilayer TMDs is described by the symmetry group D 3d with inversion symmetry and the corresponding Fermi surfaces are spin degenerate. This experimental result motivates us to study the difference between bilayer superconducting TMDs and conventional SCs.We first illustrate the difference from symmetry aspect. Although inversion symmetry exists in bilayer TMDs, the inversion center should be chosen at the center between two layers, labeled by "P" in Fig. 1a. As a result, bilayer TMDs belong to a class of materials which are globally centrosymmetric, but locally non-centrosymmetric (for each layer). The absence of local inversion symmetry can lead to the "hidden" spin polarization [18,19], the spinlayer locking [20,21] and other exotic physical phenomena [22]. The superconductivity for these materials has been studied in the CeCoIn 5 /YbCoIn 5 hybrid system [10,23], SrPtAs [23][24][25][26] and other ...

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“…Another important aspect to consider is the strong spin-orbit coupling in Sr 2 RuO 4 , as pointed out by Haverkort et al 23 The k-dependent orientation of the expectation value of the spin strongly mixes the spinsinglet and spin-triplet pairings as seen in the superconductors with inversion symmetry breaking. 24,25 However, the Knight shift corresponding to the spin-singlet component should decrease in the SC state. Thus, even in this case, the unchanged Knight shift suggests that the spin-triplet component is dominant in Sr 2 RuO 4 .…”

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Absence of the O17 Knight-shift changes across the first-order phase transition line in Sr2RuO4

et al. 2016

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We performed17 O nuclear magnetic resonance measurements on superconducting (SC) Sr2RuO4 under in-plane magnetic fields. We found that no new signal appears in the SC state and that the 17 O Knight shifts obtained from the double-site measurements remain constant across the first-order phase-transition line, as well as across the second-order phase-transition line as already reported. The present results indicate that the SC spin susceptibility does not decrease in the high-field region, although a magnetization jump in the SC state was reported at low temperatures. Because the spin susceptibility is unchanged in the SC state in Sr2RuO4, we suggest that the first-order phase transition across the upper critical field should be interpreted as a depairing mechanism other than the conventional Pauli-paramagnetic effect.The layered perovskite Sr 2 RuO 4 has attracted special attention, because it has been suggested that Sr 2 RuO 4 may be a chiral p-wave spin-triplet superconductor.1 The chiral state is shown from the broken time-reversal symmetry probed by µSR 2 and Kerr-effect 3 measurements. The existence of spin-triplet equal-spin pairing is based on experimental results that show the spin susceptibility is unchanged on passing through the superconducting (SC) transition temperature T c , as revealed by nuclear magnetic resonance (NMR) Knight-shift measurements at the Ru and O sites 4-8 and polarized neutron scattering measurements.9 The chiral p-wave spin-triplet state would be an SC state analogous to the superfluid 3 He A-phase with two dimensionality.However, several recent experimental results are difficult to interpret with the above SC state. The firstorder (FO) SC-normal (S-N) transition 10 accompanied by a clear magnetization jump 11 is observed in a lowtemperature region near the upper critical field H c2 for fields parallel to the ab plane. This abrupt S-N transition suggests that Sr 2 RuO 4 is a spin-singlet superconductor, because this cannot be interpreted by the conventional orbital depairing effect but seems to be explained consistently by the conventional Pauli-paramagnetic effect. Indeed, the experimental µ 0 H c2 for T → 0 nearly matches the Pauli-limiting field µ 0 H Pauli estimated using the well-1/2 ∼ 1.4 T with χ sc = 0, where E cond is the SC condensation energy and χ n and χ sc are the spin susceptibilities in the normal and SC states, respectively. Here, χ sc = 0 means that the spin susceptibility totally vanishes in the SC state, which contradicts the above spin-susceptibility results showing χ n = χ sc .Recently, we performed 99 Ru Knight-shift ( 99 K) measurements again to re-examine the previous results, and found a new phenomenon that the spin susceptibility slightly increases in the SC state at lower magnetic fields.12 We reported that this experimental result further suggests the spin-triplet equal-spin pairing state. 13Because the hyperfine coupling constant A hf at the 99 Ru site is largest among the nuclei that are feasible for NMR in Sr 2 RuO 4 ( 99 A hf ∼ −25 T/µ B ), 6 the s...

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Superconductors with Staggered Non-centrosymmetricity

Cited by 68 publications

References 33 publications

Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films

Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films

Unconventional Superconductivity in Bilayer Transition Metal Dichalcogenides

Absence of the O17 Knight-shift changes across the first-order phase transition line in Sr2RuO4

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