Nematic Skyrmions in Odd-Parity Superconductors

Anisotropic superconductivity in the quasi-one-dimensional superconductor V2Ga5

The intermetallic quasi-one-dimensional binary superconductor V2Ga5 was recently found to exhibit a topologically nontrivial normal state, making it a natural candidate for a topological superconductor. By combining dc-magnetization, nuclear magnetic resonance, and muon-spin rotation ([Formula: see text]SR) measurements on high-quality V2Ga5 single crystals, we investigate the electronic properties of its normal- and superconducting ground states. NMR measurements in the normal state indicate a strong anisotropy in both the line shifts and the relaxation rates. Such anisotropy persists also in the superconducting state, as shown by the magnetization- and [Formula: see text]SR-spectroscopy results. In the latter case, data collected at different temperatures, pressures, and directions of the magnetic field evidence a fully-gapped, strongly anisotropic superconductivity. At the same time, hydrostatic pressure is shown to only lower the [Formula: see text] value, but not to change the superfluid density nor its temperature dependence. Lastly, we discuss the search for topological signatures in the normal state of V2Ga5, as well as a peak splitting in the FFT of the [Formula: see text]SR spectrum, possibly related to an unconventional vortex lattice. Our results suggest that V2Ga5 is a novel system, whose anisotropy plays a key role in determining its unusual electronic properties.

Magnetisation control of the nematicity direction and nodal points in a superconducting doped topological insulator

We study the effects of magnetisation on the properties of the doped topological insulator of theBi2Se3family with nematic superconductivity. We found that the direction of the in-plane magnetisation fixes the direction of the nematicity in the system. The chiral state is more favourable than the nematic state for large values of out-of-plane magnetisation. Overall, the critical temperature of the nematic superconductivity is robust against magnetisation. We explore in detail the spectrum of the system with the pinned direction of the nematic order parameterΔy. Without magnetisation, there is a full gap in the spectrum because of finite hexagonal warping. At an out-of-planemzor orthogonal in-planemxmagnetisation that is strong enough, the spectrum is closed at the nodal points that are split by the magnetisation. Flat Majorana surface states connect such split bulk nodal points. Parallel magnetisationmylifts the nodal points and opens a full gap in the spectrum. We discuss relevant experiments and propose experimental verifications of our theory.

Chiral Higgs Mode in Nematic Superconductors

Nematic superconductivity with spontaneously broken rotation symmetry has recently been reported in doped topological insulators, M_{x}Bi_{2}Se_{3} (M=Cu, Sr, Nb). Here we show that the electromagnetic (EM) response of these compounds provides a spectroscopy for bosonic excitations that reflect the pairing channel and the broken symmetries of the ground state. Using quasiclassical Keldysh theory, we find two characteristic bosonic modes in nematic superconductors: the nematicity mode and the chiral Higgs mode. The former corresponds to the vibrations of the nematic order parameter associated with broken crystal symmetry, while the latter represents the excitation of chiral Cooper pairs. The chiral Higgs mode softens at a critical doping, signaling a dynamical instability of the nematic state towards a new chiral ground state with broken time reversal and mirror symmetry. Evolution of the bosonic spectrum is directly captured by EM power absorption spectra. We also discuss contributions to the bosonic spectrum from subdominant pairing channels to the EM response.

Nematic Superconductivity in Doped Bi2Se3 Topological Superconductors

Nematic superconductivity is a novel class of superconductivity characterized by spontaneous rotational-symmetry breaking in the superconducting gap amplitude and/or Cooper-pair spins with respect to the underlying lattice symmetry. Doped Bi 2 Se 3 superconductors, such as Cu x Bi 2 Se 3 , Sr x Bi 2 Se 3 , and Nb x Bi 2 Se 3 , are considered as candidates for nematic superconductors, in addition to the anticipated topological superconductivity. Recently, various bulk probes, such as nuclear magnetic resonance, specific heat, magnetotransport, magnetic torque, and magnetization, have consistently revealed two-fold symmetric behavior in their in-plane magnetic-field-direction dependence, although the underlying crystal lattice possesses three-fold rotational symmetry. More recently, nematic superconductivity was directly visualized using scanning tunneling microscopy and spectroscopy. In this short review, we summarize the current research on the nematic behavior in superconducting doped Bi 2 Se 3 systems and discuss issues and perspectives.