Abstract
The search for unconventional superconductivity has been focused on materials with strong spin-orbit coupling and unique crystal lattices. Doped bismuth selenide (Bi$_2$Se$_3$) is a strong candidate given the topological insulator nature of the parent compound and its triangular lattice. The coupling between the physical properties in the superconducting state and its underlying crystal symmetry is a crucial test for unconventional superconductivity. In this paper, we report direct evidence that the superconducting magnetic response couples strongly to the underlying 3-fold crystal symmetry in the recently discovered superconductor with trigonal crystal structure, niobium (Nb)-doped bismuth selenide (Bi$_2$Se$_3$). More importantly, we observed that the magnetic response is greatly enhanced along one preferred direction spontaneously breaking the rotational symmetry. Instead of a simple 3-fold crystalline symmetry, the superconducting hysteresis loop shows dominating 2-fold and 4-fold symmetry. This observation confirms the breaking of the rotational symmetry and indicates the presence of nematic order in the superconducting ground state of Nb-doped Bi$_2$Se$_3$. Further, heat capacity measurements display an exponential decay in superconducting state and suggest that there is no line node in the superconducting gap. These observations provide strong evidence of odd-parity topological superconductivity.
Highlights
Unconventional superconductors are characterized by superconducting order parameters that are noninvariant under crystal symmetry operations
No direct thermodynamic signature of rotational symmetry breaking due to superconductivity has been observed in any crystal
Strong spin-orbit coupling in the triangular lattice produces a topologically insulating ground state [1,2]
Summary
Unconventional superconductors are characterized by superconducting order parameters that are noninvariant under crystal symmetry operations. Here the first direct observation of rotational symmetry breaking in the superconducting state of Nb-doped Bi2Se3 [12], a new member of the superconducting doped topological insulators. There is not clear evidence showing the anisotropy in the superconducting properties deep in the superconducting state at H much smaller than Hc2 To solve this problem, we chose a unique method, torque magnetometry, to resolve the anisotropy of the superconducting critical current in these materials. Rotational symmetry-breaking measurements require noncontact methods such as heat capacity, magnetization, or NMR since contact itself may generate a preferred axis. Torque magnetometry has been used to detect nematicity only in the normal state in superconductors URu2Si2 or BaFe2ðAs1−xPxÞ2 [17,18]. Rotational symmetry breaking in the superconducting state suggests spin-triplet pairing and topological superconductivity. Our approach and (d) results greatly impact future research on topological superconductivity
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