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Register a new userPhase transition with trivial quantum criticality in anisotropic Weyl semimetal
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When a metal undergoes continuous quantum phase transition, the correlation length diverges at the critical point and the quantum fluctuation of order parameter behaves as a gapless bosonic mode. Generically, the coupling of this boson to fermions induces a variety of unusual quantum critical phenomena, such as non-Fermi liquid behavior and various emergent symmetries. Here, we perform a renormalization group analysis of the semimetal-superconductor quantum criticality in a three-dimensional anisotropic Weyl semimetal. Surprisingly, distinct from previously studied quantum critical systems, the anomalous dimension of anisotropic Weyl fermions flows to zero very quickly with decreasing energy, and the quasiparticle residue takes a nonzero value. These results indicate that, the quantum fluctuation of superconducting order parameter is irrelevant at low energies, and a simple mean-field calculation suffices to capture the essential physics of the superconducting transition. We thus obtain a phase transition that exhibits trivial quantum criticality, which is unique comparing to other invariably nontrivial quantum critical systems. Our theoretical prediction can be experimentally verified by measuring the fermion spectral function and specific heat.
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We present a holographic model of a topological Weyl semimetal. A key ingredient is a time-reversal breaking parameter and a mass deformation. Upon varying the ratio of mass to time-reversal breaking parameter the model undergoes a quantum phase transition from a topologically nontrivial semimetal to a trivial one. The topological nontrivial semimetal is characterised by the presence of an anomalous Hall effect. The results can be interpreted in terms of the holographic renormalization group (RG) flow leading to restoration of time-reversal at the end point of the RG flow in the trivial phase.
Two-dimensional (2D) transition-metal dichalcogenide (TMDs) MoTe2 has attracted much attention due to its predicted Weyl semimetal (WSM) state and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that the superconductivity in MoTe2 single crystal can be much enhanced by the partial substitution of the Te ions by the S ones. The maximum of the superconducting temperature TC of MoTe1.8S0.2 single crystal is about 1.3 K. Compared with the parent MoTe2 single crystal (TC=0.1 K), nearly 13-fold in TC is improved in MoTe1.8S0.2 one. The superconductivity has been investigated by the resistivity and magnetization measurements. MoTe2-xSx single crystals belong to weak coupling superconductors and the improvement of the superconductivity may be related to the enhanced electron-phonon coupling induced by the S-ion substitution. A dome-shape superconducting phase diagram is obtained in the S-doped MoTe2 single crystals. MoTe2-xSx materials may provide a new platform for our understanding of superconductivity phenomena and topological physics in TMDs.
MoTe_2, with the orthorhombic T_d phase, is a new type (type-II) of Weyl semimetal, where the Weyl Fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance (MR), and superconductivity were also observed in T_d-MoTe_2. Understanding the superconductivity in T_d-MoTe_2, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure (p_max = 1.3 GPa) muon spin rotation experiments on the temperature-dependent magnetic penetration depth in T_d-MoTe_2. A substantial increase of the superfluid density n_s/m^* and a linear scaling with T_c is observed under pressure. Moreover, the superconducting order parameter in T_d-MoTe_2 is determined to be two gap (s+s)-wave symmetric. We also excluded time reversal symmetry breaking in the SC state with sensitive zero-field ${mu}$SR experiments. Considering the previous report cite{Balicas1} on the strong suppression of T_c in T_d-MoTe_2 by disorder, we suggest that s^{+-} (topological order parameter) state is more likely to be realized in MoTe_2 than the s^{++} (trivial) state. Should s^{+-} be the SC gap symmetry, the T_d-MoTe_2 is, to our knowledge, the first known example of a time reversal invariant topological (Weyl) superconductor.
We report the pressure (p_max = 1.5 GPa) evolution of the crystal structure of the Weyl semimetal T_d-MoTe_2 by means of neutron diffraction experiments. We find that the fundamental non-centrosymmetric structure T_d is fully suppressed and transforms into a centrosymmertic 1T structure at a critical pressure of p_cr = 1.2 GPa. This is strong evidence for a pressure induced quantum phase transition (QPT) between topological to a trivial electronic state. Although the topological QPT has strong effect on magnetoresistance, it is interesting that the superconducting critical temperature T_c, the superfluid density, and the SC gap all change smoothly and continuously across p_cr and no sudden effects are seen concomitantly with the suppression of the T_d structure. This implies that the T_c, and thus the SC pairing strength, is unaffected by the topological QPT. However, the QPT requires the change in the SC gap symmetry from non-trivial s+- to a trivial s++ state, which we discuss in this work. Our systematic characterizations of the structure and superconducting properties associated with the topological QPT provide deep insight into the pressure induced phase diagram in this topological quantum material.
We study a holographic model which exhibits a quantum phase transition from the strongly interacting Weyl semimetal phase to an insulating phase. In the holographic insulating phase there is a hard gap in the real part of frequency dependent diagonal conductivities. However, the anomalous Hall conductivity is nonzero at zero frequency, indicting that it is a Chern insulator. This holographic quantum phase transition is always of first order, signified by a discontinuous anomalous Hall conductivity at the phase transition, in contrast to the very continuous holographic Weyl semimetal/trivial semimetal phase transition. Our work reveals the novel phase structure of strongly interacting Weyl semimetal.
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