arXiv subjects

Xiong-Jun Liu

Publications and source records attributed to Xiong-Jun Liu.

At least 91 records · Page 5Linked to original sources

arXiv2017

Dirac, Rashba and Weyl type spin-orbit couplings: toward experimental realization in ultracold atoms

We propose a hierarchy set of minimal optical Raman lattice schemes toward the experimental realization of spin-orbit (SO) couplings of various types for ultracold atoms, including two-dimensional (2D) Dirac type of $C_4$ symmetry, 2D Rashba and 3D Weyl types. These schemes are well accessible with current cold-atom technology, and in particular, a long-lived Bose-Einstein condensation of the 2D Dirac SO coupling has been experimentally proved. The generation of 2D Rashba and 3D Weyl types has an exquisite request that two sources of laser beams have distinct frequencies of factor-two difference. Surprisingly, we find that $^{133}$Cs atoms provide an ideal candidate for the realization. A common and essential feature is the absence of any fine-tuning and phase-locking in the realization, and the resulted SO coupled ultracold atoms have a long lifetime. These schemes essentially improve over the current experimental accessibility and controllability, and open new experimental platforms to study high-dimensional SO physics and novel topological phases with ultracold atoms.

cond-mat.mes-hall↗

arXiv2017

A generic theory for Majorana zero modes in 2D superconductors

It is well known that non-Abelian Majorana zero modes (MZM) harbor at vortex cores in a $p_{x}+\text{i}p_{y}$ topological superconductor, which can be realized in a 2D spin-orbit coupled system with a single Fermi surface and by proximity coupling to an $s$-wave superconductor. Here we show that existence of non-Abelian MZMs is unrelated to the bulk topology of a 2D superconductor, and propose that such exotic modes can be resulted in much broader range of superconductors, being topological or trivial. For a generic 2D system with multiple Fermi surfaces and gapped out by superconducting pairings, we show that at least a single MZM survives if there are only odd number of Fermi surfaces of which the corresponding superconducting orders have vortices, and such MZM is protected by an emergent Chern-Simons invariant, irrespective of the bulk topology of the superconductor. This result may enrich new experimental schemes for realizing non-Aelian MZMs. In particular, we propose a minimal scheme to realize the MZMs in a 2D superconducting Dirac semimetal with trivial bulk topology, which can be well achieved based on the recent cold atom experiments.

cond-mat.supr-con↗

arXiv2017

From semimetal to chiral Fulde-Ferrell superfluids

The recent realization of two-dimensional (2D) synthetic spin-orbit (SO) coupling opens a broad avenue to study novel topological states for ultracold atoms. Here, we propose a new scheme to realize exotic chiral Fulde-Ferrell superfluid for ultracold fermions, with a generic theory being shown that the topology of superfluid pairing phases can be determined from the normal states. The main findings are two fold. First, a semimetal is driven by a new type of 2D SO coupling whose realization is even simpler than the recent experiment, and can be tuned into massive Dirac fermion phases with or without inversion symmetry. Without inversion symmetry the superfluid phase with nonzero pairing momentum is favored under an attractive interaction. Furthermore, we show a fundamental theorem that the topology of a 2D chiral superfluid can be uniquely determined from the unpaired normal states, with which the topological chiral Fulde-Ferrell superfluid with a broad topological region is predicted for the present system. This generic theorem is also useful for condensed matter physics and material science in search for new topological superconductors.

cond-mat.supr-con↗

arXiv2016

Spin-orbit coupled two-electron Fermi gases of ytterbium atoms

We demonstrate all-optical implementation of spin-orbit coupling (SOC) in a two-electron Fermi gas of $^{173}$Yb atoms by coupling two hyperfine ground states with a narrow optical transition. Due to the SU($N$) symmetry of the $^1$S$_0$ ground-state manifold which is insensitive to external magnetic fields, an optical AC Stark effect is applied to split the ground spin states, which exhibits a high stability compared with experiments on alkali and lanthanide atoms, and separate out an effective spin-1/2 subspace from other hyperfine levels for the realization of SOC. The dephasing spin dynamics when a momentum-dependent spin-orbit gap being suddenly opened and the asymmetric momentum distribution of the spin-orbit coupled Fermi gas are observed as a hallmark of SOC. The realization of all-optical SOC for ytterbium fermions should offer a new route to a long-lived spin-orbit coupled Fermi gas and greatly expand our capability in studying novel spin-orbit physics with alkaline-earth-like atoms.

cond-mat.quant-gas↗

arXiv2016

Non-Abelian Majorana modes protected by an emergent second Chern number

The search for topological superconductors and non-Abelian Majorana modes ranks among the most fascinating topics in condensed matter physics. There now exist several fundamental superconducting phases which host symmetry protected or chiral Majorana modes. The latter, namely the chiral Majorana modes are protected by Chern numbers in even dimensions. Here we propose to observe novel chiral Majorana modes by realizing Fulde-Ferrell-Larkin-Ovchinnikov state, i.e. the pairing density wave (PDW) phase in a Weyl semimetal which breaks time-reversal symmetry. Without symmetry protection, the 3D gapped PDW phase is topologically trivial. However, a vortex line generated in such phase can host chiral Majorana modes, which are shown to be protected by an emergent second Chern number of a synthetic 4D space generalized from the PDW phase. We further show that these chiral modes in the vortex rings obey 3D non-Abelian loop-braiding statistics, which can be applied to topological quantum computation.

cond-mat.supr-con↗

arXiv2016

Symmetry-protected non-Abelian braiding of Majorana Kramers' pairs

We develop the complete theory for non-Abelian braiding of Majorana Kramers' pairs (MKPs) in time-reversal (TR) invariant topological superconductors. By introducing an effective Hamiltonian approach to describe the braiding of MKPs, we show that the non-Abelian braiding is protected when the effective Hamiltonian exhibits a new TR like anti-unitary symmetry, which is satisfied if the system is free of dynamical noise. Importantly, even the dynamical noise may not cause error in braiding, unless the noise correlation function breaks a dynamical TR symmetry, which generalizes the TR symmetry protection of MKPs to dynamical regime. Moreover, the resulted error by noise is shown to be a higher order effect, compared with the decoherence of Majorana qubits without TR symmetry protection. These results show that the non-Abelian braiding of MKPs is observable and may have versatile applications to future quantum computation technologies.

cond-mat.supr-con↗

arXiv2016

Majorana spintronics

We propose a systematic magnetic-flux-free approach to detect, manipulate and braid Majorana fermions in a semiconductor nanowire-based topological Josephson junction by utilizing the Majorana spin degree of freedom. We find an intrinsic $π$-phase difference between spin-triplet pairings enforced by the Majorana zeros modes (MZMs) at the two ends of a one-dimensional spinful topological superconductor. This $π$-phase is identified to be a spin-dependent superconducting phase, referred to as the spin-phase, which we show to be tunable by controlling spin-orbit coupling strength via electric gates. This electric controllable spin-phase not only affects the coupling energy between MZMs but also leads to a fractional Josephson effect in the absence of any applied magnetic flux, which enables the efficient topological qubit readout. We thus propose an all-electrically controlled superconductor-semiconductor hybrid circuit to manipulate MZMs and to detect their non-Abelian braiding statistics properties. Our work on spin properties of topological Josephson effects potentially opens up a new thrust for spintronic applications with Majorana-based semiconductor quantum circuits.

cond-mat.supr-con↗

arXiv2016

Reply to Comment on "Tip induced unconventional superconductivity on Weyl semimetal TaAs"

Recently, we presented a paper about the tip-induced superconducting phase on TaAs single crystal (arXiv:1607.00513). A conductance plateau with double conductance peaks at finite bias, sharp double dips and a zero bias conductance peak were observed in the point contact spectra (PCS). These three features in one PCS suggest the possibility of p-wave like superconductivity and Majorana zero modes in the surface. Soon after our paper posted on arXiv, Gayen el al. (arXiv:1607.01405) questioned the underlying mechanism of our observations based on their experimental measurements in conventional superconductors(Pb and Nb) and conclusively indicated that the superconductivity observed in our TaAs crystal is conventional. We disagree with their quick conclusion without any specific study on the TaAs materials. We will explain in the following that for the clean (Z=0) point contact (PC) in ballistic limit, the superconducting transition feature in temperature dependence of PC resistance is expected. And the critical current effect claimed by Gayen el al. cannot explain the observed PCS in our paper.

cond-mat.supr-con↗

arXiv2016

Tip induced unconventional superconductivity on Weyl semimetal TaAs

Weyl fermion is a massless Dirac fermion with definite chirality, which has been long pursued since 1929. Though it has not been observed as a fundamental particle in nature, Weyl fermion can be realized as low-energy excitation around Weyl point in Weyl semimetal, which possesses Weyl fermion cones in the bulk and nontrivial Fermi arc states on the surface. As a firstly discovered Weyl semimetal, TaAs crystal possesses 12 pairs of Weyl points in the momentum space, which are topologically protected against small perturbations. Here, we report for the first time the tip induced superconductivity on TaAs crystal by point contact spectroscopy. A conductance plateau and sharp double dips are observed in the point contact spectra, indicating p-wave like unconventional superconductivity. Furthermore, the zero bias conductance peak in low temperature regime is detected, suggesting potentially the existence of Majorana zero modes. The experimentally observed tunneling spectra can be interpreted with a novel mirror-symmetry protected topological superconductor induced in TaAs, which can exhibit zero bias and double finite bias peaks, and double conductance dips in the measurements. Our work can open a broad avenue in search for new topological superconducting phases from topological Weyl materials and trigger intensive investigations for pursuing Majorana fermions.

cond-mat.supr-con↗

arXiv2016

Superconductivity in topologically nontrivial material Au2Pb

The search for nontrivial superconductivity in novel quantum materials is currently a most attractive topic in condensed matter physics and material science. The experimental studies have progressed quickly over the past couple of years. In this article, we report systematic studies of superconductivity in Au2Pb single crystals. The bulk superconductivity (onset transition temperature, Tconset= 1.3 K) of Au2Pb is characterized by both transport and diamagnetic measurements, where the upper critical field Hc2 shows unusual quasi-linear temperature dependence. The superconducting gap is revealed by point contact measurement with gold tip. However, when using tungsten (W) tip, which is much harder, the superconducting gap probed is largely enhanced as demonstrated by the increases of both Tconset and upper critical field (Hc2). This can be interpreted as a result of increase in density of states under external anisotropic stress imposed by the tip, as revealed by first-principles calculations. Furthermore, novel phase winding of the pseudospin texture along k-space loops around the Fermi energy is uncovered from the calculations, indicating that the observed superconductivity in Au2Pb may have nontrivial topology.

cond-mat.supr-con↗

arXiv2016

A Scaling Behavior of Bloch Oscillation in Weyl Semimetals

We predict a linear logarithmical scaling law of Bloch oscillation dynamics in Weyl semimetals (WSMs), which can be applied to detect Weyl nodal points. Applying the semiclassical dynamics for quasiparticles which are accelerated bypassing a Weyl point, we show that transverse drift exhibits asymptotically a linear log-log relation with respect to the minimal momentum measured from the Weyl point. This linear scaling behavior is a consequence of the monopole structure nearby the Weyl points, thus providing a direct measurement of the topological nodal points, with the chirality and anisotropy being precisely determined. We apply the present results to two lattice models for WSMs which can be realized with cold atoms in experiment, and propose realistic schemes for the experimental detection. With the analytic and numerical results we show the feasibility of identifying topological Weyl nodal points based on the present prediction.

cond-mat.mes-hall↗

arXiv2015

Symmetry Protected and Topologically Protected Crossed Andreev Reflection

Crossed Andreev reflection (CAR) is considered to probe the non-locality of the Majorana quasiparticles in topological superconductors. We propose here to study the CAR in multiband one-dimensional (1D) topological superconductors which may or may not have chiral symmetries, and show how the CAR can be applied to identify Majorana modes in such systems. In particular, for a multiband 1D topological superconductor with approximate chiral symmetry, both the multiple Majorana and low-energy Andreev bound modes can drive the nearly zero-bias CAR, which is shown to be protected by the chiral symmetry and is stable against non-magnetic disorder scatterings. The emergence of symmetry protected CAR makes it hard to identify exotic Majorana fermions out of low-energy Andreev bound states. It is interesting that the magnetic disorders, which break chiral symmetry, can fully kill the non-locality of low-energy Andreev bound states, while cannot affect the Majorana modes, leading to the topologically protected CAR which is solely contributed by the Marjorana modes. The experimentally relevant systems, including the semiconductor nanowires and the Fe chains in the top of an $s$-wave superconductor, have been considered.

cond-mat.str-el↗

arXiv2015

Realization of Two-Dimensional Spin-orbit Coupling for Bose-Einstein Condensates

Cold atoms with laser-induced spin-orbit (SO) interactions provide intriguing new platforms to explore novel quantum physics beyond natural conditions of solids. Recent experiments demonstrated the one-dimensional (1D) SO coupling for boson and fermion gases. However, realization of 2D SO interaction, a much more important task, remains very challenging. Here we propose and experimentally realize, for the first time, 2D SO coupling and topological band with $^{87}$Rb degenerate gas through a minimal optical Raman lattice scheme, without relying on phase locking or fine tuning of optical potentials. A controllable crossover between 2D and 1D SO couplings is studied, and the SO effects and nontrivial band topology are observed by measuring the atomic cloud distribution and spin texture in the momentum space. Our realization of 2D SO coupling with advantages of small heating and topological stability opens a broad avenue in cold atoms to study exotic quantum phases, including the highly-sought-after topological superfluid phases.

cond-mat.quant-gas↗

arXiv2015

Chern Kondo Insulator in an Optical Lattice

We propose to realize and observe Chern Kondo insulators in an optical superlattice with laser-assisted $s$ and $p$ orbital hybridization and synthetic gauge field, which can be engineered based on the recent cold atom experiments. Considering a double-well square optical lattice, the localized $s$ orbitals are decoupled from itinerant $p$ bands and are driven into a Mott insulator due to strong Hubbard interaction. Raman laser beams are then applied to induce tunnelings between $s$ and $p$ orbitals, and generate a staggered flux simultaneously. Due to the strong Hubbard interaction of $s$ orbital states, we predict the existence of a critical Raman laser-assisted coupling, beyond which the Kondo screening is achieved and then a fully gapped Chern Kondo phase emerges, with the topology characterized by integer Chern numbers. Being a strongly correlated topological state, the Chern Kondo phase is different from the single-particle quantum anomalous Hall state, and can be identified by measuring the band topology and double occupancy of $s$ orbitals. The experimental realization and detection of the predicted Chern Kondo insulator are also proposed.

cond-mat.str-el↗

arXiv2015

Symmetry Reduction and Boundary Modes for Fe-Chains on an s-wave Superconductor

We investigate the superconducting phase diagram and boundary modes for a quasi-1D system formed by three Fe-Chains on an s-wave superconductor, motivated by the recent Princeton experiment. The $\vec l\cdot\vec s$ onsite spin-orbit term, inter-chain diagonal hopping couplings, and magnetic disorders in the Fe-chains are shown to be crucial for the superconducting phases, which can be topologically trivial or nontrivial in different parameter regimes. For the topological regime a single Majorana and multiple Andreew bound modes are obtained in the ends of the chain, while for the trivial phase only low-energy Andreev bound states survive. Nontrivial symmetry reduction mechanism induced by the $\vec l\cdot\vec s$ term, diagonal hopping couplings, and magnetic disorder is uncovered to interpret the present results. Our study also implies that the zero-bias peak observed in the recent experiment may or may not reflect the Majorana zero modes in the end of the Fe-chains.

cond-mat.str-el↗

arXiv2015

Quantum Dynamical Phase Transition in a Spin-Orbit Coupled Bose Condensate

Spin-orbit coupled bosons can exhibit rich equilibrium phases at low temperature and in the presence of particle-particle interactions. In the case with a 1D synthetic spin-orbit interaction, it has been observed that the ground state of a Bose gas can be a normal phase, stripe phase, or magnetized phase in different experimentally controllable parameter regimes. The magnetized states are doubly degenerate and consist of a many-particle two-state system. In this work, we investigate the nonequilibrium quantum dynamics by switching on an external perturbation to induce resonant couplings between the magnetized phases, and predict the novel quantum spin dynamics which cannot be obtained in the single-particle systems. In particular, due to particle-particle interactions, the transition of the Bose condensate from one magnetized phase to the other is forbidden when the strength of external perturbation is less than a critical value, and a full transition can occur only when the perturbation exceeds such critical strength. This phenomenon manifests itself a quantum dynamical phase transition, with the critical point behavior being exactly solvable. From the numerical simulations and exact analytic studies we show that the predicted many-body effects can be well observed with the current experiments.

cond-mat.quant-gas↗

arXiv2015

Demonstrating Lattice-Symmetry-Protection in Topological Crystalline Superconductors

We propose to study the lattice-symmetry protection of Majorana zero bound modes in topological crystalline superconductors (SCs). With an induced $s$-wave superconductivity in the $(001)$-surface of the topological crystalline insulator Pb$_{1-x}$Sn$_x$Te, which has a C$_4$ rotational symmetry, we show a new class of 2D topological SC with four Majorana modes obtained in each vortex core, while only two of them are protected by the cyclic symmetry. Furthermore, applying an in-plane external field can break the four-fold symmetry and lifts the Majorana modes to finite energy states in general. Surprisingly, we show that even the C$_4$ symmetry is broken, two Majorana modes are restored exactly one time whenever the in-plane field varies $π/2$, i.e. $1/4$-cycle in the direction. This novel phenomenon has a profound connection to the four-fold cyclic symmetry of the original crystalline SC and uniquely demonstrates the lattice-symmetry protection of the Majorana modes. We further generalize these results to the system with generic C$_{2N}$ symmetry, and show that the symmetry class of the topological crystalline SC can be demonstrated by the $2N$ times of restoration of two Majorana modes when the external symmetry-breaking field varies one cycle in direction.

cond-mat.str-el↗

arXiv2015

Observation of superconductivity in 3D Dirac semimetal Cd3As2 crystal

Lately, the three-dimensional (3D) Dirac semimetal, which possesses 3D linear dispersion in electronic structure as a bulk analogue of graphene, has generated widespread interests in both material science and condensed matter physics. Very recently, crystalline Cd3As2 has been proposed and proved to be one of 3D Dirac semimetals which can survive in atmosphere. Here, by controlled point contact (PC) measurement, we observe the exotic superconductivity around point contact region on the surface of Cd3As2 crystal. The observation of zero bias conductance peak (ZBCP) and double conductance peaks (DCPs) symmetric to zero bias further reveal p-wave like unconventional superconductivity in Cd3As2 quantum matter. Considering the topological property of the 3D Dirac semimetal, our findings may indicate that the Cd3As2 crystal under certain conditions is a candidate of the topological superconductor, which is predicted to support Majorana zero modes or gapless Majorana edge/surface modes in the boundary depending on the dimensionality of the material.

cond-mat.supr-con↗