arXiv subjects

Qi-Feng Liang

Publications and source records attributed to Qi-Feng Liang.

At least 19 recordsLinked to original sources

arXiv2024

Spontaneous polarized phase transitions and symmetry breaking of an ultracold atomic ensemble in a Raman-assisted cavity

We investigate the ground-state properties and quantum phase transitions of an ensemble consisting of $N$ four-level atoms within an optical cavity coupled to the single cavity mode and external laser fields. The system is described by an extended imbalanced Dicke model, in which the co- and counterrotating coupling terms are allowed to have different coupling strengths. Some novel polarized phases characterized by the phase differences between the cavity field or the atomic spin excitation and the Raman laser are found analytically. Meanwhile, the full phase diagram and quantum phase transitions are also revealed. Finally, the breaking or restoration of the intrinsic symmetry in this system is addressed. It is found that besides the continuous $U(1)$ and discrete $\mathbb{Z}_2$ symmetries, the system also exhibits two reflection symmetries $σ_v$s, a central symmetry $C_2$ in the abstract position-momentum representation, and a discrete reflection parity-time ($\mathcal{PT}$) symmetry, a parameter exchange symmetry $\mathcal{T}_\mathrm{ex}$ in the parameters space. These additional symmetries are governed by two Coxeter groups.

quant-ph↗

arXiv2018

Exotic Node Line in ternary MgSrSi-type crystals

Node line (NL) band-touchings protected by mirror symmetry (named as $m$-NL), the product of inversion and time reversal symmetry $\mathcal{S=PT}$ ($s$-NL), or nonsymmorphic symmetry are nontrivial topological objects of topological semimetals (SMs) in the Brillouin Zone (BZ). In this work, we screened the family of MgSrSi-type crystals using first principles calculations, and discovered that more than 70 members are NLSMs. A new type of multi-loop structure was found in AsRhTi that a $s$-NL touches robustly with a $m$-NL at some "nexus point", and in the meanwhile a second $m$-NL crosses with the $s$-NL to form a Hopf-link. Unlike the previously proposed Hopf-link formed by two $s$-NLs or two $m$-NLs, a Hopf-link formed by a $s$-NL and a $m$-NL requires a minimal three-band model to characterize its essential electronic structure. The associated topological surface states on different surfaces of AsRhTi crystal were also obtained. Even more complicated and exotic multi-loop structure of NLs were predicted in AsFeNa and PNiNb. Our work may shed light on search for exotic multi-loop NLSMs in real materials.

cond-mat.mtrl-sci↗

arXiv2017

Critical Current Anomaly at the Topological Quantum Phase Transition in a Majorana Josephson Junction

Majorana bound states in topological Josephson junctions induce a 4? period current-phase relation. Direct detection of the 4? periodicity is complicated by the quasiparticle poisoning. We reveal that Majorana bound states are also signaled by the anomalous enhancement on the critical current of the junction. We show the landscape of the critical current for a nanowire Josephson junction under a varying Zeeman field, and reveal a sharp step feature at the topological quantum phase transition point, which comes from the anomalous enhancement of the critical current at the topological regime. In multi-band wires, the anomalous enhancement disappears for an even number of bands, where the Majorana bound states fuse into Andreev bound states. This anomalous critical current enhancement directly signals the existence of the Majorana bound states, and also provides a valid signature for the topological quantum phase transition.

cond-mat.supr-con↗

arXiv2017

Interaction-driven Quantum Anomalous Hall Effect in Halogenated Hematite Nanosheets

Based on first-principle calculations and $k\cdot p$ model analysis, we show that the quantum anomalous Hall (QAH) insulating phase can be realized in the functionalized hematite (or $α$-Fe$_2$O$_3$) nanosheet and the obtained topological gap can be as large as $\sim$300 meV. The driving force of the topological phase is the strong interactions of localized Fe 3$d$ electrons operating on the quadratic band crossing point of the non-interacting band structures. Such interaction driven QAH insulator is different from the single particle band topology mechanism in experimentally realized QAH insulator, the magnetic ion doped topological insulator film. Depending on the thickness of the nanosheet, topological insulating state with helical-like or chiral edge states can be realized. Our work provides a realization of the interaction-driven QAH insulating state in a realistic material.

cond-mat.mes-hall↗

arXiv2017

Transition between strong and weak topological insulator in ZrTe$_5$ and HfTe$_5$

ZrTe$_5$ and HfTe$_5$ have attracted increasingly attention recently since the theoretical prediction of being topological insulators (TIs). However, subsequent works show many contradictions about their topological nature.Three possible phases, i.e. strong TI, weak TI, and Dirac semi-metal, have been observed in different experiments until now. Essentially whether ZrTe$_5$ or HfTe$_5$ has a band gap or not is still a question. Here, we present detailed first-principles calculations on the electronic and topological properties of ZrTe$_5$ and HfTe$_5$ on variant volumes and clearly demonstrate the topological phase transition from a strong TI, going through an intermediate Dirac semi-metal phase, then to a weak TI when the crystal expands.Our work might give a unified explain about the divergent experimental results and propose the crucial clue to further experiments to elucidate the topological nature of these materials.

cond-mat.mtrl-sci↗

arXiv2016

Landau-Zener-Stückelberg Interferometry for Majorana Qubit

Stimulated by a very recent experiment observing successfully two superconducting states with even- and odd-number of electrons in a nanowire topological superconductor as expected from the existence of two end Majorana quasiparticles (MQs) [Albrecht \textit{et al.}, Nature \textbf{531}, 206 (2016)], we propose a way to manipulate Majorana qubit exploiting quantum tunneling effects. The prototype setup consists of two one-dimensional (1D) topological superconductors coupled by a tunneling junction which can be controlled by gate voltage. We show that, upon current injection, the time evolution of superconducting phase difference at the junction induces an oscillation in energy levels of the Majorana parity states, whereas the level-crossing is avoided by a small coupling energy of MQs in the individual 1D superconductors. This results in a Landau-Zener-Stückelberg (LZS) interference between the Majorana parity states. Adjusting the current pulse and gate voltage, one can build a LZS interferometry which provides an arbitrary manipulation of the Majorana qubit. The LZS rotation of Majorana qubit can be monitored by the microwave radiated from the junction.

cond-mat.supr-con↗

arXiv2016

Quantum topological Hall effect and noncoplanar antiferromagnetism in K$_{0.5}$RhO$_2$

Quantum anomalous Hall (QAH) phase is a two-dimensional bulk ferromagnetic insulator with a nonzero Chern number in presence of spin-orbit coupling (SOC) but absence of applied magnetic fields. Associated metallic chiral edge states host dissipationless current transport in electronic devices. This intriguing QAH phase has recently been observed in magnetic impurity-doped topological insulators, {\it albeit}, at extremely low temperatures. Based on first-principles density functional calculations, here we predict that layered rhodium oxide K$_{0.5}$RhO$_2$ in noncoplanar chiral antiferromagnetic state is an unconventional three-dimensional QAH insulator with a large band gap and a Neel temperature of a few tens Kelvins. Furthermore, this unconventional QAH phase is revealed to be the exotic quantum topological Hall effect caused by nonzero scalar spin chirality due to the topological spin structure in the system and without the need of net magnetization and SOC.

cond-mat.str-el↗

arXiv2016

Node-Surface and Node-Line Fermions From Nonsymmorphic Lattice Symmetries

We propose a kind of novel topological quantum state of semimetals in a quasi-one-dimensional (1D) crystals BaMX$_3$ (M = V, Nb or Ta; X = S or Se) family by using symmetry analysis and first principles calculation. We find that in BaVS$_3$ the valence and conduction bands are degenerate in the $k_z=π/c$ plane ($c$ is the lattice constant along $\hat{z}$ axis) of the Brillouin Zone (BZ). These nodal points form a node-surface and they are protected by a nonsymmorphic crystal symmetry consisting of a two-fold rotation about the $\hat{z}$ axis and a half-translation along the same $\hat{z}$ axis. The band degeneracy in the node-surface is lifted in BaTaS$_3$ by including strong spin-orbit coupling (SOC) of Ta. The node-surface is reduced into 1D node-lines along the high-symmetry paths $k_x=0$ and $k_x$ = $\pm{\sqrt{3}}k_y$ on the $k_z=π/c$ plane. These node-lines are robust against SOC and guaranteed by the symmetries of $P6_3/mmc$ space group. These node-line states are entirely different from previous proposals which are based on the accidental band touchings. We also propose a useful material design for realizing topological node-surface and node-line semimetals.

cond-mat.mes-hall↗

arXiv2015

Quantum Anomalous Hall Effect in a Perovskite and Inverse-Perovskite Sandwich Structure

Based on first-principles calculations, we propose a sandwich structure composed of a G-type anti-ferromagnetic (AFM) Mott insulator LaCrO$_3$ grown along the [001] direction with one atomic layer replaced by an inverse-perovskite material Sr$_3$PbO. We show that the system is in a topologically nontrivial phase characterized by simultaneous nonzero charge and spin Chern numbers, which can support a spin-polarized and dissipationless edge current in a finite system. Since these two materials are stable in bulk and match each other with only small lattice distortions, the composite material is expected easy to synthesize.

cond-mat.mes-hall↗

arXiv2015

Manipulating the Majorana Qubit with the Landau-Zener-Stückelberg Interference

Constructing a universal operation scheme for Majorana qubits remains a central issue for the topological quantum computation. We study the Landau-Zener-Stückelberg interference in a Majorana qubit and show that this interference can be used to achieve controllable operations. The Majorana qubit consists of an rf SQUID with a topological nanowire Josephson junction which hosts Majorana bound states. In the SQUID, a magnetic flux pulse can drive the quantum evolution of the Majorana qubit. The qubit experiences two Landau-Zener transitions when the amplitude of the pulse is tuned around the superconducting flux quanta $2e/\hbar$. The Landau-Zener-Stückelberg interference between the two transitions rotates the Majorana qubit, with the angle controlled by the time scale of the pulse. This rotation operation implements a high-speed single-qubit gate on the Majorana qubit, which is a necessary ingredient for the topological quantum computation.

cond-mat.supr-con↗

arXiv2015

Proposal for a flux qubit in a dc SQUID with the $4π$ period Josephson effect

Constructing qubits which are suitable for quantum computation remains a notable challenge. Here, we propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero energy Majorana bound states transport $4π$ period Josephson currents in the nanowire junction. The interplay between this $4π$ period Josephson effect and the convectional $2π$ period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest energy levels of the SQUID are isolated from other levels. These two levels show contradicting circulating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.

cond-mat.supr-con↗

arXiv2014

Topological states of non-Dirac electrons on triangular lattice

We demonstrate the possibility of topological states for non-Dirac electrons. Specifically it is shown that, because of the $C_{\rm 3}$ crystal symmetry and time reversal symmetry, $p_x$ and $p_y$ orbits accommodated on triangular lattice exhibit a quadratic band touching at $Γ$ point at the Fermi level. When the atomic spin-orbit coupling (SOC) is taken into account, a gap is opened resulting in a quantum spin Hall effect state. As revealed explicitly by a $k\cdot p$ model, the topology is associated with a meron structure in the pseudo spin texture with vorticity two, a mechanism different from honeycomb lattice and the band inversion. One possible realization of this scheme is the 1/3 coverage by Bi atom adapted on the Si[111] surface. First-principle calculations are carried out, and a global gap of $\sim 0.15$eV is observed. With the Si substrate taking part in realizing the nontrivial topology, the present template is expected to make the integration of topological states into existing electronics and photonics technologies promising.

cond-mat.mes-hall↗

arXiv2014

Viewing Majorana Bound States by Rabi Oscillations

Rabi oscillation is a pure quantum phenomenon where the system jumps forth and back between two quantum levels under stimulation of a microwave, and a resonance occurs when the energy difference is matched by the photon energy. Rabi oscillations have been observed in various quantum systems so far, and especially are used to demonstrate the quantum coherence of quantum bits. In the present work, we explore Rabi oscillation in a system accommodating the elusive Majorana bound states (MBSs) under intensive search recently. The proposed setup is constructed by a quantum dot (QD) and a superconducting quantum interference device (SQUID), where a semiconductor nanowire with spin-orbital coupling in the topological state is introduced to form Josephson junction. When the coupling between QD and the Josephson junction is tuned by an ac gate voltage, Rabi oscillation takes place among quantum states formed by QD and MBSs, which makes it possible to reveal bizarre properties of MBSs by sensing the electron occupation on QD. Especially, one should be able to observe the fractional Josephson relation $\cos(πΦ/Φ_0)$ unique to MBSs with $Φ$ the magnetic flux applied in the SQUID and $Φ_0=hc/2e$ the flux quantum. The system has been investigated in terms of the analytic Floquet theorem and numerical simulations with fine agreement.

cond-mat.mes-hall↗

arXiv2013

Fidelity in topological superconductors with end Majorana fermions

Fidelity and fidelity susceptibility are introduced to investigate the topological superconductors with end Majorana fermions. A general formalism is established to calculate the fidelity and fidelity susceptibility by solving Bogoliubov-de Gennes equations. It is shown that the fidelity susceptibility manifest itself as a peak at the topological quantum phase transition point for homogeneous Kitaev wire, thus serves as a valid indicator for the topological quantum phase transition which signals the appearance of Majorana fermions. The effect of disorders is investigated within this formalism. We consider three disordered systems and observe fidelity susceptibility peak in all of them. By analyzing the susceptibility peak, we notice that the local potential disorders and the hopping disorders can shift the phase transition point, while off-diagonal disorders have no obvious influence. Our results confirm that the existence of topological quantum phase transition is robust to these disorders, while the behavior of the phase transition might be influenced by disorders.

cond-mat.supr-con↗

arXiv2013

Proposal for Manipulation of Majorana Fermions in Nano-Patterned Semiconductor-Superconductor Heterostructure

We investigate a heterostructure system with a spin-orbit coupled semiconductor sandwiched by an s-wave superconductor and a ferromagnetic insulator, which supports Majorana fermions (MFs) at the superconducting vortex cores. We propose a scheme of transporting and braiding the MFs, which only requires application of point-like gate voltages in a system with nano-meter patterns. By solving the time-dependent Bogoliubov-de Gennes equation numerically, we monitor the time evolutions of MF wave-functions and show that the braiding of MFs with non-Abelian statistics can be achieved by adiabatic switching within several nano seconds.

cond-mat.mes-hall↗

arXiv2013

Topological Single Electron Pumping Assisted by Majorana Fermions

Single electron pumping based on the topological property of Majorana fermions (MFs) is proposed. The setup consists of a quantum dot and four nano topological superconductors (TSs) connected by constriction junctions, with an additional vortex located in the loop of TSs. Operation is performed by gate voltages at constriction junctions. Simulations with Bogloliubov-de Gennes equation demonstrate successfully quantum protection during switching operation.

cond-mat.mes-hall↗

arXiv2013

Electrically Tunable Topological State in [111] Perovskite Materials with Antiferromagnetic Exchange Field

A topological state with simultaneous nonzero Chern number and spin Chern number is possible for electrons on honeycomb lattice based on band engineering by staggered electric potential and antiferromagnetic exchange field in presence of intrinsic spin-orbit coupling. With first principles calculation we confirm that the scheme can be realized by material modification in perovskite G-type antiferromagnetic insulators grown along [111] direction, where d electrons hop on a single buckled honeycomb lattice. This material is ideal for spintronics applications, since it provides a spin-polarized quantized edge current, robust to both nonmagnetic and magnetic defects, with the spin polarization tunable by inverting electric field.

cond-mat.mes-hall↗

arXiv2012

Ratchet potential and rectification effect in Majorana fermion SQUID

Motivated by a recent experimental progress in realizing Majorana fermions (MFs) in a heterostructure of a spin-orbit coupling nanowire and superconductor (V. Mourik et al., Science.1222360), we investigate a SQUID formed by the novel superconductor-nanowire-superconductor Josephson junction which contains MFs and a conventional superconductor-insulator-superconductor junction. It is shown that the critical current of the SQUID is different for the two current directions. Since the asymmetric Josepshon current forms a ratchet potential for the dynamics of superconducting phase, a rectification effect is expected when the SQUID is driven by an ac current. These novel properties are expected to be useful for probing the elusive MFs as well as for their dynamics.

cond-mat.supr-con↗