Search

Intense highly charged ion beam production is essential for high-power heavy ion accelerators. A novel movable Vlasov launcher for superconducting high charge state Electron Cyclotron Resonance (ECR) ion source has been devised that can affect the microwave power effectiveness by a factor of about 4 in terms of highly charged ion beam production. This approach based on a dedicated microwave launching system instead of the traditional coupling scheme has led to new insight on microwave-plasma interaction. With this new understanding, the world record highly charged xenon ion beam currents have been enhanced by up to a factor of 2, which could directly and significantly enhance the performance of heavy ion accelerators and provide many new research opportunities in nuclear physics, atomic physics and other disciplines.

The dynamics described by the non-Hermitian Hamiltonian typically capture the short-term behavior of open quantum systems before quantum jumps occur. In contrast, the long-term dynamics, characterized by the Lindblad master equation (LME), drive the system towards a non-equilibrium steady state (NESS), which is an eigenstate with zero energy of the Liouvillian superoperator, denoted as $\mathcal{L}$. Conventionally, these two types of evolutions exhibit distinct dynamical behaviors. However, in this study, we challenge this common belief and demonstrate that the effective non-Hermitian Hamiltonian can accurately represent the long-term dynamics of a critical two-level open quantum system. The criticality of the system arises from the exceptional point (EP) of the effective non-Hermitian Hamiltonian. Additionally, the NESS is identical to the coalescent state of the effective non-Hermitian Hamiltonian. We apply this finding to a series of critical open quantum systems and show that a local dissipation channel can induce collective alignment of all spins in the same direction. This direction can be well controlled by modulating the quantum jump operator. The corresponding NESS is a product state and maintains long-time coherence, facilitating quantum control in open many-body systems. This discovery paves the way for a better understanding of the long-term dynamics of critical open quantum systems., Comment: 13 pages, 7 figures

Topological characteristics of quantum systems are typically determined by the closing of a gap, while the dynamical quantum phase transition (DQPT) during quantum real-time evolution has emerged as a nonequilibrium analog to the quantum phase transition (QPT). In this paper, we illustrate that the system dynamics can be elucidated by considering the precession of a collection of free-pseudo spins under a magnetic field based on the exact results of extended Kitaev chains. The topology of the driven Hamiltonian is determined by the average winding number of the nonequilibrium state. Furthermore, we establish that the singularity of the DQPT arises from two perpendicular pseudo-spin vectors associated with the pre- and post-quenched Hamiltonians. Moreover, we investigate the distinct behaviors of the dynamic pairing order parameter in both topological and non-topological regions. These findings offer valuable insights into the non-equilibrium behavior of topological superconductors, contributing to the understanding of the resilience of topological properties in driven quantum systems., Comment: 6+7 pages, 3+5 figures

A many-body quantum system with varying parameters can exhibit two distinct quantum states within the same energy shell. This allows for a dynamic transition from the ground state of the pre-quench Hamiltonian to a steady state of the post-quench Hamiltonian. We investigate the dynamic response of the ground states in a two-layer half-filled Hubbard model to a perpendicular electric field. We demonstrate that the steady state exhibits conductivity when the field is in resonance with the on-site repulsion, while the initial state is a Mott-insulating state. Additionally, the two layers exhibit identical conducting behavior due to the formation of long-lived dopings, as evidenced by the charge fluctuation. The key factor in achieving this dynamic transition is the cooperative interplay between on-site interactions and the resonant field, rather than the individual roles they play. Our findings offer an alternative mechanism for field-induced conductivity in strongly correlated systems., Comment: 12 pages, 5 figures

We show that a resonant pulsed field can induce a steady superconducting state even in the deep Mott insulating phase of the dimerized Hubbard model. The superconductivity found here in the non-equilibrium steady state is due to the $\eta $-pairing mechanism, characterized by the existence of the off-diagonal long-range order (ODLRO), and is absent in the ground-state phase diagram. The key of the scheme lies in the generation of the field-induced charge density wave (CDW) state that is from the valence bond solid. The dynamics of this state resides in the highly-excited subspace of dimerized Hubbard model and is dominated by a $\eta $-spin ferromagnetic model. The decay of such long-living excitation is suppressed because of energy conservation. We also develop a dynamical method to detect the ODLRO of the non-equilibrium steady state. Our finding demonstrates that the non-equilibrium many-body dynamics induced by the interplay between the resonant external field and electron-electron interaction is an alternative pathway to access a new exotic quantum state, and also provides an alternative mechanism for enhancing superconductivity., Comment: 13 pages, 9 figures

We study an anisotropic two-dimensional non-Hermitian Lieb lattice, where the staggered gain and loss present in the horizontal and vertical directions, respectively. The intra-cell nonreciprocal coupling generates magnetic flux enclosed in the unit cell of the Lieb lattice and creates nontrivial topology. The active and dissipative topological edge states are along the horizontal and vertical directions, respectively. The two-dimensional non-Hermitian Lieb lattice also supports passive topological corner state. At appropriate magnetic flux, the non-Hermiticity can alter the corner state from one corner to the opposite corner as the non-Hermiticity increases. The gapless phase of the Lieb lattice is characterized by different configurations of exceptional points in the Brillouin zone. The topology of the anisotropic non-Hermitian Lieb lattices can be verified in many experimental platforms including the optical waveguide lattices, photonic crystals, and electronic circuits., Comment: 11 pages, 9 figures

Electron--electron repulsion, on the one hand, can result in bound pair, which has heavy effective mass. On the other hand, it is also the cause of Mott insulator. We study the effect of a staggered magnetic field on a Hubbard model. We find that a bound pair with large energy bandwidth can be formed under the resonant staggered field, being the half of Hubbard repulsion strength. Accordingly, the system exhibits following dynamical behaviors: (i) When an electric field is applied, fast bound pair Bloch oscillation occurs, while a single electron is frozen. (ii) When a quenching resonant field is applied to an initial antiferromagnetic Mott insulating state, the final state becomes doublon conducting state manifested by the non-zero $\eta$ correlator and large charge fluctuation. Our finding indicates that the cooperation of electron-electron correlation and modulated external field can induce novel quench dynamics., Comment: 9 pages, 6 figures

We study the response of a thermal state of the Hubbard-like system to either global or local non-Hermitian perturbation, which coalesces the degenerate ground state within the $U(1)$ symmetry breaking phase. We show that the dynamical response of the system is strongly sensitive to the underlying quantum phase transition (QPT) from a Mott insulator to a superfluid state. The Uhlmann fidelity in the superfluid phase decays to a steady value determined by the order of the exceptional point (EP) within the subspace spanned by the degenerate ground states but remains almost unchanged in the Mott insulating phase. It demonstrates that the phase diagram at zero temperature is preserved even though a local probing field is applied. Specifically, two celebrated models including the Bose-Hubbard model and the Jaynes-Cummings-Hubbard model are employed to demonstrate this property in the finite-size system, wherein fluctuations of the boson and polariton number are observed based on EP dynamics. This work presents an alternative approach to probe the superfluid-insulator QPT at non-zero temperature., Comment: 8 pages, 5 figures

The introduction of non-Hermiticity has greatly enriched the research field of traditional condensed matter physics, and eventually led to a series of discoveries of exotic phenomena. We investigate the effect of non-Hermitian imaginary hoppings on the attractive Hubbard model. The exact bound-pair solution shows that the electron-electron correlation suppresses the non-Hermiticity, resulting in off-diagonal long-range order (ODLRO) ground state. In a large negative $U $ limit, the ODLRO ground state corresponds to $\eta $-spin ferromagnetic states. We also study the system with mixed hopping configuration. The numerical result indicates the existence of the transition from normal to $\eta $-pairing ground states by increasing the imaginary hopping strength. Our results provide a promising approach for the non-Hermitian strongly correlated system., Comment: 10 pages, 4 figures

The cooperation between non-Hermiticity and interaction brings about a lot of counterintuitive behaviors, which are impossible to exist in the framework of the Hermitian system. We study the effect of a non-Hermitian impurity on the Hubbard model in the context of $\eta $ symmetry. We show that the non-Hermitian Hubbard Hamiltonian can respect a full real spectrum even if a local non-Hermitian impurity is applied to. The balance between dissipation of single fermion and on-site pair fluctuation results in a highest-order coalescing state with off-diagonal long-range order (ODLRO). Based on the characteristic of High-order EP, the critical non-Hermitian Hubbard model allows the generation of such a steady superconducting-like state through the time evolution from an arbitrary initial state, including the vacuum state. Remarkably, this dynamic scheme is insensitive to the on-site interaction and entirely independent of the locations of particle dissipation and pair fluctuation. Our results lay the groundwork for the dynamical generation of a steady ODLRO state through the critical non-Hermitian strongly correlated system., Comment: 8 pages, 2 figures

We report a global effect induced by the local complex field, associated with the spin-exchange interaction. High-order exceptional point up to ($N+1$)-level coalescence is created at the critical local complex field applied to the $N$-size quantum spin chain. The ($N+1$)-order coalescent level is a saturated ferromagnetic ground state in the isotropic spin system. Remarkably, the final state always approaches the ground state for an arbitrary initial state with any number of spin flips; even if the initial state is orthogonal to the ground state. Furthermore, the switch of macroscopic magnetization is solely driven by the time and forms a hysteresis loop in the time domain. The retentivity and coercivity of the hysteresis loop mainly rely on the non-Hermiticity. Our findings highlight the cooperation of non-Hermiticity and the interaction in quantum spin system, suggest a dynamical framework to realize magnetization, and thus pave the way for the non-Hermitian quantum spin system., Comment: 10 pages, 4 figures

We employ the intertwining operator technique to synthesize a supersymmetric (SUSY) array of arbitrary size $N$. The synthesized SUSY system is equivalent to a spin-$(N-1)/2$ under an effective magnetic field. By considering an additional imaginary magnetic field, we obtain a generalized parity-time-symmetric non-Hermitian Hamiltonian that describes a SUSY array of coupled resonators or waveguides under a gradient gain and loss; all the $N$ energy levels coalesce at an exceptional point (EP), forming the isotropic high-order EP with $N$ states coalescence (EPN). Near the EPN, the scaling exponent of phase rigidity for each eigenstate is $(N-1)/2$; the eigen frequency response to the perturbation $\epsilon$ acting on the resonator or waveguide couplings is $\epsilon^{1/N}$. Our findings reveal the importance of the intertwining operator technique for the spectral engineering and exemplify the practical application in non-Hermitian physics., Comment: 10 pages, 7 figures

Unlike a Chern number in $2$D and $3$D topological system, Zak phase takes a subtle role to characterize the topological phase in $1$D. On the one hand, it is not a gauge invariant, on the other hand, the Zak phase difference between two quantum phases can be used to identify the topological phase transitions. A non-Hermitian system may inherit some characters of a Hermitian system, such as entirely real spectrum, unitary evolution, topological energy band, etc. In this paper, we study the influence of non-Hermitian term on the Zak phase for a class of non-Hermitian systems. We show exactly that the real part of the Zak phase remains unchanged in a bipartite lattice. In a concrete example, $1$D Su-Schrieffer-Heeger (SSH) model, we find that the real part of Zak phase can be obtained by an adiabatic process. To demonstrate this finding, we investigate a scattering problem for a time-dependent scattering center, which is a magnetic-flux-driven non-Hermitian SSH ring. Owing to the nature of the Zak phase, the intriguing features of this design are the wave-vector independence and allow two distinct behaviors, perfect transmission or confinement, depending on the timing of a flux impulse threading the ring. When the flux is added during a wavepacket travelling within the ring, the wavepacket is confined in the scatter partially. Otherwise, it exhibits perfect transmission through the scatter. Our finding extends the understanding and broaden the possible application of geometric phase in a non-Hermitian system., Comment: 11 pages, 7 figures

We systematically study the topology of the exceptional point (EP) in the finite non-Hermitian system. Based on the concrete form of the Berry connection, we demonstrate that the exceptional line (EL), at which the eigenstates coalesce, can act as a vortex filament. The direction of the EL can be identified by the corresponding Berry curvature. In this context, such a correspondence makes the topology of the EL clear at a glance. As an example, we apply this finding to the non-Hermitian Rice-Mele (RM) model, the non-Hermiticity of which arises from the staggered on-site complex potential. The boundary ELs are topological, but the non-boundary ELs are not. Each non-boundary EL corresponds to two critical momenta that make opposite contributions to the Berry connection. Therefore, the Berry connection of the many-particle quantum state can have classical correspondence, which is determined merely by the boundary ELs. Furthermore, the non-zero Berry phase, which experiences a closed path in the parameter space, is dependent on how the curve surrounds the boundary EL. This also provides an alternative way to investigate the topology of the EP and its physical correspondence in a finite non-Hermitian system., Comment: 8 pages, 4 figures

We study the dynamical behavior of a non-Hermitian moire superlattice system, which consists of two-coupled SSH chains with staggered imaginary on-site potentials. There are two main spatial regions, in which systems are in unbroken symmetric phases with fully real spectrum, appearing periodically along the ladder. We show that the two quantum phases are dimerized and tetramerized, which determine the distinct dynamical behaviors. Dirac probability can oscillate periodically, increase quadratically and increase exponentially, which correspond to the unbroken phase, exceptional point and the broken phase of the tetramerized region. In comparison, the Dirac probability can exhibit high-frequency oscillation in the dimerized region. These phenomena demonstrate the dynamical signature and provide insightful information of the moire pattern in the non-Hermitian regime., Comment: 10 pages, 5 figures

We study a non-Hermitian Rice-Mele model without breaking time-reversal symmetry, with the non-Hermiticity arising from imbalanced hopping rates. The Berry connection, Berry curvature and Chern number are introduced in the context of biorthonormal inner product. It is shown that for a bulk system, although the Berry connection can be complex numbers, the Chern number is still quantized, as topological invariant. For an opened chain system, the mid-gap edge modes are obtained exactly, obeying the bulk-edge correspondence. Furthermore, we also introduce a local current in the context of biorthonormal inner product to measure the pumping charge generated by a cyclic adiabatic evolution. Analytical analysis and numerical simulation of the time evolution of the mid-gap states show that the pumping charge can be a dynamical topological invariant in correspondence with the Chern number. It indicates that the geometric concepts for Hermitian topological insulator can be extended to the non-Hermitian regime.

We study an optomechanical transistor, where an input field can be transferred and amplified unidirectionally in a cyclic three-mode optomechanical system. In this system, the mechanical resonator is coupled simultaneously to two cavity modes. We show that it only requires a finite mechanical gain to achieve the nonreciprocal amplification. Here the nonreciprocity is caused by the phase difference between the linearized optomechanical couplings that breaks the time-reversal symmetry of this system. The amplification arises from the mechanical gain, which provides an effective phonon bath that pumps the mechanical mode coherently. This effect is analogous to the stimulated emission of atoms, where the probe field can be amplified when its frequency is in resonance with that of the anti-Stokes transition. We show that by choosing optimal parameters, this optomechanical transistor can reach perfect unidirectionality accompanied with strong amplification. In addition, the presence of the mechanical gain can result in ultra-long delay in the phase of the probe field, which provides an alternative to controlling light transport in optomechanical systems., Comment: 8 pages, 5 figures

Nodal line in parameter space, at which the energy gap closes up, can either be the boundary separating two topological quantum phases or two conventional phases. We study the topological feature of nodal line in parameter space via a dimerized Kitaev spin chain with staggered transverse field, which can be mapped onto the system of dimerized spinless fermions with p-wave superconductivity. The quantum phase boundaries are straight crossing degeneracy lines in 3D parameter space. We show that the nodal line acts as a vortex filament associated with a vector field, which is generated from the Zak phase of Bogoliubov-de Gennes band. We also investigate the topological invariant in Majorana fermion representation for open chain. The Majorana edge modes are not zero mode, but is still protected by energy gap. The exact mid-gap states of the Majorana lattice allows to obtain the corresponding Majorana probability distribution for whole 3D parameter space, which can establish a scalar field to identify the topological feature of the nodal lines. Furthermore, when we switch on a weak tunneling between two ends of the Majorana lattice, the topological invariants of the nodal lines can be obtained by the pumped Majorana probability from an adiabatic passage encircling the lines. Numerical simulations of quasi-adiabatic time evolution is performed in small system. We compute the current to monitor the transport of Majorana fermion, which exhibits evident character of topological pumping. Our results link the bulk topological invariant to the dynamical behavior of Majorana edge mode, extending the concept of bulk-edge correspondence.

The Hartree-Fock-Bogolyubov (HFB) plus proton-neutron quasiparticle random phase approximation (pnQRPA) approach based on Skyrme interaction is applied to study the nuclear $\beta^+$/EC decay for nuclei near the proton magic numbers $Z=$20, 28, and 50. With properly selected Skyrme interactions that include the tensor terms, and the $T=0$ pairing interaction, the experimental $\beta^+$/EC decay half-lives of these nuclei can be systematically reproduced quite well. It is shown that the tensor and $T=0$ pairing interactions play different roles in different nuclei. The specific effect is relevant to the configurations that contribute to the decay. The attractive and repulsive properties of tensor interaction for the GT state is also studied. The present results indicate that it works repulsively on the GT states dominated by the configurations from $\pi j_\gtrless$ to $\nu j_\gtrless$ orbits, while working attractively for those mainly composed of the configurations from $\pi j_\gtrless$ to $\nu j_\lessgtr$ orbits., Comment: 6 pages, 2 figures, and 4 tables

We investigate herein the existence of spectral singularities (SSs) in composite systems that consist of two separate scattering centers A and B embedded in one-dimensional free space, with at least one scattering center being non-Hermitian. We show that such composite systems have an SS at $k_{c}$ if the reflection amplitudes $r^{A}\left( k_{c}\right) $ and $r^{B}\left( k_{c}\right) $ of the two scattering centers satisfy the condition $r_{\mathrm{R}% }^{A}\left( k_{c}\right) r_{\mathrm{L}}^{B}\left( k_{c}\right) e^{i2k_{c}\left( x_{B}-x_{A}\right) }=1$. We also extend the condition to the system with multi-scattering centers. As an application, we construct a simple system to simulate a resonant lasing cavity., Comment: 12 pages, 5 figures

We systematically study a Kitaev chain with imbalanced pair creation and annihilation, which is introduced by non-Hermitian pairing terms. Exact phase diagram shows that the topological phase is still robust under the influence of the conditional imbalance. The gapped phases are characterized by a topological invariant, the extended Zak phase, which is defined by the biorthonormal inner product. Such phases are destroyed at the points where the coalescence of groundstates occur, associating with the time-reversal symmetry breaking. We find that the Majorana edge modes also exist for the open chain within unbroken time-reversal symmetric region, demonstrating the bulk-edge correspondence in such a non-Hermitian system., Comment: 8 pages, 4 figures

We study inter-chain pair tunnelling dynamics based on an exact two-particle solution for a two-leg ladder. We show that the Hermitian Hamiltonian shares a common two-particle eigenstate with a corresponding non-Hermitian Hubbard Hamiltonian in which the non-Hermiticity arises from an on-site interaction of imaginary strength. Our results provides that the dynamic processes of two-particle collision and across-legs tunnelling are well described by the effective non-Hermitian Hubbard Hamiltonian based on the eigenstate equivalence. We also find that any common eigenstate is always associated with the emergence of spectral singularity in the non-Hermitian Hubbard model. This result is valid for both Bose and Fermi systems and provides a clear physical implication of the non-Hermitian Hubbard model., Comment: 10 pages, 4 figures

We study the directional amplification of an optical probe field in a three-mode optomechanical system, where the mechanical resonator interacts with two linearly-coupled optical cavities and the cavities are driven by strong optical pump fields. The optical probe field is injected into one of the cavity modes, and at the same time, the mechanical resonator is subject to a mechanical drive with the driving frequency equal to the frequency difference between the optical probe and pump fields. We show that the transmission of the probe field can be amplified in one direction and de-amplified in the opposite direction. This directional amplification or de-amplification results from the constructive or destruction interference between different transmission paths in this three-mode optomechanical system., Comment: 8 pages, 4 figures

The fully self-consistent Hartree-Fock (HF) plus random phase approximation (RPA) based on Skyrme-type interaction is used to study the existence problem of proton semi-bubble structure in the $2_1^+$ state of $^{34}$Si. The experimental excitation energy and the B(E2) strength of the $2_1^+$ state in $^{34}$Si can be reproduced quite well. The tensor effect is also studied. It is shown that the tensor interaction has a notable impact on the excitation energy of the $2_1^+$ state and a small effect on the B(E2) value. Besides, its effect on the density distributions in the ground and $2_1^+$ state of $^{34}$Si is negligible. Our present results with T36 and T44 show that the $2_1^+$ state of $^{34}$Si is mainly caused by proton transiton from $\pi 1d_{5/2}$ orbit to $\pi 2s_{1/2}$ orbit, and the existence of a proton semi-bubble structure in this state is very unlikely., Comment: 6 pages, 3 figures, 3 tables

In contrast to a Hermitian system, in which zero modes are usually degenerate and localized edge state in the thermodynamic limit, the zero mode of a finite non-Hermitian system can be a single nontrivial long-range entangled state at the exceptional point (EP). In this work, we demonstrate this feature with a concrete example based on exact solutions. Numerical simulations show that the entangled state can be generated through the dynamic process with a high fidelity.

Parity-time (PT) symmetry is of great interest. The reciprocal and unidirectional features are intriguing besides the PT symmetry phase transition. Recently, the reciprocal transmission, unidirectional reflectionless and invisibility are intensively studied. Here, we show the reciprocal reflection/transmission in PT -symmetric system is closely related to the type of PT symmetry, that is, the axial (reflection) PT symmetry leads to reciprocal reflection (transmission). The results are further elucidated by studying the scattering of rhombic ring form coupled resonators with enclosed synthetic magnetic flux. The nonreciprocal phase shift induced by the magnetic flux and gain/loss break the parity (P) and time-reversal (T) symmetry but keep the parity-time (PT) symmetry. The reciprocal reflection (transmission) and unidirectional transmission (reflection) are found in the axial (reflection) PT-symmetric ring center. The explorations of symmetry and asymmetry from PT symmetry may shed light on novel one-way optical devices and application of PT metamaterials., Comment: 19 Pages, 5 figures + SI of 4 Pages, 2 figures

We study the dynamics of the collision between two fermions in Hubbard model with on-site interaction strength $U$. The exact solution shows that the scattering matrix for two-wavepacket collision is separable into two independent parts, operating on spatial and spin degrees of freedom, respectively. The S-matrix for spin configuration is equivalent to that of Heisenberg-type pulsed interaction with the strength depending on $U$ and relative group velocity $\upsilon _{r}$. This can be applied to create distant EPR pair, through a collision process for two fermions with opposite spins in the case of $\left\vert \upsilon _{r}/U\right\vert =1$,\ without the need for temporal control and measurement process. Multiple collision process for many particles is also discussed., Comment: 7 pages, 3 figures

Short-range interaction can give rise to particle pairing with a short-range correlation, which may be destroyed in the presence of an external field. We study the transition between correlated and uncorrelated particle states in the framework of one-dimensional Hubbard model driven by a field. We show that the long time-scale transfer rate from an initial correlated state to final uncorrelated particle states is sensitive to the quench field strength and exhibits a periodic behavior. This process involves an irreversible energy transfer from the field to particles, leading to a quantum electrothermal effect., Comment: 9 pages, 8 figures

In the Hermitian regime, a Berry phase is always the real number. It may be imaginary for a non-Hermitian system, which leads to amplitude amplification or attenuation of an evolved quantum state. We study the dynamics of the non-Hermitian Rice-Mele model driven by a time-dependent external field. The exact results show that it can have full real spectrum for any value of the field. Several rigorous results are presented for the Berry phase with respect to the varying field. We show that the Berry phase is the same complex constant for any initial state in a single sub-band. Numerical simulation indicates that the amplitude control of a state can be accomplished by a quasi-adiabatic process within a short time.

We study the dynamics of bound pairs in the extended Hubbard model driven by a linear external field. It is shown that two interacting bosons or singlet fermions with nonzero on-site and nearest-neighbor interaction strengths can always form bound pairs in the absence of an external field. There are two bands of bound pairs, one of which may have incomplete wave vectors when it has an overlap with the scattering band, referred to as an imperfect band. In the presence of the external field, the dynamics of the bound pair in the perfect band exhibits distinct Bloch-Zener oscillation (BZO), while in the imperfect band the oscillation presents a sudden death. The pair becomes uncorrelated after the sudden death and the BZO never comes back. Such dynamical behaviors are robust even for the weak-coupling regime and thus can be used to characterize the phase diagram of the bound states.

We study the effect of PT-symmetric imaginary potentials embedded in the two arms of an Aharonov-Bohm interferometer on the transmission phase by finding an exact solution for a concrete tight-binding system. It is observed that the spectral singularity always occurs at k=${\pm}${\pi}/2 for a wide range of fluxes and imaginary potentials. Critical behavior associated with the physics of the spectral singularity is also investigated. It is demonstrated that the quasi-spectral singularity corresponds to a transmission maximum and the transmission phase jumps abruptly by {\pi} when the system is swept through this point. Moreover, We find that there exists a pulse-like phase lapse when the imaginary potential approaches the boundary value of the spectral singularity., Comment: 8 pages, 4 figures, Accepted by PRA

We study the possibility of asymmetric transmission induced by a non-Hermitian scattering center embedded in a one-dimensional waveguide, motivated by the aim of realizing quantum diode in a non-Hermitian system. It is shown that a $\mathcal{PT}$ symmetric non-Hermitian scattering center always has symmetric transmission although the dynamics within the isolated center can be unidirectional, especially at its exceptional point. We propose a concrete scheme based on a flux-controlled non-Hermitian scattering center, which comprises a non-Hermitian triangular ring threaded by an Aharonov-Bohm flux. The analytical solution shows that such a complex scattering center acts as a diode at the resonant energy level of the spectral singularity, exhibiting perfect unidirectionality of the transmission. The connections between the phenomena of the asymmetric transmission and reflectionless absorption are also discussed., Comment: 6 pages, 5 figures

A conventional quantum phase transition (QPT) can be accessed by varying a real parameter at absolute zero temperature. Motivated by the discovery of the pseudo-Hermiticity of non-Hermitian systems, we explore the QPT in non-Hermitian PT-symmetric Ising model, which is driven by a staggered complex transverse field. Exact solution shows that the Laplacian of the groundstate energy density, with respect to real and imaginary components of the transverse field, diverges on the boundary in the complex plane. The phase diagram indicate that the imaginary transverse field has the effect of shrinking the paramagnet phase. In addition, we also investigate the connection between the geometric phase and the QPT., Comment: 7 pages,4 figures

Motivated by the physical relevance of a spectral singularity of interacting many-particle system, we explore the dynamics of two bosons as well as fermions in one-dimensional system with imaginary delta interaction strength. Based on the exact solution, it shows that the two-particle collision leads to amplitude-reduction of the wave function. For fermion pair, the amplitude-reduction depends on the spin configuration of two particles. In both cases, the residual amplitude can vanish when the relative group velocity of two single-particle Gaussian wave packets with equal width reaches the magnitude of the interaction strength, exhibiting complete particle-pair annihilation at the spectral singularity., Comment: 8 pages, 2 figures

We study the geometric phase for the ground state of a generalized one-dimensional non-Hermitian quantum XY model, which has transverse-field-dependent intrinsic rotation-time reversal symmetry. Based on the exact solution, this model is shown to have full real spectrum in multiple regions for the finite size system. The result indicates that the phase diagram or exceptional boundary, which separates the unbroken and broken symmetry regions corresponds to the divergence of the Berry curvature. The scaling behaviors of the groundstate energy and Berry curvature are obtained in an analytical manner for a concrete system., Comment: 6 pages, 3 figures