Superconducting state generated dynamically from distant pair source and drain

It has been well established that the origin of p-wave superconductivity is the balance between pair creation and annihilation, described by the spin-less fermionic Kitaev chain model. In this work, we study the dynamics of a composite system where the pair source and drain are spatially separated by a long distance. We show that this non-Hermitian system possesses a high-order exceptional point (EP) when only a source or drain is considered. The EP dynamics provide a clear picture: A pair source can fully fill the system with pairs, while a drain can completely empty the system. When the two coexist simultaneously, the dynamics depend on the distance and the relative phase between the pair creation and annihilation terms. Analytical analysis and numerical simulation results show that the superconducting state can be dynamically established at the resonant pair source and drain: from an initial empty state to a stationary state with the maximal pair order parameter. It provides an alternative way of understanding the mechanism of the nonequilibrium superconducting state.


I. INTRODUCTION
Motivated by recent advances in experimental capability [1][2][3][4][5][6], the nonequilibrium dynamics of quantum manybody systems has emerged as a fundamental and attractive topic in condensed-matter physics.As one of the potential applications, nonequilibrium many-body dynamics provide an alternative way to access a new exotic quantum state with energy far from the ground state [7][8][9][10][11][12][13][14].Unlike traditional protocols based on the cooling mechanism, quench dynamics have a wide range of potential applications since they provide many ways to take a system out of equilibrium, such as applying a driving field or pumping energy or particles in the system through external reservoirs [15][16][17].This makes it possible to design interacting many-body systems to prepare some desirable many-body quantum states by a quenching process.Much effort [7,[18][19][20][21][22][23][24][25][26][27][28][29] has been devoted to developing various nonequilibrium protocols for the generation of the η-pairing-like state [30] in the Hubbard model.
In parallel, the Kitaev model is a lattice model of a pwave superconducting wire, which realize Majorana zero modes at the ends of the chain [31].This has been demonstrated by unpaired Majorana modes exponentially localized at the ends of open Kitaev chains [32][33][34].The main feature of this model originates from the pairing term, which violates the conservation of the fermion number but preserves its parity, leading to the superconducting phase.The amplitudes for pair creation and annihilation play an important role in the existence of the gapped superconducting phase.Compared to the Hubbard model, the Kitaev model has an advantage for the task since it is exactly solvable.In recent work on a simple 1D Kitaev model [14,35], it has been shown that a nonequilibrium superconducting state can be obtained through time evolution from an initially prepared vacuum state, providing * songtc@nankai.edu.cn an alternative approach to dynamically generate a superconducting state from an easily prepared trivial state.
Due to the translational symmetry of such a system, every pair term contributes equally to the formation of the superconducting state.A question arises as to what happens if there is only a single pair term in the Hamiltonian.In this work, we study the dynamics of a composite system where the pair source and drain are considered individually or spatially separated by a long distance.In the framework of quantum mechanics, it corresponds to a non-Hermitian Hamiltonian [36,37].Many contributions have been devoted to non-Hermitian Kitaev models [38][39][40][41][42][43] and Ising models [44][45][46][47][48] within the pseudo-Hermitian framework.We show that this non-Hermitian system possesses a high-order exceptional point (EP) when only a source or drain is considered.It admits peculiar dynamics: the final state is a particular eigenstate, coalescing state [49][50][51][52].The EP dynamics provide a clear physical picture: A pair source can eventially fully fill the system with pairs, while a drain can completely empty the system.However, both final states are trivial.When the two coexist simultaneously, the dynamics depend on the distance and the relative phase between the pair creation and annihilation terms.Analytical analysis and numerical simulation results for a finite system show that a perfect superconducting state can be dynamically established at the resonant pair source and drain, i.e., an initial empty state evolves to a stationary state, which is a perfect superconducting state with the maximal pair order parameter.We consider a composite system with nonhomogenous pair terms in the present work in comparison to previous works.This provides an alternative mechanism for forming nonequilibrium superconducting state.
This paper is organized as follows.In Section II, we describe the model Hamiltonian and introduce the order parameter.In Section III, based on the exact solutions of a toy model, we demonstrate that its ground state has the maximal order parameter.In Section IV, we study the dynamics in the non-Hermitian system, where only pair creation or annihilation terms are considered.
In Section V, we investigate the dynamics driven by a single source or drain.In Section VI, we investigate the dynamics driven by spatially separated source and drain at resonance.Finally, we give a summary and discussion in Section VII.
Pair source 1. Schematic of the 1D Kitaev models for spinless fermions with pair terms across two adjacent sites, red and green dimers indicating specially separated pair creation (source) and annihilation (drain), respectively.Two resonant pair terms are embedded in a (a) ring lattice and (b) open chain.The goal of this work is to investigate the effect of the setup on the nonequilibrium state after sufficiently long time.

II. HAMILTONIAN AND ORDER PARAMETER
We start with a generalized Hamiltonian where the distribution functions f r j and g r j determine the location of creation and annihilation of a pair with size r, respectively.In the case with f r j = g r j = ∆δ r,1 and iT → T , the model as a paradigm of p wave topological superconductivity has been well studied.Here, we take the imaginary hopping strength for the sake of convenience in the following discussion, and iT → T can be realized by local gauge transformation.However, the origin of the phase in the hopping cannot be considered as the magnetic flux since it still takes effect on the eigenstates even if the open boundary condition is taken due to the existence of the pair term.One of the possible origins is the phase gradient on the pair term [53][54][55][56][57].
The main purposes of the following discussion are (i) to present a Hermitian system with a superconducting ground state, which possesses the maximal BCS-pair order parameter, and (ii) to provide a scheme to achieve such a state dynamically by a non-Hermitian system with local pair source and drain.To this end, we will consider several types of functions f r j , g r j , which correspond to different types of pairing processes, including local and extensive sized pairing.In the case with spatially separated creation and annihilation of pairs, the non-Hermitian term is naturally involved, and then some particular dynamic behaviors may emerge that never appear in a Hermitian system.One of them is the EP dynamics, which provides a mechanism for a relaxation process in the framework of quantum mechanics.In fact, in the case with f r j = g r j = ∆δ r,1 , the model has been studied systematically [58].It has been shown that the ground states near the critical point µ = 0 possess ODLRO in association with the maximum of BCS-pair order parameter.
In the framework of the Kitaev model, the pair number is not suitable for characterizing a superconducting state, since the fully filled pair state is an insulating state.To quantitatively characterize the superconductivity of a given state |ψ , we introduce the operator Obviously, for a given state |ψ , quantity measures the rate of transition for a pair at k channel and the population of pairs.Then, O is defined by the average magnitude over all channels.In general, nonzero O means that state |ψ is a superconducting state.

III. PERFECT SUPERCONDUCTING STATE
In this section, we consider a specific distribution function which is the key of the following toy model.Such a toy model allows us to obtain an exact solution, which is ultimately related to the main goal of this work.To this end, we introduce a set of pseudo spin operators which obey the SU(2) commutation relation, [s + , s − ] = 2s z .Particularly, in real space, the spin operator has the form in the thermodynamic limit N −→ ∞.Then, taking f r j = g r j * = D * (r), we have and the Hermitian extended Kitaev Hamiltonian in the form Here, we neglect a constant µN/2 for the sake of simplicity.In the BCS-pair invariant subspace spanned by the pair states where {k} denotes the 2 N/2 dimensional set of configuration of the BCS-pair filling, we always have (c This indicates that all 2 N/2 states Ψ {k} are zero-energy eigenstates of H T , i.e., the first term of H ext .Then, we have the equivalent Hamiltonian of H ext in the subspace where the magnetic field B =(2∆, 0, 2µ).Obviously, both s 2 and s z are commutative to H ext .Based on this fact, one can further construct multi-invariant subspaces by the common eigenstates of s 2 and s z .We are interested in a set {|ψ n } with n ∈ [0, N/2] and be expressed in the form satisfying the equation The coefficient d l n (µ) can be obtained exactly, but here, we only list two of them explicitly which lead to with tan δ = −∆/µ.We note that the two above eigenstates reduce to k>0 (1∓ , which supports that the corresponding order parameter reaches the maximum 0.5 when taking the chemical potential µ = 0. We refer to such states as perfect superconducting states.
It is clear that these two states are not unique perfect superconducting states.In fact, state has the same feature for any distribution of {γ k }.This inspires us to find another way to prepare a superconducting state.Now we consider the dynamic generation of such states in the case with zero µ.For the initial state |ψ (0) = |ψ 0 , which is actually an empty state, the time evolution under a quench Hamiltonian can be expressed as which is essentially a rotation around the x-axis.Obviously |ψ (t) can be easily obtained by local rotation for each k, where θ is a function of time θ(t) = ∆t.Direct derivation shows that the order parameter is a periodic function of time which reaches 0.5 at instants t = (m + 1/2)π/(2∆) with integer m.This indicates that a perfect superconducting state can be established via a dynamic process.The physics seems to be clear that the oscillating O(t) is a resultant effect of both pair creation and annihilation terms.

IV. HIGH-ORDER EP AND DYNAMICS
Now, we consider a question of what happens if only the pair annihilation (creation) terms are taken.It is a first step to investigate the effect of spatially separated source and drain.Naturally, a non-Hermitian Hamiltonian by taking f r j = 0 but g r j = D(r) is involved, i.e., The equivalent Hamiltonian in the invariant subspace spanned by the set of states {|ψ n } becomes The dynamics of H ext are slightly little special and can be captured from the matrix representation of Hamiltonian H ext .It is an (N/2 + 1) × (N/2 + 1) matrix M , with nonzero matrix elements with n = [0, N/2 − 1].Note that M is a nilpotent matrix, i.e.M N/2+1 = 0, or an (N/2 + 1)-order Jordan block.
The dynamics for any state in this subspace {|ψ n } is governed by the time evolution operator Then for the initial state |ψ (0) = ψ N/2 , we have the normalized evolved state which turns to the coalescing state, i.e., |ψ (∞) −→ |ψ 0 .Obviously, |ψ (t) has maximal O at instant 1/∆.The above analysis is still true when we take f r j = D * (r) but g r j = 0 and |ψ (0) = |ψ 0 , which corresponds to a time reversal process.As expected, the physical picture is clear that the pair term takes the role of not only pair generation but also reduction.Intuitively, a local pair term should have a similar effect.This is the aim of the next section.

V. SINGLE SOURCE OR DRAIN
The results obtained in the last section are exact and explicit due to the translational symmetry of the model.In this section, we will show that a similar result can be obtained approximately when only a single pair term is considered, i.e., f r j = 0 but g r j = δ j,j0 D(r), or vice versa.First, states {|ψ n } have translational symmetry with zero momentum.This originates from the translational symmetry of the system, i. e., [H T , T 1 ] = 0, where operator T 1 is defined by T 1 c j T −1 1 = c j+1 .Obviously, we have T 1 |ψ 0 = |ψ 0 , which results in based on the relation Obviously, the perturbation matrix is still in (N + 1)order Jordan block form.The time evolution under such a system should obey the EP dynamics.Furthermore, a Jordan block matrix does not restrict the values of the nonzero matrix elements.Then the relation may also result in EP dynamics, based on the following analysis.Actually, considering a more generalized form of H P = i,j λ ij c i c j with arbitrary factor {λ ij }, states |ψ 0 and |ψ N are two degenerate states of the Hermitian Hamiltonian H T , and we always have due to the facts This means that two states |ψ 0 and that ψ N/2 are mutually biorthogonal conjugates and ψ 0 ψ N/2 is their biorthogonal norm.Importantly, the vanishing norm ψ 0 ψ N/2 = 0 indicates that state |ψ 0 ( ψ N/2 ) is the coalescing state of H(H † ) or Hamiltonians H and H † obtain an EP.From the perspective of dynamics, we have for a sufficiently long time t.Although both states |ψ 0 and ψ N/2 are trivial states, e −iHt ψ N/2 and e −iH † t |ψ 0 may have pair currents at finite t.

VI. RESONANT DISTANT SOURCE AND DRAIN
The above result at least indicates that the local pair terms can be regarded as particle sources or drains, which can fully fill the empty state (|ψ 0 −→ ψ N/2 ) or empty FIG. 2. Plots of the time evolution of O in Eq. ( 43) for several representative ϕ under the Hamiltonian with HT in Eq. ( 2) and Hquen in Eq. ( 42) on the lattice, which is schematically illustrated in Fig. 1 (a) and (b), respectively.The initial state is the vacuum state and the parameters are N = 9, T = 1, µ = 0 and ∆ = 0.005.(a1), (a2) and (a3) are the situations for a ring lattice with N0 = 3, 4 and 5, respectively.(b1), (b2) and (b3) are the situations for an open chain with N0 = 3, 5 and 8, respectively.We find that for all cases, O(t) tends to stabilize at O(∞) after a sufficiently long time.The results show that the stable final state has maximal O for different ϕ.O(∞) is not sensitive to N0 for the ring system, and ϕ affects the rate of convergence of the evolved state.For the chain system, O(∞) reaches the maximum when the pair source and drain are located at the ends of the chain.
the fully filled state ( ψ N/2 −→ |ψ 0 ).This inspires us to investigate the dynamics with balanced local pair terms that are spatially separated by a distance.To this end, we consider the pair term in the resonant form or explicitly which acts as separated local pair sources and drains.
Here, as the resonant condition, the amplitudes of the pair annihilation and creation terms are the same, while there is a phase difference ϕ between them, which is crucial for the dynamics, as shown in the following.In the small ∆ limit, based on the perturbation method, the matrix representation of Hamiltonian H ext in the subspace spanned by the set of states e inϕ/2 |ψ n is an (N/2 + 1) × (N/2 + 1) matrix H ext with nonzero matrix elements with n = [0, N/2 − 1].We note that matrix H ext is the same as that in (21) but with a complex strength constant.The corresponding eigenenergy is complex with n = [0, N/2], and its imaginary part is Im (E n ) = ∆ (2n/N − 1/2) sin ϕ 2 .Unlike a Hermitian system, the imaginary part of the eigenvalue can amplify or reduce the corresponding amplitude of the wave function in the dynamic process.For the given initial state |ψ (0) = |ψ 0 when the evolution time is sufficiently long, the final state is the eigenstate of H ext with the maximum imaginary part.The corresponding approximate eigenstate is where σ = sgn(sin ϕ 2 ).Obviously, |ψ (∞) is a perfect superconducting state.When the off-resonant case is considered, the expression is the same as |ψ (∞) , but an imaginary part should be added in ϕ, which will reduce the order parameter from 0.5.Now we consider a more practical case with where the pairing terms reduce to the simplest case.In addition, we also consider the case with an open boundary condition, which is closer to the real sample in the experiment.The physical intuition for this setup is simple.Term c † 1 c † 2 acts as a source of pair at one end of the chain, while c N −1 c N takes the role of drain at the other end.According to the analysis of the pair term i,j λ ij c i c j in the last section, it is expected that the nearest neighboring pair terms share a similar feature, i.e., a stable state with the order parameter close to that of state (41) emerges when the source and drain are balanced.Numerical simulation is performed to verify our predictions.We compute the time evolution |ψ (t) = e −i(HT+Hquen)t |ψ (0) by exact diagonalization.The geometries of finite systems are schematically illustrated in Fig. 1.We consider an N = 9 site system with periodic and open boundary conditions.In this case, the order parameter has the following explicit form: We plot O(t) as a function of t and ϕ in Fig. 2 for the finite size cases schematically illustrated in Fig. 1, obtained by numerical simulations.The numerical results agree with our prediction, that for all cases, O(t) tends to stabilize at O(∞) after a sufficiently long time.In addition, we find that the relaxation process and the final O depend on the geometry and ϕ. (i) O(∞) is not sensitive to N 0 for the ring system, and O(∞) can reach the maximum 0.5.(ii) O(∞) depends on N 0 for the chain system, and O(∞) can reach the maximum 0.45 when the pair source and drain are located at the ends of the chain.This implies that the balanced edge pair source and drain benefit to forming a superconducting state.For both boundary conditions, the converging time depends on the value of ϕ, but in two different ways.

VII. SUMMARY
In summary, we have studied several types of toy models with deliberately engineered pair terms to explore the possibility of realizing nonequilibrium superconducting state in a nonhomogeneous Kitaev model, which is essentially a non-Hermitian extension of the Kitaev chain.In the framework of quantum mechanics, based on the analysis of the exact solution and perturbation method, we find that the EP dynamics provides a clear picture for the action of a single pair source or drain.When the two coexist simultaneously, the dynamics depend on the distance and the relative phase between the pair creation and annihilation terms.Analytical analysis and numerical simulation results show that the superconducting state can be dynamically established at the resonant local pair source and drain.Two spatially separated pair terms can drive an initial empty state to a stationary state with a near maximal pair order parameter.It provides an alternative way of understanding the mechanism of the nonequilibrium superconducting state.