arXiv subjects

Xiao-Xue Zhang

Publications and source records attributed to Xiao-Xue Zhang.

6 recordsLinked to original sources

arXiv2026

Single-photon scattering in a dissipative superconducting-qubit--SSH lattice hybrid

We study single-photon scattering in a Su--Schrieffer--Heeger (SSH) photonic lattice locally coupled to a superconducting qubit with tunable loss or gain. Working in the single-excitation sector, we derive an explicit real-space scattering formulation for the full energy-dependent scattering matrix $S(E)$ and identify how its eigenvalues encode coherent perfect absorption, amplification, and spectral singular behavior. The analytical results are benchmarked against time-domain wave-packet simulations, which reproduce the stationary scattering probabilities with high accuracy. We show that the SSH dimerization, the qubit-induced non-Hermitian self-energy, and the synthetic gauge phase cooperate to reshape the reflection and transmission spectra in a highly selective way. In particular, changing the dimerization can switch the system between transmission-dominated and reflection-dominated regimes, while the flux provides a direct handle on interference and symmetry-controlled response. We also find a robust loss--gain correspondence in the reflection landscape and show that the linewidth broadening is governed predominantly by the magnitude $|γ|$ of the non-Hermitian coupling. These results establish a compact and experimentally relevant framework for topological scattering in superconducting quantum networks.

quant-ph↗

arXiv2026

Non-Hermitian scattering in SSH superconducting waveguides: exact Green-function reduction and dimerization-sensitive microwave functionalities

We formulate an exact Green-function theory for non-Hermitian single-microwave-photon scattering by finite superconducting circuit subsystems embedded in an SSH waveguide. The structured SSH environment is integrated out exactly and enters the local scattering problem as an energy-dependent matrix self-energy, reducing the full open system to a finite-dimensional effective non-Hermitian Hamiltonian. This reduction places scattering amplitudes, exceptional-point diagnostics, coherent-perfect-absorption conditions, and lasing thresholds within one unified framework. Within this approach we analyze two superconducting devices. A flux-controlled two-qubit interferometric scatterer exhibits a broad bright branch and a narrow quasi-dark branch whose interference is reshaped by the SSH environment and changes qualitatively across the two dimerizations. A mediator-assisted two-qubit scatterer generates an additional energy-dependent complex coupling, reorganizes the dressed spectrum, and produces clearer dimerization-sensitive transparency-versus-absorption windows together with a pronounced separation between zero-like and pole-like scattering branches. In the active regime, near-exceptional-point hybridization enhances the pole-dominated response while deepening the singular-value valley associated with near-coherent perfect absorption. These results show how structured topological waveguides can be used not only to host scattering, but also to design non-Hermitian superconducting microwave functionalities.

quant-ph↗

arXiv2017

Spin-accumulation capacitance and its application to magnetoimpedance

It has been known that spin-dependent capacitances usually coexist with geometric capacitances in a magnetic multilayer. However, the charge and energy storage of the capacitance due to spin accumulation has not been fully understood. Here, we resolve this problem starting from the charge storage in the spin degree of freedom: spin accumulation manifests itself as an excess of electrons in one spin channel and an equal deficiency in the other under the quasi-neutrality condition. This enables us to model the two spin channels as the two plates of a capacitor. Taking a ferromagnet/nonmagnet junction as an example and using a method similar to that for treating quantum capacitance, we find that a spin-accumulation (SA) capacitance can be introduced for each layer to measure its ability to store spins. A spatial charge storage is not essential for the SA capacitor and the energy stored in it is the splitting energy of the spin-dependent chemical potentials instead of the electrostatic energy. The SA capacitance is essentially a quantum capacitance due to spin accumulation on the scale of the spin-diffusion length. The SA capacitances can be used to reinterpret the imaginary part of the low-frequency magnetoimpedance.

cond-mat.mtrl-sci↗

arXiv2017

An alternative to the spin-coupled interface resistance for describing heat generation

Using a macroscopic approach, we studied theoretically the heat generation in a typical spin valve with nonmagnetic spacer layer of finite thickness. Our analysis shows that the spin-dependent heat generation cannot be interpreted as the Joule heating of the spin-coupled interface resistance except for some special segments. Moreover, the spin-coupled interface resistance can be negative in certain situation, and thus its "Joule heating" should be understood instead as the work done by the extra field in the ferromagnetic layers and at the spin-selective interfaces. Effective resistances are proposed as alternatives so that the spin-dependent heat generation can still be expressed in a form resembling Joule's law.

cond-mat.mtrl-sci↗

arXiv2017

Mechanisms of spin-dependent heat generation in spin valves

The extra heat generation in spin transport is usually interpreted in terms of the spin relaxation. By reformulating the heat generation rate, we found alternative current-force pairs without cross effects, which enable us to interpret the product of each pair as a distinct mechanism of heat generation. The results show that the spin-dependent part of the heat generation includes two terms. One of them is proportional to the square of the spin accumulation and arises from the spin relaxation. However, the other is proportional to the square of the spin-accumulation gradient and should be attributed to another mechanism, the spin diffusion. We illustrated the characteristics of the two mechanisms in a typical spin valve with a finite nonmagnetic spacer layer.

cond-mat.mtrl-sci↗

arXiv2017

Heat generation due to spin transport in spin valves

Using a macroscopic approach, we studied theoretically the heat generation due to spin transport in a typical spin valve with nonmagnetic spacer layer of finite thickness. Our analysis shows that the spin-dependent heat generation can also be caused by another mechanism, the spin-conserving scattering in the presence of spin accumulation gradient, in addition to the well-known spin-flip scattering. The two mechanisms have equal contributions in semi-infinite layers, such as the ferromagnetic layers of the spin valve. However, in the nonmagnetic layer of a thickness much smaller than its spin-diffusion length, the spin-dependent heat generation is dominated by the spin-flip scattering in the antiparallel configuration, and by the spin-conserving scattering in the parallel configuration. We also proved that the spin-dependent heat generation cannot be interpreted as the Joule heating of the spin-coupled interface resistance in each individual layer. An effective resistance is proposed as an alternative so that the heat generation can still be described simply by applying Joule's law to an equivalent circuit.

cond-mat.mtrl-sci↗