(research)

The spin Seebeck effect provides a powerful tool to probe spin transport by generating spin currents through thermal gradients. Using thermal tensor-network algorithm based on DMRG methods, we investigate the spin Seebeck effect in the triangular lattice quantum antiferromagnets. We focus on the low-temperature scaling behaviors of the normalized spin current across the interface. Spin frustration dramatically enhances the low-temperature spin Seebeck effect with distinct spin-current signatures, particularly the sign reversal and characteristic temperature dependence, which distinguishes different spin states. Most importantly, we discover a persistent, negative spin current in the spin supersolid phase, which saturates to a nonzero value in the low-temperature limit. This behavior can be interpreted as direct evidence of a spin supercurrent.
In addition, for the 1D Heisenberg chain, we find a negative spinon spin current in the bulk with algebraic temperature scaling. These results are consistent with field-theoretical analysis.

Y. Gao, Y. Huang, S. Maekawa and W. Li, Phys. Rev. Lett. 135, 236504 (2025)

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We simulate the time-dependent rotational, vibrational, and electronic dynamics of H2+ in an intense laser pulse using a multiconfiguration theory. The results agree well with a reference close-coupling simulation and open the possibility of time-dependent rovibronic simulations of other, larger molecules.

E. Lötstedt, T. Kato, and K. Yamanouchi, J. Phys. B: At. Mol. Opt. Phys. 57, 235602 (2024).

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Sine-square deformation (SSD) is a technique by which the local energy scale of the Hamiltonian in open-boundary systems is spatially modulated by a sine-square profile, thereby suppressing boundary effects and approximately reproducing the physics of periodic boundary conditions. In certain models, the ground state of the SSD system is known to coincide with that of the corresponding system under periodic boundary conditions. By exploiting this property, we propose a new approach for efficiently analyzing critical phenomena in quantum phase transitions. Moreover, we show that by appropriately arranging Rydberg atoms in a zigzag geometry, the SSD-modulated Hamiltonian of a model including next-nearest-neighbor interactions can be approximately realized experimentally.

Y. Miyazaki, S. Tanigawa, G. Marmorini, N. Furukawa, and D. Yamamoto,　arXiv:2512.14149.

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We consider a situation in which the entire system is described by a pure quantum state and is bipartitioned into two subsystems, A and B. When quantum entanglement exists between A and B, tracing out subsystem B results in a mixed state for subsystem A, which is described by the reduced density matrix. Although this reduced density matrix depends on the specific pure state of the total system and on the detailed structure of entanglement between A and B, its general properties are not known a priori.
In this work, we study a case where the total system is in the ground state (a pure state) of a Heisenberg model, and the bipartition into subsystems A and B covers the entire volume of the subsystem. There is an interaction between A and B, whose strength controls the degree of quantum entanglement between them. Through numerical calculations, we find that the reduced density matrix of subsystem A is well approximated by a thermal state of subsystem A at an effective temperature. This effective temperature is defined using the entanglement entropy between subsystems A and B and the energy of subsystem A, in a manner analogous to the definition of temperature in thermodynamics.

K. Seki and S. Yunoki, Phys. Rev. Research 2, 043087 (2020).

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The Dicke mapping, in which spin states are represented by superpositions of qubits states, leads to a compact representation of the Heisenberg Hamiltonian. We demonstrate the usefulness of the Dicke mapping in simulations using Quantinuum’s H1-1 trapped-ion quantum computer.

E. Lötstedt and K. Yamanouchi, Phys. Rev A 111, 062416 (2025)

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The topological superconducting state is a highly sought-after quantum state hosting topological order and Majorana excitations. In this Letter, we explore the mechanism to realize the topological superconductivity in the doped Mott insulators with time-reversal symmetry. Through large-scale DMRG study of an extended triangular-lattice 𝑡−𝐽 model, we identify a 𝑑+𝑖𝑑-wave chiral topological superconductivity with spontaneous TRS breaking, which is characterized by a Chern number 𝐶 =2 and quasi-long-range superconducting order.
We map out the quantum phase diagram with by tuning the next-nearest-neighbor electron hopping and spin interaction. In the weaker coupling regime, we identify a pseudogaplike phase with a charge stripe order coexisting with fluctuating superconductivity, which can be tuned into 𝑑-wave superconductivity by increasing the doping level and system width. The topological superconductivity emerges in the intermediate-coupling regime, which has a transition to a 𝑑-wave superconducting phase with larger NNN couplings.
The emergence of the topological superconductivity is driven by geometrical frustrations and hole dynamics which suppress spin correlation and charge order, leading to a topological quantum phase transition.

Y. Huang, S. S. Gong and D. N. Sheng, Phys. Rev. Lett. 130, 136003 (2023)

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In strongly correlated electron systems, electrons interact with each other so strongly that they no longer behave as simple, freely moving particles. As a result, the two fundamental degrees of freedom—charge and spin—become tightly coupled, giving rise to optical responses that differ markedly from those of ordinary metals or semiconductors. In particular, in Mott insulators, which do not conduct electricity under normal conditions, light irradiation can generate doublon–holon pairs. Their motion disturbs the surrounding spin order, producing dynamics characteristic of strong electron correlations.
In this process, antiferromagnetic spin correlations provide an effective attractive interaction between the photo-generated electron and hole. This leads to a characteristic peak near the absorption edge of the optical spectrum, known as an exciton. Such a feature cannot be explained by simple interband transitions; rather, it is a direct manifestation of the cooperative motion of spin and charge. By examining the optical conductivity in detail, one can probe how electrons interact and move collectively, both in energy and in time.
In recent years, non-perturbative numerical methods such as the time-dependent density matrix renormalization group (tDMRG) have made it possible to quantitatively compute the optical response of two-dimensional Hubbard models. The resulting spectra agree well with experimental measurements in cuprate Mott insulators and related materials. This agreement provides concrete insight into how the coupling between spin and charge appears in the optical response of strongly correlated systems.
Thus, the study of optical responses in strongly correlated electron systems offers a direct way to observe and understand new quantum phenomena that emerge from collective electron behavior. It not only deepens our understanding of fundamental electronic states in solids but also opens pathways toward discovering novel optoelectronic materials and quantum functionalities.

K. Shinjo, Y. Tamaki, S. Sota, T. Tohyama, Phys. Rev. B 104, 205123 (2021)

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Luttinger’s theorem is a sum rule that relates the particle number of a fermionic system to the volume in momentum space enclosed by all low-energy single-particle excitations (the Fermi surface volume). Although it is often referred to as a “theorem,” it is not an identity. Rather, whether this sum rule holds in interacting fermionic systems is closely connected to the applicability of Fermi-liquid theory and remains one of the central issues in the study of strongly correlated electron systems.
Traditionally, in interacting fermionic systems, the Fermi surface volume has been defined through the sign of the real part of the single-particle Green’s function at the Fermi level, which characterizes single-particle excitations. In this work, we reformulate the quantity corresponding to the Fermi surface volume in terms of the winding number of the determinant of the single-particle Green’s function matrix in the complex frequency plane, which is a topological quantity. This formulation clarifies its mathematical structure. By employing tools from complex analysis, such as the argument principle and Rouché’s theorem, we demonstrate that the quantity corresponding to the Fermi surface volume can be expressed as the difference between the number of poles and zeros of the Green’s function below the Fermi level, and we derive a sufficient condition for the breakdown of the sum rule.

K. Seki and S. Yunoki, Phys. Rev. B 96, 085124 (2017).

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