Finite-Size Scaling Theory at a Self-Dual Quantum Critical Point

The nondivergence of the generalized Gr¨uneisen ratio(GR)at a quantum critical point(QCP)has been proposed to be a universal thermodynamic signature of self-duality.We study how the Kramers–Wannier-type self-duality manifests itself in the finite-size scaling behavior of thermodynamic quantities in the quantum critical regime.While the self-duality cannot be realized as a unitary transformation in the total Hilbert space for the Hamiltonian with the periodic boundary condition,it can be implemented in certain symmetry sectors with proper boundary conditions.Therefore,the GR and the transverse magnetization of the one-dimensional transversefield Ising model exhibit different finite-size scaling behaviors in different sectors.This implies that the numerical diagnosis of self-dual QCP requires identification of the proper symmetry sectors.

Lowest-energy moiré band formed by Dirac zero modes in twisted bilayer graphene

The recent discovery of an unconventional insulating phase and an adjacent superconducting (SC) phase in the twisted bilayer graphene (TBG)[1,2] has triggered great excitement (3–11)The two layers of graphene are rotated relatively by an angle θ.

High temperature magnetic semiconductors: narrow band gaps and two-dimensional systems

Magnetic semiconductors have been demonstrated to work at low temperatures, but not yet at room temperature for spin electronic applications. In contrast to the p-type diluted magnetic semiconductors, n-type diluted magnetic semiconductors are few. Using a combined method of the density function theory and quantum Monte Carlo simulation, we briefly discuss the recent progress to obtain diluted magnetic semiconductors with both p- and n-type carriers by choosing host semiconductors with a narrow band gap. In addition, the recent progress on two-dimensional intrinsic magnetic semiconductors with possible room temperature ferromangetism and quantum anomalous Hall effect are also discussed.

Possible Room-Temperature Ferromagnetic Semiconductors

Magnetic semiconductors integrate the dual characteristics of magnets and semiconductors.It is difficult to manufacture magnetic semiconductors that function at room temperature.Here,we review a series of our recent theoretical predictions on room-temperature ferromagnetic semiconductors.Since the creation of two-dimensional(2D)magnetic semiconductors in 2017,there have been numerous developments in both experimental and theoretical investigations.By density functional theory calculations and model analysis,we recently predicted several2D room-temperature magnetic semiconductors,including CrGeSe_(3)with strain,CrGeTe_(3)/PtSe_(2) heterostructure,and technetium-based semiconductors(TcSiTe_(3),TcGeSe_(3),and TcGeTe_(3)),as well as PdBr_(3)and PtBr_(3)with a potential room-temperature quantum anomalous Hall effect.Our findings demonstrated that the Curie temperature of these 2D ferromagnetic semiconductors can be dramatically enhanced by some external fields,such as strain,construction of heterostructure,and electric field.In addition,we proposed appropriate doping conditions for diluted magnetic semiconductors,and predicted the Cr doped GaSb and InSb as possible room-temperature magnetic semiconductors.

Fluctuating magnetic droplets immersed in a sea of quantum spin liquid

The search of quantum spin liquid(QSL),an exotic magnetic state with strongly fluctuating and highly entangled spins down to zero temperature,is a main theme in current condensed matter physics.However,there is no smoking gun evidence for deconfined spinons in any QSL candidate so far.The disorders and competing exchange interactions may prevent the formation of an ideal QSL state on frustrated spin lattices.Here we report comprehensive and systematic measurements of the magnetic susceptibility,ultralow-temperature specific heat,muon spin relaxation(μSR),nuclear magnetic resonance(NMR),and thermal conductivity for NaYbSe2 single crystals,in which Yb3+ions with effective spin-1/2 form a perfect triangular lattice.All these complementary techniques find no evidence of long-range magnetic order down to their respective base temperatures.Instead,specific heat,μSR,and NMR measurements suggest the coexistence of quasi-static and dynamic spins in NaYbSe2.The scattering from these quasi-static spins may cause the absence of magnetic thermal conductivity.Thus,we propose a scenario of fluctuating ferrimagnetic droplets immersed in a sea of QSL.This may be quite common on the way pursuing an ideal QSL,and provides a brand new platform to study how a QSL state survives impurities and coexists with other magnetically ordered states.

Possible quantum paraelectric state in Kitaev spin liquid candidate H3LiIr2O6

A new quantum spin liquid(QSL)candidate material H3LiIr2O6 was synthesized recently and was found not to show any magnetic order or phase transition down to low temperatures.In this work,we study the quantum dynamics of the hydrogen ions,i.e.,protons,in this material by combining first-principles calculations and theoretical analysis.We show that each proton and its adjacent oxygen ions form an electric dipole.The dipole interactions and the proton tunneling are captured by a transverse-field Ising model with a quantum disordered paraelectric ground state.The dipole excitations have an energy gap△d=60 meV,and can be probed by the infrared optical spectroscopy and the dielectric response.We argue that the electric dipole fluctuations renormalize the magnetic interactions in H3LiIr2O6 and lead to a Kitaev QSL state.

Thermodynamics of spin-1/2 Kagomé Heisenberg antiferromagnet:algebraic paramagnetic liquid and finite-temperature phase diagram

Quantum fluctuations from frustration can trigger quantum spin liquids(QSLs) at zero temperature.However, it is unclear how thermal fluctuations affect a QSL. We employ state-of-the-art tensor network-based methods to explore the ground state and thermodynamic properties of the spin-1=2 kagomé Heisenberg antiferromagnet(KHA). Its ground state is shown to be consistent with a gapless QSL by observing the absence of zero-magnetization plateau as well as the algebraic behaviors of susceptibility and specific heat at low temperatures, respectively. We show that there exists an algebraic paramagnetic liquid(APL) that possesses both the paramagnetic properties and the algebraic behaviors inherited from the QSL. The APL is induced under the interplay between quantum fluctuations from geometrical frustration and thermal fluctuations. By studying the temperature-dependent behaviors of specific heat and magnetic susceptibility, a finite-temperature phase diagram in a magnetic field is suggested, where various phases are identified. This present study gains useful insight into the thermodynamic properties of the spin-1/2 KHA with or without a magnetic field and is helpful for relevant experimental studies.