arXiv subjects

Gang Su

Publications and source records attributed to Gang Su.

At least 73 records · Page 4Linked to original sources

arXiv2023

Possible Intermediate Quantum Spin Liquid Phase in $α$-RuCl$_3$ under High Magnetic Fields up to 100 T

Pursuing the exotic quantum spin liquid (QSL) state in the Kitaev material $α$-RuCl$_3$ has intrigued great research interest recently. A fascinating question is on the possible existence of field-induced QSL phase in this compound. Here we perform a high-field measurement of the magnetization process of $α$-RuCl$_3$ up to 102 T employing the non-destructive and destructive pulsed magnets. Under the out-of-plane field along the c$^*$ axis (i.e., perpendicular to the honeycomb plane), two quantum phase transitions are uncovered at respectively 36 T and about 83 T, between which there lies an intermediate phase as the predicted QSL. By measuring the magnetization data with fields tilted from the c$^*$ axis up to $90^\circ$ (i.e., in-plane direction), we obtain the field-angle phase diagram that contains the zigzag, paramagnetic, and QSL phases. Based on the accurate $K$-$J$-$Γ$-$Γ^{\prime}$ model of $α$-RuCl$_3$, we perform density matrix renormalization group simulations and reproduce the quantum phase diagram in excellent agreement with experiments.

cond-mat.str-el↗

arXiv2023

Efficient Quantum Mixed-State Tomography with Unsupervised Tensor Network Machine Learning

Quantum state tomography (QST) is plagued by the ``curse of dimensionality'' due to the exponentially-scaled complexity in measurement and data post-processing. Efficient QST schemes for large-scale mixed states are currently missing. In this work, we propose an efficient and robust mixed-state tomography scheme based on the locally purified state ansatz. We demonstrate the efficiency and robustness of our scheme on various randomly initiated states with different purities. High tomography fidelity is achieved with much smaller numbers of positive-operator-valued measurement (POVM) bases than the conventional least-square (LS) method. On the superconducting quantum experimental circuit [Phys. Rev. Lett. 119, 180511 (2017)], our scheme accurately reconstructs the Greenberger-Horne-Zeilinger (GHZ) state and exhibits robustness to experimental noises. Specifically, we achieve the fidelity $F \simeq 0.92$ for the 10-qubit GHZ state with just $N_m = 500$ POVM bases, which far outperforms the fidelity $F \simeq 0.85$ by the LS method using the full $N_m = 3^{10} = 59049$ bases. Our work reveals the prospects of applying tensor network state ansatz and the machine learning approaches for efficient QST of many-body states.

quant-ph↗

arXiv2023

Family of binary transition metal pnictide superconductors

Superconductivity in transition metal nitrides (TMNs) has been investigated for a long time, such as zirconium nitride (ZrN) with a superconducting transition temperature Tc of 10 K. Recently, a phase diagram has been revealed in ZrNx with different nitrogen concentrations, which is very similar to that of high-temperature copper oxide superconductors. Here, we study the TMNs with face-centered cubic lattice, where ZrN and HfN have been experimentally obtained, and predict eight new stable superconductors by the first-principle calculations. We find that CuN has a high Tc of 30 K with a very strong electron-phonon coupling (EPC) strength. In contrast to ZrN, CuN has softening acoustic phonons at the high symmetry point L, which accounts for its much stronger EPC. In addition, the highly symmetrical structure leads to topological protected nodal points and lines, such as the hourglass Weyl loop in kx/y/z = 0 plane and Weyl points in kx/y/z = 2π/a plane, as well as quadratic band touch at Γ point. CuN could be a topological superconductor. Our results expand the transition metal nitrides superconductor family and would be helpful to guide the search for high temperature topological superconductors.

cond-mat.supr-con↗

arXiv2023

Superconducting, topological and transport properties of kagome metals CsTi$ _{3} $Bi$ _{5} $ and RbTi$ _{3} $Bi$ _{5} $

The recently discovered ATi$_3$Bi$_5$ (A=Cs, Rb) exhibit intriguing quantum phenomena including superconductivity, electronic nematicity, and abundant topological states, which provide promising platforms for studying kagome superconductivity, band topology, and charge orders. In this work, we comprehensively study various properties of ATi$_3$Bi$_5$ including superconductivity under pressure and doping, band topology under pressure, thermal conductivity, heat capacity, electrical resistance, and spin Hall conductivity (SHC) using first-principles calculations. Calculated superconducting transition temperature ($\mathrm{ T_{c}}$) of CsTi$_3$Bi$_5$ and RbTi$_3$Bi$_5$ at ambient pressure are about 1.85 and 1.92K. When subject to pressure, $\mathrm{ T_{c}}$ of CsTi$_3$Bi$_5$ exhibits a special valley and dome shape, which arises from quasi-two-dimensional to three-dimensional isotropic compression within the context of an overall decreasing trend. Furthermore, $\mathrm{ T_{c}}$ of RbTi$_3$Bi$_5$ can be effectively enhanced up to 3.09K by tuning the kagome van Hove singularities (VHSs) and flat band through doping. Pressure can also induce abundant topological surface states at the Fermi energy ($\mathrm{E}_{\mathrm{F}}$) and tune VHSs across $\mathrm{E}_{\mathrm{F}}$. Additionally, our transport calculations are in excellent agreement with recent experiments, confirming the absence of charge density wave. Notably, SHC of CsTi$_3$Bi$_5$ can reach as large as 226$ \hbar\cdot (e\cdot Ω\cdot cm) ^{-1} $ at $\mathrm{E}_{\mathrm{F}}$. Our work provides a timely and detailed analysis of the rich physical properties for ATi$_3$Bi$_5$, offering valuable insights for further explorations and understandings on these intriguing superconducting materials.

cond-mat.supr-con↗

arXiv2023

Controllable spin splitting in 2D Ferroelectric few-layer ${$γ}$-GeSe

$γ$-GeSe is a new type of layered bulk material that was recently successfully synthesized. By means of density functional theory first-principles calculations, we systematically studied the physical properties of two-dimensional (2D) few-layer $γ$-GeSe. It is found that few-layer $γ$-GeSe are semiconductors with band gaps decreasing with increasing layer number; and 2D $γ$-GeSe with layer number $n \geq 2$ are ferroelectric with rather low transition barriers, consistent with the sliding ferroelectric mechanism. Particularly, spin-orbit coupling induced spin splitting is observed at the top of the valence band, which can be switched by the ferroelectric reversal; furthermore, their negative piezoelectricity also enables the regulation of spin splitting by strain. Finally, excellent optical absorption was also revealed. These intriguing properties make 2D few-layer $γ$-GeSe promising in spintronic and optoelectric applications.

cond-mat.mtrl-sci↗

arXiv2023

Nature of the 1/9-magnetization plateau in the spin-1/2 kagome Heisenberg antiferromagnet

The nature of the 1/9-magnetization plateau of the spin-1/2 kagome Heisenberg antiferromagnet remains controversial due to the exotic physical properties and high complexity induced by the geometrical frustration. Instead of a Z3 quantum spin liquid revealed on a cylinder, we show on an infinite-size lattice that the 1/9 plateau can be described by a valence bond crystal (VBC) that breaks spatial translational invariance. Consistent results are achieved by two accurate tensor network methods, namely the full-update infinite projected-entangled pair states and the projected-entangled simplex states. The VBC exhibits an hourglass pattern with $\sqrt{3}\times\sqrt{3}$ spatial symmetry, demonstrated by magnetizations, bond energies, and three-body correlators. The spatial inversion symmetry in the $\sqrt{3}\times\sqrt{3}$ VBC is instantly broken with the presence of the difference between the coupling strengths in the up and down triangles, suggesting the existence of gapless excitations. The gapless nature of the 1/9 plateau is further indicated by the scaling behaviors of the entanglement entropy and the correlation length, which indicate a c=1 conformal field theory.

cond-mat.str-el↗

arXiv2023

Photogalvanic effect and second harmonic generation from radio to infrared region in WTe$_2$ monolayer

Second-order nonlinear optical responses, including photogalvanic effect (PGE) and second harmonic generation (SHG), are important physical phenomena in nonlinear optics. The PGE (SHG) related to linearly and circularly polarized light are called the linear and circular PGE (LPGE and CPGE) [linear and circular SHG (LSHG and CSHG)], respectively. In this work, we use the quantum kinetics under relaxation time approximation to study the dependence of second-order nonlinear optical responses on Fermi level and frequency under different out-of-plane electric fields in WTe$_2$ monolayer from radio to infrared region. We find that the maximum frequency at which the Berry curvature dipole mechanism for the nonlinear Hall effect plays a major role is about 1 THz. In radio and microwave regions, two large peaks of nonlinear conductivities occur when the Fermi level is equal to the energy corresponding to gap-opening points. In terms of frequency, in radio region, LPGE and SHG conductivities maintain a large constant while the CPGE conductivity disappears. In microwave region, LPGE and SHG start to decrease with increasing frequency while the CPGE is large. In 125-300 THz region and in y direction, the presence of DC current without the disturbance of second harmonic current under circularly polarized light may be useful for fabricating new optoelectronic devices. Moreover, we illustrate that when calculating the nonlinear optical responses of practical materials, the theories in the clean limit fail and it is necessary to use a theory that considers scattering effects. We also point out that for materials with femtosecond-scale relaxation times and complex energy band structures, the quantum kinetics is more accurate than the semi-classical Boltzmann equation method. Besides, phenomenological expressions of PGE and SHG are provided.

cond-mat.mes-hall↗

arXiv2023

Intelligent diagnostic scheme for lung cancer screening with Raman spectra data by tensor network machine learning

Artificial intelligence (AI) has brought tremendous impacts on biomedical sciences from academic researches to clinical applications, such as in biomarkers' detection and diagnosis, optimization of treatment, and identification of new therapeutic targets in drug discovery. However, the contemporary AI technologies, particularly deep machine learning (ML), severely suffer from non-interpretability, which might uncontrollably lead to incorrect predictions. Interpretability is particularly crucial to ML for clinical diagnosis as the consumers must gain necessary sense of security and trust from firm grounds or convincing interpretations. In this work, we propose a tensor-network (TN)-ML method to reliably predict lung cancer patients and their stages via screening Raman spectra data of Volatile organic compounds (VOCs) in exhaled breath, which are generally suitable as biomarkers and are considered to be an ideal way for non-invasive lung cancer screening. The prediction of TN-ML is based on the mutual distances of the breath samples mapped to the quantum Hilbert space. Thanks to the quantum probabilistic interpretation, the certainty of the predictions can be quantitatively characterized. The accuracy of the samples with high certainty is almost 100$\%$. The incorrectly-classified samples exhibit obviously lower certainty, and thus can be decipherably identified as anomalies, which will be handled by human experts to guarantee high reliability. Our work sheds light on shifting the ``AI for biomedical sciences'' from the conventional non-interpretable ML schemes to the interpretable human-ML interactive approaches, for the purpose of high accuracy and reliability.

q-bio.QM↗

arXiv2023

Field-induced Berry connection and planar Hall effect in tilted Weyl semimetals

We propose the linear and nonlinear planar Hall effect (PHE) in tilted Weyl semimetals in the presence of an in-plane magnetic and electric field, where the field-induced Berry connection (FBC) plays a key role. We show that the PHE is ascribed to the quantum metric, distinct from the well-known chiral anomaly-induced PHE arising from the Berry curvature. Using a tilting vector to describe the model, we demonstrate the constrains on the linear and nonlinear PHE by the tilting directions. The linear PHE is intrinsic that is determined by the topological properties of energy bands, whereas the nonlinear PHE is extrinsic. The predicted linear and nonlinear PHE are inherently different from others and may shed light on a deeper understanding on transport nature of the tilted Weyl semimetals.

cond-mat.mes-hall↗

arXiv2023

Dipolar Spin Liquid Ending with Quantum Critical Point in a Gd-based Triangular Magnet

By performing experiment and model studies on a triangular-lattice dipolar magnet KBaGd(BO$_3$)$_2$ (KBGB), we find the highly frustrated magnet with a planar anisotropy hosts a strongly fluctuating dipolar spin liquid (DSL), which originates from the intriguing interplay between dipolar and Heisenberg interactions. The DSL constitutes an extended regime in the field-temperature phase diagram, which gets lowered in temperature as field increases and eventually ends with an unconventional quantum critical point (QCP) at $B_c\simeq 0.75$~T. Based on dipolar Heisenberg model calculations, we identify the DSL as a Berezinskii-Kosterlitz-Thouless (BKT) phase with emergent U(1) symmetry. Due to the tremendous entropy accumulation that can be related to the strong BKT and quantum fluctuations, unprecedented magnetic cooling effects are observed in the DSL regime and particularly near the QCP, making KBGB a superior dipolar coolant to commercial Gd-based refrigerants. We establish the phase diagram for triangular-lattice dipolar quantum magnets where emergent symmetry plays an essential role, and provide a basis and opens an avenue for their applications in sub-Kelvin refrigeration.

cond-mat.str-el↗

arXiv2023

Two-dimensional Heisenberg models with materials-dependent superexchange interactions

The two-dimensional (2D) van der Waals ferromagnetic semiconductors, such as CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, and the 2D ferromagnetic metals, such as Fe$_3$GeTe$_2$ and MnSe$_2$, have been obtained in recent experiments and attracted a lot of attentions. The superexchange interaction has been suggested to dominate the magnetic interactions in these 2D magnetic systems. In the usual theoretical studies, the expression of the 2D Heisenberg models were fixed by hand due to experiences. Here, we propose a method to determine the expression of the 2D Heisenberg models by counting the possible superexchange paths with the density functional theory (DFT) and Wannier function calculations. With this method, we obtain a 2D Heisenberg model with six different nearest-neighbor exchange coupling constants for the 2D ferromagnetic metal Cr$_3$Te$_6$, which is very different for the crystal structure of Cr atoms in Cr$_3$Te$_6$. The calculated Curie temperature Tc = 328 K is close to the Tc = 344 K of 2D Cr$_3$Te$_6$ reported in recent experiment. In addition, we predict two stable 2D ferromagnetic semiconductors Cr$_3$O$_6$ and Mn$_3$O$_6$ sharing the same crystal structure of Cr$_3$Te$_6$. The similar Heisenberg models are obtained for 2D Cr$_3$O$_6$ and Mn$_3$O$_6$, where the calculated Tc is 218 K and 208 K, respectively. Our method offers a general approach to determine the expression of Heisenberg models for these 2D magnetic semiconductors and metals, and builds up a solid basis for further studies.

cond-mat.mtrl-sci↗

arXiv2022

Observation of Flat Band, Dirac Nodal Lines and Topological Surface States in Kagome Superconductor CsTi$_3$Bi$_5$

A kagome lattice of 3d transition metals hosts flat bands, Dirac fermions and saddle points. It provides a versatile platform for achieving topological superconductivity, anomalous Hall effect, unconventional density wave order and quantum spin liquid when the strong correlation, spin-orbit coupling or magnetic order are involved in such a lattice. Here, using laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations, we investigate the electronic structure of the newly discovered kagome superconductor CsTi$_3$Bi$_5$, which is isostructural to the AV$_3$Sb$_5$ (A=K, Rb or Cs) kagome superconductors and possesses a perfect two-dimensional kagome network of Titanium. We directly observed a strikingly flat band derived from the local destructive interferences of Bloch wave functions within the kagome lattices. We also identify the type-II Dirac nodal loops around the Brillouin zone center, the type-III Dirac nodal loops around the zone corners and type-III Dirac nodal lines along the k$_z$ direction. In addition, around the Brillouin zone center, Z2 nontrivial topological surface states are also observed which is formed from the band inversion due to strong spin orbital coupling. The simultaneous existence of such multi-sets of nontrivial band structures in one kagome superconductor not only provides good opportunities to study related physics in the kagome lattice but also makes CsTi$_3$Bi$_5$ an ideal system to realize noval quantum phenomena by manipulating its chemical potential with chemical doping or pressure.

cond-mat.supr-con↗

arXiv2022

Superconductivity and orbital-selective nematic order in a new titanium-based kagome metal CsTi3Bi5

Fabrication of new types of superconductors with novel physical properties has always been a major thread in the research of superconducting materials. An example is the enormous interests generated by the cascade of correlated topological quantum states in the newly discovered vanadium-based kagome superconductors AV3Sb5 (A=K, Rb, and Cs) with a Z2 topological band structure. Here we report the successful fabrication of single-crystals of titanium-based kagome metal CsTi3Bi5 and the observation of superconductivity and electronic nematicity. The onset of the superconducting transition temperature Tc is around 4.8 K. In sharp contrast to the charge density wave superconductor AV3Sb5, we find that the kagome superconductor CsTi3Bi5 preserves translation symmetry, but breaks rotational symmetry and exhibits an electronic nematicity. The angular-dependent magnetoresistivity shows a remarkable two-fold rotational symmetry as the magnetic field rotates in the kagome plane. The scanning tunneling microscopy and spectroscopic imaging detect rotational-symmetry breaking C2 quasiparticle interference patterns (QPI) at low energies, providing further microscopic evidence for electronic nematicity. Combined with first-principle calculations, we find that the nematic QPI is orbital-selective and dominated by the Ti dxz and dyz orbitals, possibly originating from the intriguing orbital bond nematic order. Our findings in the new "135" material CsTi3Bi5 provide new directions for exploring the multi-orbital correlation effect and the role of orbital or bond order in the electron liquid crystal phases evidenced by the symmetry breaking states in kagome superconductors.

cond-mat.supr-con↗

arXiv2022

Topological superconductivity and large spin Hall effect in the kagome family Ti6X4 (X = Bi, Sb, Pb, Tl, In)

Topological superconductors (TSC) become a focus of research due to the accompanying Majorana fermions. However, the experimentally reported TSC are extremely rare. The recent experiments reported the kagome TSC AV$_{3}$Sb$_{5}$ (A=K, Rb, Cs), which exhibit unique superconductivity, topological surface states (TSS), and Majorana bound states. More recently, the first titanium-based kagome superconductor CsTi$ _{3} $Bi$ _{5} $ with nontrivial topology was successfully synthesized as a perspective TSC. Given that Cs contributes little to the electronic structures of CsTi$ _{3} $Bi$ _{5} $ and binary compounds may be easier to be synthesized, here, by density functional theory calculations, we predict five stable non-magnetic kagome compounds Ti$ _{6} $X$ _{4} $ (X = Bi, Sb, Pb, Tl, In) which exhibit superconductivity with critical temperature Tc = 3.8$ - $5.1 K, nontrivial $\mathbb{Z}$$_2$ band topology, and TSS close to the Fermi level. In addition, the large intrinsic spin Hall effect is obtained in Ti$ _{6} $X$ _{4} $, which is caused by gapped Dirac nodes and nodal lines due to a strong spin-orbit coupling. This work offers new platforms for TSC and spintronic devices.

cond-mat.supr-con↗

arXiv2022

High-field quantum spin liquid transitions and angle-field phase diagram of Kitaev magnet $α$-RuCl$_3$

The pursuit of quantum spin liquid (QSL) in the Kitaev honeycomb magnets has drawn intensive attention recently. In particular, $α$-RuCl$_3$ has been widely recognized as a promising candidate for the Kitaev QSL. Although the compound exhibits an antiferromagnetic order under zero field, it is believed to be endowed with fractionalized excitations, and can be driven to the QSL phase by magnetic fields. Here, based on a realistic $K$-$J$-$Γ$-$Γ'$ model for $α$-RuCl$_3$ [1], we exploit the exponential tensor renormalization group approach to explore the phase diagram of the compound under magnetic fields. We calculate the thermodynamic quantities, including the specific heat, Grüneisen parameter, magnetic torque, and the magnetotropic susceptibility, etc, under a magnetic field with a tilting angle $θ$ to the $c^*$-axis perpendicular to the honeycomb plane. We find an extended QSL in the angle-field phase diagram determined with thermodynamic responses. The gapless nature of such field-induced QSL is identified from the specific heat and entropy data computed down to very low temperatures. The present study provides guidance for future high-field experiments for the QSL in $α$-RuCl$_3$ and other candidate Kitaev magnets.

cond-mat.str-el↗

arXiv2022

Large kagome family candidates with topological superconductivity and charge density waves

A group of newly discovered non-magnetic metal kagome structures AV3Sb5 (A=K, Rb, Cs) have aroused widespread interest in experiment and theory due to their unusual charge density wave (CDW) and intertwined superconductivity. However, they all possess weak electron-phonon coupling (EPC) and low superconducting transition temperature. Here, we performed high-throughput first-principles calculations on novel kagome candidates with AV3Sb5 prototype structure, and proposed 24 dynamically stable novel kagome metals. The calculation based on Bardeen-Cooper-Schrieffer theory shows that most of these metals are superconductors with much stronger EPC than the reported AV3Sb5 materials, and their superconducting transition temperatures Tc is between 0.3 and 5.0K. Additionally, several compounds, such as KZr3Pb5 with the highest Tc, are identified as Z2 topological metals with clear Dirac cone topological surface states near Fermi level. NaZr3As5 is shown to have possible CDW phases. Our results provide rich platforms for exploring various new physics with the prototype kagome structure, in which the coexistence of superconductivity and nontrivial topological nature provides promising insights on the discovery of topological superconductors.

cond-mat.supr-con↗

arXiv2022

Symmetry Dictionary on Charge and Spin Nonlinear Responses for All Magnetic Point Groups with Nontrivial Topological Nature

Recently, charge or spin nonlinear transport with nontrivial topological properties in crystal materials has attracted much attention. In this paper, we perform a comprehensive symmetry analysis for all 122 magnetic point groups (MPGs) and provide a useful dictionary for charge and spin nonlinear transport from Berry curvature dipole, Berry connection polarization and Drude term with nontrivial topological nature. The results are obtained by making a full symmetry investigation on matrix representations of six nonlinear response tensors. We further identify every MPG that can accommodate two or three of the nonlinear tensors. The present work gives a solid theoretical basis for overall understanding the second-order nonlinear responses in realistic materials.

cond-mat.mes-hall↗

arXiv2022

Titanium-based kagome superconductor CsTi_3Bi_5 and topological states

Since the discovery of a new family of vanadium-based kagome superconductor AV3Sb5 (A=K, Rb, and Cs) with topological band structures, extensive effort has been devoted to exploring the origin of superconducting states and the intertwined orders. Meanwhile, searching for new types of superconductors with novel physical properties and higher superconducting transition temperatures has always been a major thread in the history of superconductor research. Here we report a successful fabrication and the topological states of a Titanium-based kagome metal CsTi3Bi5 (CT3B5) crystal. The as-grown CT3B5 crystal is of high quality and possesses a perfect two-dimensional kagome net of Titanium. The superconductivity of the CT3B5 crystal shows that the critical temperature Tc is of ~4.8 K. First-principle calculations predict that the CT3B5 has robust topological surface states, implying that CT3B5 is a Z2 topological kagome superconductor. This finding provides a new type of superconductors and the base for exploring the origin of superconductivity and topological states in kagome superconductors.

cond-mat.supr-con↗