Normal and Superconducting Properties of La_(3)Ni_(2)O_(7)

This review provides a comprehensive overview of current research on the structural,electronic,and magnetic characteristics of the recently discovered high-temperature superconductor La_(3)Ni_(2)O_(7) under high pressures.We present the experimental results for synthesizing and characterizing this material,derived from measurements of transport,thermodynamics,and various spectroscopic techniques,and discuss their physical implications.We also explore theoretical models proposed to describe the electronic structures and superconducting pairing symmetry in La_(3)Ni_(2)O_(7),highlighting the intricate interplay between electronic correlations and magnetic interactions.Despite these advances,challenges remain in growing high-quality samples free of extrinsic phases and oxygen deficiencies and in developing reliable measurement tools for determining diamagnetism and other physical quantities under high pressures.Further investigations in these areas are essential to deepening our understanding of the physical properties of La_(3)Ni_(2)O_(7) and unlocking its superconducting pairing mechanism.

Superconductivity and Charge Density Wave in Iodine-Doped CuIr_(2)Te_(4)

We report a systematic investigation on the evolution of the structural and physical properties,including the charge density wave(CDW) and superconductivity of the polycrystalline CuIr_(2)Te_(4-x)Ix for 0.0 ≤x≤ 1.0.Xray diffraction results indicate that both of a and c lattice parameters increase linearly when 0.0 ≤ x ≤ 1.0.The resistivity measurements indicate that the CDW is destabilized with slight x but reappears at x≥0.9 with very high TCDW.Meanwhile,the superconducting transition temperature Tc enhances as x increases and reaches a maximum value of around 2.95 K for the optimal composition CuIr_(2)Te_(1.9)I_(0.1) followed by a slight decrease with higher iodine doping content.The specific heat jump(ΔC/γTc) for the optimal composition CuIr_(2)Te_(3.9)I_(0.1) is approximately 1.46,which is close to the Bardeen-Cooper-Schrieffer value of 1.43,indicating that it is a bulk superconductor.The results of thermodynamic heat capacity measurements under different magnetic fields |Cp(T,H)],magnetization M(T,H) and magneto-transport ρ(T,H) measurements further suggest that CuIr_(2)Te_(4-x)Ix bulks are type-Ⅱ superconductors.Finally,an electronic phase diagram for this CuIr_(2)Te_(4-x)Ix system has been constructed.The present study provides a suitable material platform for further investigation of the interplay of the CDW and superconductivity.

Superconductivity in octagraphene

This article reviews the basic theoretical aspects of octagraphene, an one-atom-thick allotrope of carbon, with unusual two-dimensional(2 D) Fermi nesting, hoping to contribute to the new family of quantum materials. Octagraphene has an almost strongest sp^(2)hybrid bond similar to graphene, and has the similar electronic band structure as iron-based superconductors, which makes it possible to realize high-temperature superconductivity. We have compared various possible mechanisms of superconductivity, including the unconventional s;superconductivity driven by spin fluctuation and conventional superconductivity based on electron–phonon coupling. Theoretical studies have shown that octagraphene has relatively high structural stability. Although many 2 D carbon materials with C;carbon ring and C;carbon ring structures have been reported, it is still challenging to realize the octagraphene with pure square-octagon structure experimentally.This material holds hope to realize new 2 D high-temperature superconductivity.

Magnetic excitations of diagonally coupled checkerboards

By using quantum Monte Carlo based stochastic analytic continuation(QMC-SAC)and spin wave theory,we study magnetic excitations of Heisenberg models with diagonally coupled checkerboard structures.We consider three kinds of checkerboard models(DC 2×2,DC 3×3,and CDC 3×3)consisting nearest-neighbor strong J1 and weak J2 antiferromagnetic interactions.When the coupling ratio g=J2/J1 approaches 1,all three diagonal checkerboards have the same long-range antiferromagnetic Neel order at´T=0.When g decreases,the quantum fluctuation can drive DC 2×2 model to quantum paramagnetic state,while DC 3×3 and CDC 3×3 models still have the long-range Neel order.By calculating´the magnetic excitations at different coupling ratios,we find that the low-energy part of magnetic excitations calculated by QMC-SAC can be well explained by the spin wave theory.However,the high-energy parts even deep in the long-range antiferromagnetic phase are beyond the spin wave description.Compared to the g=1 uniform square lattice,the high-energy excitations are more rich in our models.Our study may also draw the attention to the high-energy exctitaions beyond the spin wave theory.

Superexchange and charge transfer in the nickelate superconductor La_(3)Ni_(2)O_(7) under pressure

Recently, a bulk nickelate superconductor La_(3)Ni_(2)O_(7) is discovered at pressures with a remarkable high transition temperature T_(c)~ 80 K. Here, we study a Hubbard model with tight-binding parameters derived from ab initio calculations of La_(3)Ni_(2)O_(7),by employing large scale determinant quantum Monte Carlo and cellular dynamical mean-field theory. Our result suggests that the superexchange couplings in this system are comparable to that of cuprates. The system is a charge transfer insulator as the hole concentration becomes four per site at large Hubbard U. Upon hole doping, two low-energy spin-singlet bands emerge in the system exhibiting distinct correlation properties: while the one composed of the out-of-plane Ni-d_(3z^(2)-r^(2)) and O-p_(z) orbitals demonstrates strong antiferromagnetic correlations and narrow effective bandwidth, the in-plane singlet band consisting of the Ni-d_(x^(2)-y^(2)) and O-p_(x)/p_(y) orbitals is in general more itinerant. Over a broad range of hole doping, the doped holes occupy primarily the d_(x^(2)-y^(2)) and p_(x)/p_(y) orbitals, whereas the d_(3z^(2)-r^(2)) and p_(z) orbitals retain underdoped. We propose an effective t-J model to capture the relevant physics and discuss the implications of our result for comprehending the La_(3)Ni_(2)O_(7) superconductivity.

Structural transition,electric transport,and electronic structures in the compressed trilayer nickelate La_(4)Ni_(3)O_(10)

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Ruddlesden-Popper nickelate La_(3)Ni_(2)O_(7), the atomic structure and electronic band structure of the trilayer nickelate La_(4)Ni_(3)O_(10) under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P2_(1)/a space group to the tetragonal I4/mmm around 12.6-13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni 3d_(z^(2)) orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La_(4)Ni_(3)O_(10). The trilayer nickelate La_(4)Ni_(3)O_(10) shows some similarities with the bilayer La_(3)Ni_(2)O_(7) and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates.

A spin-rotation mechanism of Einstein-de Haas effect based on a ferromagnetic disk

Spin-rotation coupling(SRC)is a fundamental interaction that connects electronic spins with the rotational motion of a medium.We elucidate the Einstein-de Haas(EdH)effect and its inverse with SRC as the microscopic mechanism using the dynamic spin-lattice equations derived by elasticity theory and Lagrangian formalism.By applying the coupling equations to an iron disk in a magnetic field,we exhibit the transfer of angular momentum and energy between spins and lattice,with or without damping.The timescale of the angular momentum transfer from spins to the entire lattice is estimated by our theory to be on the order of 0.01 ns,for the disk with a radius of 100 nm.Moreover,we discover a linear relationship between the magnetic field strength and the rotation frequency,which is also enhanced by a higher ratio of Young’s modulus to Poisson’s coefficient.In the presence of damping,we notice that the spin-lattice relaxation time is nearly inversely proportional to the magnetic field.Our explorations will contribute to a better understanding of the EdH effect and provide valuable insights for magneto-mechanical manufacturing.

Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2

Vanadium dichalcogenides have attracted increasing interests for the charge density wave phenomena and possible ferromagnetism.Here,we report on the multiphase behavior and gap opening in monolayer VTe2 grown by molecular beam epitaxy.Scanning tunneling microscopy(STM)and spectroscopy study revealed the(4×4)metallic and gapped(2√3×2√3)charge-density wave(CDW)phases with an energy gap of≈40 meV.Through the in-plane condensation of vanadium atoms,the typical star-of-David clusters and truncated triangle-shaped clusters are formed in the(4×4)and(2√3×2√3)phases respectively,resulting in different surface morphologies and electronic structures as confirmed by density functional theory(DFT)calculations with on-site Coulomb repulsion.The CDW-driven reorganization of the atomic structure weakens the ferromagnetic superexchange coupling and strengthens the antiferromagnetic exchange coupling on the contrary,suppressing the long-range magnetic order in monolayer VTe2.The electron correlation is found to be important to explain the gap opening in the(2√3×2√3)phase.

Tunable nano Peltier cooling device from geometric effects using a single graphene nanoribbon

Based on the phenomenon of curvature-induced doping in graphene we propose a class of Peltier cooling devices, produced by geometrical effects, without gating. We show how a graphene nanorib- bon laid on an array of curved nano cylinders can be used to create a targeted and tunable cooling device. Using two different approaches, the Nonequilibrium Green＇s Function （NEGF） method and experimental inputs, we predict that the cooling kW/cm2, on par with the best known techniques power of such a device can approach the order of using standard superlattice structures. The structure proposed here helps pave the way toward designing graphene electronics which use geometry rather than gating to control devices.

A semiclassical approach to surface Fermi arcs in Weyl semimetals

We present a semiclassical explanation for the morphology of the surface Fermi arcs of Weyl semimetals.Viewing the surface states as a two-dimensional Fermi gas subject to band bending and Berry curvatures,we show that it is the non-parallelism between the velocity and the momentum that gives rise to the spiral structure of Fermi arcs.We map out the Fermi arcs from the velocity field for a single Weyl point and a lattice with two Weyl points.We also investigate the surface magnetoplasma of Dirac semimetals in a magnetic field,and find that the drift motion,the chiral magnetic effect and the Imbert-Fedorov shift are all involved in the formation of surface Fermi arcs.Our work not only provides an insightful perspective on the surface Fermi arcs and a practical way to find the surface dispersion,but also paves the way for the study of other physical properties of the surface states of topological semimetals,such as transport properties and orbital magnetization,using semiclassical methods.

Extremely strong coupling s-wave superconductivity in the medium-entropy alloy TiHfNbTa

Here we report a TiHfNbTa bulk medium-entropy alloy(MEA)superconductor crystallized in the body-centered cubic structure with the unit cell parameter a=3.35925?,which is synthesized by an arc melting method.Superconducting properties of the TiHfNbTa are studied by employing magnetic susceptibility,resistivity,and specific heat measurements.Experimental results show a bulk superconducting transition temperature(Tc)of around 6.75 K.The lower and upper critical fields for TiHfNbTa are45.8 m T and 10.46 T,respectively.First-principles calculations show that the d electrons of Ti,Hf,Nb,and Ta are the main contribution to the total density of states near the Fermi level.Our results indicate that the superconductivity is a conventional swave type with extremely strong coupling(△C_(el)/γ_(n)T_(c)=2.88,2△_(0)/k_(B)T_(c)=5.02,and λ_(ep)=2.77).The extremely strong coupling behavior in the s-wave type Ti Hf Nb Ta MEA superconductor is unusual because it generally happens in cuprates,pnictides,and other unconventional superconductors.

Dynamical properties of the Haldane chain with bond disorder

By using Lanczos exact diagonalization and quantum Monte Carlo combined with stochastic analytic continuation,we study the dynamical properties of the S=1 antiferromagnetic Heisenberg chain with different strengths of bond disorder.In the weak disorder region,we find weakly coupled bonds which can induce additional low-energy excitation below the one-magnon mode.As the disorder increases,the average Haldane gap closes at δ_(∆)~0.5 with more and more low-energy excitations coming out.After the critical disorder strength δ_(C)~1,the system reaches a random-singlet phase with prominent sharp peak atω=0 and broad continuum atω>0 of the dynamic spin structure factor.In addition,we analyze the distribution of random spin domains and numerically find three kinds of domains hosting effective spin-1/2 quanta or spin-1 sites in between.These“spins”can form the weakly coupled longrange singlets due to quantum fluctuation which contribute to the sharp peak atω=0.

Magnetism variation of the compressed antiferromagnetic topological insulator EuSn_(2)As_(2)

We report a comprehensive high-pressure study,up to 21.1 GPa,on the antiferromagnetic topological insulator EuSn_(2)As_(2) achieved through synchrotron X-ray diffraction,Raman scattering,electrical resistance,magnetic resistance,and Hall transport measurements in combination with first-principles calculations.The Néel temperatures determined from resistance are increased from(24±1)to(77±8)K under pressure,which is a result of enhanced magnetic exchange couplings between Eu^(2+) ions yielded by our first-principles calculations.The negative magnetoresistance of EuSn_(2)As_(2) persists to higher temperatures accordingly.However,the enhancement of the observed Néel temperatures deviates from the calculations above 10.0 GPa.In addition,the magnitude of the magnetoresistance,Hall coefficients,and charge carrier densities show abrupt changes between 6.9 and 10.0 GPa.The abrupt changes likely originate from a pressure-induced valence change of Eu ions from a divalent state to a divalent and trivalent mixed state or are related to the structural transition revealed by Raman scattering measurements.Our results provide insight into magnetism variation in EuSn_(2)As_(2) and similar antiferromagnetic topological insulators under pressure.

Magnetic order driven by orbital ordering in the semiconducting KFe1.5Se2

The two-orbital Hubbard model is studied numerically by using the Hartree-Fock approximation in both real space and momentum space, and the ground-state properties of the alkali metal iron selenide semiconducting KFel.5Se2 are investigated. A rhombus-type Fe vacancy order with stripe- type antiferromagnetic （AFM） order is found, as was observed in neutron scattering experiments [J. Zhao, et al., Phys. Rev. Lett. 109, 267003 （2012）]. Hopping parameters are obtained by fitting the experimentally observed stripe AFM phase in real space. These hopping parameters are then used to study the ground-state properties of the semiconductor in momentum space. It is found to be a strongly correlated system with a large on-site Coulomb repulsion U, similar to the AFM Mort insulator -- the parent compound of copper oxide superconductors. We also find that the electronic occupation numbers and magnetizations in the dxz and dyz orbitals become different simultaneously when U 〉 Uc （～3.4 eV）, indicating orbital ordering. These results imply that the rotational symmetry between the two orbitals is broken by orbital ordering and thus drives the strong anisotropy of the magnetic coupling that has been observed by experiments and that the stripe-type AFM order in this compound may be caused by orbital ordering together with the observed large anisotropy.

Structure and magnetic properties of the S=3/2 zigzag spin chain antiferromagnet BaCoTe_(2)O_(7)

We report a study of the structure and magnetic properties of the S=3/2 zigzag spin chain compound BaCoTe_(2)O_(7).Neutron diffraction measurements show that it crystallizes in the noncentrosymmetric space group Ama2 with a canted↑↑↓↓spin structure along the quasi-one-dimensional zigzag chain and a moment size of 1.89(2)μBat 2 K.Both magnetic susceptibility and specific heat measurements yield an antiferromagnetic phase transition at TN=6.2 K.A negative Curie-Weiss temperature,ΘCW=-74.7(2) K,and an empirical frustration parameter,f=|ΘCW|/TN≈12,are obtained by fitting the magnetic susceptibility,indicating antiferromagnetic interactions and strong magnetic frustration.From ultraviolet-visible absorption spectroscopy and first-principles calculations,an indirect band gap of 2.68(2) eV is determined.We propose that the canted zigzag spin chain of BaCoTe_(2)O_(7) may produce a change in the polarization via the exchange-striction mechanism.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor- Metal Transition in Two-Dimensional AgRuO_(3)

The intricate correlation between charge degrees of freedom and physical properties is a fascinating area of research in solid state chemistry and condensed matter physics.Herein,we report on the pressureinduced successive charge transfer and accompanied resistive evolution in honeycomb layered ruthenate AgRuO_(3).Structural revisiting and spectroscopic analyses affirm the ilmenite type R-3 structure with mixed valence cations as Ag^(+1/+2)Ru^(+4/+5)O_(3) at ambient pressure.In-situ pressure-and temperature-dependent resistance variation reveals a successive insulatormetal-insulator transition upon pressing,accompanied by unprecedented charge transfer between Ag and Ru under applied pressure,and a further structural phase transition in the insulator region at higher pressure.These phenomena are also corroborated by in-situ pressure-dependent Raman spectra,synchrotron X-ray diffraction,bond valence sums,and electronic structure calculations,emphasizing the dominated rare Ag2+,and near zero thermal expansion in the ab-plane in the metallic zone mostly due to the Jahn-Teller effect of d9-Ag2+.The multiple electronic instabilities in AgRuO_(3) may offer new possibilities toward novel and unconventionally physical and chemical behaviors in strongly correlated honeycomb lattices.

Iron-based superconductors： A new family to find the origin of high Tc superconductivity

Since the discovery of iron-based superconductors in 2008 [1], a new tide of study on high Tc superconductors spreads worldwide quickly. After a few years＇ intensive study, many new compounds of iron-based superconductors have been found and their properties have been disclosed. The great achievement is attributed to the modern experimental techniques, fast developing numerical methods and improved theories during the study of cuprate superconductors or more generally strongly correlated electron systems. For instance, the Fermi surface,