Majorana Corner Modes and Flat-Band Majorana Edge Modes in Superconductor/Topological-Insulator/Superconductor Junctions

Recently,superconductors with higher-order topology have stimulated extensive attention and research interest.Higher-order topological superconductors exhibit unconventional bulk-boundary correspondence,thus allow exotic lower-dimensional boundary modes,such as Majorana corner and hinge modes.However,higher-order topological superconductivity has yet to be found in naturally occurring materials.We investigate higher-order topology in a two-dimensional Josephson junction comprised of two s-wave superconductors separated by a topological insulator thin film.We find that zero-energy Majorana corner modes,a boundary fingerprint of higherorder topological superconductivity,can be achieved by applying magnetic field.When an in-plane Zeeman field is applied to the system,two corner modes appear in the superconducting junction.Furthermore,we also discover a two-dimensional nodal superconducting phase which supports flat-band Majorana edge modes connecting the bulk nodes.Importantly,we demonstrate that zero-energy Majorana corner modes are stable when increasing the thickness of topological insulator thin film.

Holographic insulator/superconductor phase transitions with excited states

We construct a family of solutions of the holographic insulator/superconductor phase transitions with the excited states in the AdS soliton background by using both the numerical and analytical methods. The interesting point is that the improved SturmLiouville method can not only analytically investigate the properties of the phase transition with the excited states, but also the distributions of the condensed fields in the vicinity of the critical point. We observe that, regardless of the type of the holographic model, the excited state has a higher critical chemical potential than the corresponding ground state, and the difference of the dimensionless critical chemical potential between the consecutive states is around 2.4, which is different from the finding of the metal/superconductor phase transition in the Ad S black hole background. Furthermore, near the critical point, we find that the phase transition of the systems is of the second order and a linear relationship exists between the charge density and chemical potential for all the excited states in both s-wave and p-wave insulator/superconductor models.

Tunneling Conductance in Normal Metal/Insulator/Triplet Superconductor Junction

Tunneling conductance in normal metal/insulator/triplet superconductor junctions is studied theoretically as a function of the bias voltage at zero temperature and finite temperature. The results show there are zero-bias conductance peak, zero-bias conductance dip and double-minimum structures in the spectra for p-wave superconductor junctions. The existence of such structures in the conductance spectrum can be taken as evidence that the pairing symmetry of Sr2RuO4 is p-wave symmetry.

Higher-order topological Anderson insulator on the Sierpiński lattice

Disorder effects on topological materials in integer dimensions have been extensively explored in recent years. However, its influence on topological systems in fractional dimensions remains unclear. Here, we investigate the disorder effects on a fractal system constructed on the Sierpiński lattice in fractional dimensions. The system supports the second-order topological insulator phase characterized by a quantized quadrupole moment and the normal insulator phase. We find that the second-order topological insulator phase on the Sierpiński lattice is robust against weak disorder but suppressed by strong disorder. Most interestingly, we find that disorder can transform the normal insulator phase to the second-order topological insulator phase with an emergent quantized quadrupole moment. Finally, the disorder-induced phase is further confirmed by calculating the energy spectrum and the corresponding probability distributions.

Ultrafast dynamics in photo-excited Mott insulator Sr_(3)Ir_(2)O_7 at high pressure

High-pressure ultrafast dynamics,as a new crossed research direction,are sensitive to subtle non-equilibrium state changes that might be unresolved by equilibrium states measurements,providing crucial information for studying delicate phase transitions caused by complex interactions in Mott insulators.With time-resolved transient reflectivity measurements,we identified the new phases in the spin–orbit Mott insulator Sr_(3)Ir_(2)O_7 at 300 K that was previously unidentified using conventional approaches such as x-ray diffraction.Significant pressure-dependent variation of the amplitude and lifetime obtained by fitting the reflectivity?R/R reveal the changes of electronic structure caused by lattice distortions,and reflect the critical phenomena of phase transitions.Our findings demonstrate the importance of ultrafast nonequilibrium dynamics under extreme conditions for understanding the phase transition of Mott insulators.

Data augmentation method for insulators based on Cycle-GAN

Data augmentation is an important task of using existing data to expand data sets.Using generative countermeasure network technology to realize data augmentation has the advantages of high-quality generated samples,simple training,and fewer restrictions on the number of generated samples.However,in the field of transmission line insulator images,the freely synthesized samples are prone to produce fuzzy backgrounds and disordered samples of the main insulator features.To solve the above problems,this paper uses the cycle generative adversarial network(Cycle-GAN)used for domain conversion in the generation countermeasure network as the initial framework and uses the self-attention mechanism and channel attention mechanism to assist the conversion to realize the mutual conversion of different insulator samples.The attention module with prior knowledge is used to build the generation countermeasure network,and the generative adversarial network(GAN)model with local controllable generation is built to realize the directional generation of insulator belt defect samples.The experimental results show that the samples obtained by this method are improved in a number of quality indicators,and the quality effect of the samples obtained is excellent,which has a reference value for the data expansion of insulator images.

Optical study of magnetic topological insulator MnBi_(4)Te_7

We present an infrared spectroscopy study of the magnetic topological insulator MnBi_(4)Te_7 with antiferromagnetic(AFM) order below the Neel temperature TN= 13 K. Our investigation reveals that the low-frequency optical conductivity consists of two Drude peaks, indicating a response of free carriers involving multiple bands. Interestingly, the narrow Drude peak grows strongly as the temperature decreases, while the broad Drude peak remains relatively unchanged. The onset of interband transitions starts around 2000 cm^(-1), followed by two prominent absorption peaks around 10000 cm^(-1) and 20000 cm^(-1). Upon cooling, there is a notable transfer of spectral weight from the interband transitions to the Drude response. Below TN, the AFM transition gives rise to small anomalies of the charge response due to a band reconstruction.These findings provide valuable insights into the interplay between magnetism and the electronic properties in MnBi_(4)Te_7.

Three-dimensional topological crystalline insulator without spin-orbit coupling in nonsymmorphic photonic metacrystal

By including certain point group symmetry in the classification of band topology,Fu proposed a class of threedimensionaltopological crystalline insulators(TCIs)without spin-orbit coupling in 2011.In Fu’s model,surface states(ifpresent)doubly degenerate atГandM when time-reversal and C_(4) symmetries are preserved.The analogs of Fu’s modelwith surface states quadratically degenerate atM are widely studied,while surface states with quadratic degeneracy atГare rarely reported.In this study,we propose a three-dimensional TCI without spin-orbit coupling in a judiciously designednonsymmorphic photonic metacrystal.The surface states of photonic TCIs exhibit quadratic band degeneracy in the(001)surface Brillouin zone(BZ)center(Гpoint).The gapless surface states and their quadratic dispersion are protected by C4and time-reversal symmetries,which correspond to the nontrivial band topology characterized by Z_(2)topological invariant.Moreover,the surface states along lines fromГto the(001)surface BZ boundary exhibit zigzag feature,which is interpretedfrom symmetry perspective by building composite operators constructed by the product of glide symmetries with timereversalsymmetry.The metacrystal array surrounded with air possesses high order hinge states with electric fields highlylocalized at the hinge that may apply to optical sensors.The gapless surface states and hinge states reside in a cleanfrequency bandgap.The topological surface states emerge at the boundary of the metacrystal and perfect electric conductor(PEC),which provide a pathway for topologically manipulating light propagation in photonic devices.

A Simple and Effective Surface Defect Detection Method of Power Line Insulators for Difficult Small Objects

Insulator defect detection plays a vital role in maintaining the secure operation of power systems.To address the issues of the difficulty of detecting small objects and missing objects due to the small scale,variable scale,and fuzzy edge morphology of insulator defects,we construct an insulator dataset with 1600 samples containing flashovers and breakages.Then a simple and effective surface defect detection method of power line insulators for difficult small objects is proposed.Firstly,a high-resolution featuremap is introduced and a small object prediction layer is added so that the model can detect tiny objects.Secondly,a simplified adaptive spatial feature fusion(SASFF)module is introduced to perform cross-scale spatial fusion to improve adaptability to variable multi-scale features.Finally,we propose an enhanced deformable attention mechanism(EDAM)module.By integrating a gating activation function,the model is further inspired to learn a small number of critical sampling points near reference points.And the module can improve the perception of object morphology.The experimental results indicate that concerning the dataset of flashover and breakage defects,this method improves the performance of YOLOv5,YOLOv7,and YOLOv8.In practical application,it can simply and effectively improve the precision of power line insulator defect detection and reduce missing detection for difficult small objects.

Evolution from the Parent Mott Insulator to a Superconductor in Lightly Hole-Doped Bi2Sr2CaCu2O8+δ

High temperature superconductivity in cuprates is realized by doping the Mott insulator with charge carriers.A central issue is how such an insulating state can evolve into a conducting or superconducting state when charge carriers are introduced.Here,by in situ vacuum annealing and Rb deposition on the Bi2Sr2Ca0.6Dy0.4Cu2O8+δ(Bi2212)sample surface to push its doping level continuously from deeply underdoped(Tc=25K,doping level p^0.066)to the near-zero doping parent Mott insulator,angle-resolved photoemission spectroscopy measurements are carried out to observe the detailed electronic structure evolution in the lightly hole-doped region for the first time.Our results indicate that the chemical potential lies at about l eV above the charge transfer band for the parent state at zero doping,which is quite close to the upper Hubbard band.With increasing hole doping,the chemical potential moves continuously towards the charge transfer band and the band structure evolution exhibits a rigid band shift-like behavior.When the chemical potential approaches the charge transfer band at a doping level of^0.05,the nodal spectral weight near the Fermi level increases,followed by the emergence of the coherent quasiparticle peak and the insulator-superconductor transition.Our observations provide key insights in understanding the insulator-superconductor transition in doping the parent cuprate compound and for establishing related theories.

Quantitative determination of the critical points of Mott metal–insulator transition in strongly correlated systems

Mottness is at the heart of the essential physics in a strongly correlated system as many novel quantum phenomena occur in the metallic phase near the Mott metal–insulator transition. We investigate the Mott transition in a Hubbard model by using the dynamical mean-field theory and introduce the local quantum state fidelity to depict the Mott metal–insulator transition. The local quantum state fidelity provides a convenient approach to determining the critical point of the Mott transition. Additionally, it presents a consistent description of the two distinct forms of the Mott transition points.

Tunneling Conductance in Quantum-Wire/Ferromagnetic-Insulator/d-Wave Superconductor Junction

We have studied the quasiparticle transport in quantum-wire /ferromagnetic-insulator/d wave super- conductor Junction （q/FI/d） in the framework of the Blonder-Tinkham-Klapwijk model. We calculate the tunneling conductance in q/FI/d as a function of the bias voltage at zero temperature and finite temperature based on Bogoliubov- de Gennes equations. Different from the case in normal-metal/insulator/d wave superconductor Junctions, the zero-bias conductance peaks vanish for the single-mode case. The tunneling conductance spectra depend on the magnitude of the exchange interaction at the ferromagnetic-insulator.

dc Josephson Effect in s-Wave Superconductor/Ferromagnet Insulator/p-Wave Superconductor Junctions

The Josephson currents in s-wave superconductor/ferromagnet insulator/p-wave superconductor（s/FI/p） junctions are calculated as a function of temperature and the phase taking into account the roughness scattering effect at interface. The phase dependence of the Josephson current I （φ） between s-wave and px-wave superconductor is predicted to be sin（2φ）. The ferromagnet scattering effect, the barrier strength, and the roughness strength at interface suppress the dc currents in s/FI/p junction.

Differential conductance in a normal metal/insulator/metal/d-wave superconductor junction carrying a supercurrent

This paper applies the Bogoliubov-de Gennes equation and the Blonder-Tinkham-Klapwijk approach to study the oscillatory behaviour of differential conductance in a normal metal/insulator/metal/d-wave superconductor junction carrying a supercurrent Is. We find that （i） a three-humped structure appears at a nearly critical supercurrent Is and z ≈ 0.5 for the normal metal/insulator/metal/dx^2＋y^2-wave superconductor junction; （ii） the zero-bias conductance peak splits into two peaks with sufficiently large applied current for the normal metal/insulator/metal/dxy-wave superconductor junction; （iii） the conductance spectrum exhibits oscillating behaviour with the bias voltage and the peaks of the resonances are suppressed by increasing supercurrent Is.

Tunneling conductance in quantum wire/insulator/d_(x2-y2) + id_(xy) mixed wave superconductor junctions

Using the extended Blonder-Tinkham-Klapwijk （BTK） theory, this paper calculates the tunnelling conductance in quantum wire/insulator/dx2-y2 ＋ idly mixed wave superconductor （q/I/dx2-y2 ＋ idly） junctions. That is different from the case in d- and p-wave superconductor junctions. When the angle α between a-axis of the dx2-y2 wave superconductor and the interface normal is π/4, there follows a rather distinctive tunnelling conductance. The zero-bias conductance peak （ZBCP） may or may not appear in the tunnelling conductance. Both the interface potential z and the quasi-particle lifetime factor F are smaller, there is no ZBCP. Otherwise, the ZBCP will appear. The position of bias conductance peak （BCP） depends strongly on the amplitude ratio of two components for dx2-y2 ＋ idxy mixed wave. The low and narrow ZBCP may coexist with the BCP in the tunnelling conductance. Using those features in the tunnelling conductance of q/I/dx2-y2 ＋ idxy junctions, it can distinguish dx2-y2 ＋ idxy mixed wave superconductor from d- and p-wave one.

Suppression of Andreev conductance in a topological insulator–superconductor nanostep junction

When two three-dimensional topological insulators （TIs） are brought close to each other with their surfaces aligned, the surfaces form a line junction. Similarly, three TI surfaces, not lying in a single plane, can form an atomic-scale nanostep junction. In this paper, Andreev reflection in a TI-TI-superconductor nanostep junction is investigated theoretically. Be- cause of the existence of edge states along each line junction, the conductance for a nanostep junction is suppressed. When the incident energy （e） of an electron is larger than the superconductor gap （A）, the Andreev conductance in a step junction is less than unity while for a plane junction it is unity. The Andreev conductance is found to depend on the height of the step junction. The Andreev conductance exhibits oscillatory behavior as a function of the junction height with the amplitude of the oscillations remaining unchanged when e = 0, but decreasing for e = A, which is different from the case of the plane junction. The height of the step is therefore an important parameter for Andreev reflection in nanostep junctions, and plays a role similar to that of the delta potential barrier in normal metal-superconductor plane junctions.

Josephson Current in Superconductor-Ferromagnet／Insulator／d-Wave Superconductor Junctions

Solving the Bogoliubov-de Gennes equation, the energy levels of bound statesare obtained in the ferromagnetic superconductor. The Josephson currents in a ferromagneticsuperconductor/Insulator/d-wave superconductor junction are calculated as a function of the exchangefield, temperature, and insulating barrier strength. It is found that the Josephson criticalcurrent is always suppressed by the presence of exchange Geld h and depends on crystalline axisorientation of d-wave superconductor.

Heat Transport in Graphene Ferromagnet-Insulator-Superconductor Junctions

We study heat transport in a graphene ferromagnet-insulator-superconducting junction.It is found that the thermal conductance of the graphene ferromagnet-insulator-superconductor(FIS)junction is an oscillatory function of the barrier strength X in the thin-barrier limit.The gate potential Uo decreases the amplitude of thermal conductance oscillation.Both the amplitude and phase of the thermal conductance oscillation varies with the exchange energy Eh.The thermal conductance of a graphene FIS junction displays the usual exponential dependence on temperature,reflecting the s-wave symmetry of superconducting graphene.

Holographic Insulator/Superconductor Phase Transition in Born–Infeld Electrodynamics

We study holographic insulator/superconductor phase transition in the framework of Born-Infeld electrodynamics both numerically and analytically. First we numerically study the effects of the Born-Infeld electrodynamics on the phase transition, find that when the Born-Infeld parameter increases, the critical chemical potential keeps invariant but the gap frequency becomes larger. Then we employ the variational method for the Sturm-Liouville eigenvalue problem to study the phase transition analytically. The analytical results obtained are found to be consistent with the numerical results.

Nonequilibrium Effect in Ferromagnet-Insulator-Superconductor Tunneling Junction Currents

Nonequilibrium effect due to the imbalance in the number of the ? and ? spin electrons has been studied for the tunneling currents in the ferromagnet-insulator-superconductor (FIS) tunneling junctions within a phenomenological manner. It has been stated how the nonequilibrium effect should be observed in the spin-polarized quasiparticle tunneling currents, and pointed out that the detectable nonequilibrium effect could be found in the FIS tunneling junction at 77 K using HgBa2Ca2Cu3O8+? (Hg-1223) high-Tc superconductor rather than Bi2Sr2CaCu2O8+? (Bi-2212) one.