Observation of In-Gap States in a Two-Dimensional CrI/NbSe Heterostructure.

Low-dimensional magnetic structures coupled with superconductors are promising platforms for realizing Majorana zero modes, which have potential applications in topological quantum computing. Here, we report a two-dimensional (2D) magnetic-superconducting heterostructure consisting of single-layer chromium diiodide (CrI) on a niobium diselenide (NbSe) superconductor. Single-layer CrI nanosheets, which hold antiferromagnetic (AFM) ground states by our first-principles calculations, were epitaxially grown on the layered NbSe substrate.

Measuring the Boundary Gapless State and Criticality via Disorder Operator.

The disorder operator is often designed to reveal the conformal field theory (CFT) information in quantum many-body systems. By using large-scale quantum Monte Carlo simulation, we study the scaling behavior of disorder operators on the boundary in the two-dimensional Heisenberg model on the square-octagon lattice with gapless topological edge state. In the Affleck-Kennedy-Lieb-Tasaki phase, the disorder operator is shown to hold the perimeter scaling with a logarithmic term associated with the Luttinger liquid parameter K.

Orbital-dependent electron correlation in double-layer nickelate LaNiO.

The latest discovery of high temperature superconductivity near 80 K in LaNiO under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking.

Spin-degree manipulation for one-dimensional room-temperature ferromagnetism in a haldane system.

The intricate correlation between lattice geometry, topological behavior and charge degrees of freedom plays a key role in determining the physical and chemical properties of a quantum-magnetic system. Herein, we investigate the introduction of the unusual oxidation state as an alternative pathway to modulate the magnetic ground state in the well-known = 1 Haldane system nickelate YBaNiO (YBNO). YBNO is topologically reduced to incorporate d-Ni ( = 1/2) in the one-dimensional Haldane chain system.

Superconductivity in the High-Entropy Ceramics Ti Zr Nb Mo Ta C with Possible Nontrivial Band Topology.

Topological superconductors have drawn significant interest from the scientific community due to the accompanying Majorana fermions. Here, the discovery of electronic structure and superconductivity (SC) in high-entropy ceramics Ti Zr Nb Mo Ta C (x = 1 and 0.8) combined with experiments and first-principles calculations is reported.

Bilayer Two-Orbital Model of La_{3}Ni_{2}O_{7} under Pressure.

The newly discovered Ruddlesden-Popper bilayer La_{3}Ni_{2}O_{7} reaches a remarkable superconducting transition temperature T_{c}≈80  K under a pressure of above 14 GPa. Here we propose a minimal bilayer two-orbital model of the high-pressure phase of La_{3}Ni_{2}O_{7}. Our model is constructed with the Ni-3d_{x^{2}-y^{2}}, 3d_{3z^{2}-r^{2}} orbitals by using Wannier downfolding of the density functional theory calculations, which captures the key ingredients of the material, such as band structure and Fermi surface topology.

Superconductivity determined by the S-H framework in CH4-inserted S-H framework hydrides under high pressures.

Recently, a debate is raising the concern of possible carbonaceous sulfur hydrides with room-temperature superconductivity around 270 GPa. In order to systematically investigate the structural information and relevant natures of C-S-H superconductors, we performed an extremely extensive structure search and first-principles calculations under high pressures. As a result, the metastable stoichiometries of CSH7, C2SH14, CS2H10, and CS2H11 were unveiled under high pressure, which can be viewed as CH4 units inserted into the S-H framework.

Interplay between Charge-Density-Wave, Superconductivity, and Ferromagnetism in CuIrCrTe Chalcogenides.

We report the crystal structure, charge-density-wave (CDW), superconductivity (SC), and ferromagnetism (FM) in CuIrCrTe (0 ≤ ≤ 2) chalcogenides. Powder x-ray diffraction (PXRD) results reveal that the CuIrCrTe series are distinguished between two structural types and three different regions: (i) layered trigonal structure region, (ii) mixed phase regions, and (iii) spinel structure region. Besides, Cr substitution for Ir site results in rich physical properties including the collapse of CDW, the formation of dome-shaped like SC, and the emergence of magnetism.

NbSeTe-a new layered transition metal dichalcogenide superconductor.

Transition metal dichalcogenides (TMDCs) usually exhibit layered polytypic structures due to the weak interlayer coupling. 2H-NbSe is one of the most widely studied in the pristine TMDC family due to its high superconducting transition temperature (T   =  7.3 K) and the occurrence of a charge-density wave (CDW) order below 33 K.

Detecting Crystallographic Lattice Chirality using Resonant Inelastic X-ray Scattering.

The control and detection of crystallographic chirality is an important and challenging scientific problem. Chirality has wide ranging implications from medical physics to cosmology including an intimate but subtle connection in magnetic systems, for example MnFeSi. X-ray diffraction techniques with resonant or polarized variations of the experimental setup are currently utilized to characterize lattice chirality.

Topological phase transition induced by magnetic proximity effect in two dimensions.

We study the magnetic proximity effect on a two-dimensional topological insulator in a CrI/SnI/CrI trilayer structure. From first-principles calculations, the BiI-type SnI monolayer without spin-orbit coupling has Dirac cones at the corners of the hexagonal Brillouin zone. With spin-orbit coupling turned on, it becomes a topological insulator, as revealed by a non-vanishing Z invariant and an effective model from symmetry considerations.

Stabilities and novel electronic structures of three carbon nitride bilayers.

We predict three novel phases of the carbon nitride (CN) bilayer, denoted α-CN, β-CN and γ-CN, respectively. All of them consist of two CN sheets connected by C-C covalent bonds. The phonon dispersions reveal that all these phases are dynamically stable, because no imaginary frequency is present.

Gradual enhancement of stripe-type antiferromagnetism in the spin-ladder material BaFeS under pressure.

We report pressure-dependent neutron diffraction and muon spin relaxation/rotation measurements combined with first-principles calculations to investigate the structural, magnetic, and electronic properties of BaFeS under pressure. The experimental results reveal a gradual enhancement of the stripe-type ordering temperature with increasing pressure up to 2.6 GPa and no observable change in the size of the ordered moment.

Anomalous Quantum-Critical Scaling Corrections in Two-Dimensional Antiferromagnets.

We study the Néel-paramagnetic quantum phase transition in two-dimensional dimerized S=1/2 Heisenberg antiferromagnets using finite-size scaling of quantum Monte Carlo data. We resolve the long-standing issue of the role of cubic interactions arising in the bond-operator representation when the dimer pattern lacks a certain symmetry. We find nonmonotonic (monotonic) size dependence in the staggered (columnar) dimerized model, where cubic interactions are (are not) present.

XXZ-Ising model on the triangular kagome lattice with spin 1 on the decorated trimers.

We consider the triangular kagome XXZ-Ising model (TKL XXZ-Ising model) formed by inserting small triangles ("a-trimers") with XXZ spin-1 inside the triangles of the kagome lattice ("b-trimers"). It is a mixed spin system and can be solved exactly by transforming into the kagome lattice with the general transformation method for decorated spin systems. In the absence of an external field, we integrate out the quantum spins of the a-trimers and map the TKL model to the kagome Ising model exactly.

Spatial Rabi oscillations between Majorana bound states and quantum dots.

A Majorana bound state is a superconducting quasiparticle that is the superposition of particle and hole with equal amplitude. We propose a verification of this amplitude equality by analyzing the spatial Rabi oscillations of the quantum states of a quantum dot that is tunneling-coupled to the Majorana bound states. We find two resonant Rabi driving energies that correspond to the energy splitting due to the coupling of two spatially separated Majorana bound states.

Electronic Structure and Band Gap Engineering of Two-Dimensional Octagon-Nitrogene.

A new phase of nitrogen with octagon structure has been predicted in our previous study, which we referred to as octagon-nitrogene (ON). In this work, we make further investigations of its stability and electronic structures. The phonon dispersion has no imaginary phonon modes, which indicates that ON is dynamically stable.

Indirect K-edge bimagnon resonant inelastic x-ray scattering spectrum of α-FeTe.

We calculate the K-edge indirect bimagnon resonant inelastic x-ray scattering (RIXS) intensity spectra of the bicollinear antiferromagnetic order known to occur in the α-FeTe chalcogenide system. Utilizing linear spin wave theory for this large-S spin system we find that the bimagnon spectrum contains four scattering channels (two intraband and two interband). We find from our calculations that for suitable energy-momentum combination the RIXS spectra can exhibit a one-, two- or three- peak structure.

Band Gap Engineering of Two-Dimensional Nitrogene.

In our previous study, we have predicted the novel two-dimensional honeycomb monolayers of pnictogen. In particular, the structure and properties of the honeycomb monolayer of nitrogen, which we call nitrogene, are very unusual. In this paper, we make an in-depth investigation of its electronic structure.

Itinerancy-enhanced quantum fluctuation of magnetic moments in iron-based superconductors.

We investigate the influence of itinerant carriers on the dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly, against the common lore, instead of enhancing the (π,0) order, itinerant carriers with well-nested Fermi surfaces are found to induce a significant amount of spatial and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be an intrapocket nesting-associated long-range coupling rather than the previously believed ferromagnetic double-exchange effect.

Viewing Majorana Bound States by Rabi Oscillations.

We propose to use Rabi oscillation as a probe to view the fractional Josepshon relation (FJR) associated with Majorana bound states (MBSs) expected in one-dimensional topological superconductors. The system consists of a quantum dot (QD) and an rf-SQUID with MBSs at the Josephson junction. Rabi oscillations between energy levels formed by MBSs are induced by ac gate voltage controlling the coupling between QD and MBS when the photon energy proportional to the ac frequency matches gap between quantum levels formed by MBSs and QD.

Two-Dimensional Pnictogen Honeycomb Lattice: Structure, On-Site Spin-Orbit Coupling and Spin Polarization.

Because of its novel physical properties, two-dimensional materials have attracted great attention. From first-principle calculations and vibration frequencies analysis, we predict a new family of two-dimensional materials based on the idea of octet stability: honeycomb lattices of pnictogens (N, P, As, Sb, Bi). The buckled structures of materials come from the sp(3) hybridization.

Linear alkane C-C bond chemistry mediated by metal surfaces.

Linear alkanes undergo different C-C bond chemistry (coupling or dissociation) thermally activated on anisotropic metal surfaces depending on the choice of the substrate material. Owing to the one-dimensional geometrical constraint, selective dehydrogenation and C-C coupling (polymerization) of linear alkanes take place on Au(110) surfaces with missing-row reconstruction. However, the case is dramatically different on Pt(110) surfaces, which exhibit similar reconstruction as Au(110).