arXiv subjects

Zheng Gai

Publications and source records attributed to Zheng Gai.

35 records · Page 2Linked to original sources

arXiv2020

Realizing gapped surface states in magnetic topological insulator MnBi$_{2-x}$Sb$_{x}$Te$_{4}$

The interplay between magnetism and non-trivial topology in magnetic topological insulators (MTI) is expected to give rise to a variety of exotic topological quantum phenomena, such as the quantum anomalous Hall (QAH) effect and the topological axion states. A key to assessing these novel properties is to tune the Fermi level in the exchange gap of the Dirac surface band. MnBi$_2$Te$_4$ possesses non-trivial band topology with intrinsic antiferromagnetic (AFM) state that can enable all of these quantum states, however, highly electron-doped nature of the MnBi$_2$Te$_4$ crystals obstructs the exhibition of the gapped topological surface states. Here, we tailor the material through Sb-substitution to reveal the gapped surface states in MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ (MBST). By shifting the Fermi level into the bulk band gap of MBST, we access the surface states and show a band gap of 50 meV at the Dirac point from quasi-particle interference (QPI) measured by scanning tunneling microscopy/spectroscopy (STM/STS). Surface-dominant conduction is confirmed below the Néel temperature through transport spectroscopy measured by multiprobe STM. The surface band gap is robust against out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. The realization of bulk-insulating MTI with the large exchange gap offers a promising platform for exploring emergent topological phenomena.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic anisotropy in single crystal high entropy perovskite oxide La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 films

Local configurational disorder can have a dominating role in the formation of macroscopic functional responses in strongly correlated materials. Here, we use entropy-stabilization synthesis to create single crystal epitaxial ABO3 perovskite thin films with equal atomic concentration of 3d transition metal cations on the B-site sublattice. X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy of La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 (L5BO) films demonstrate excellent crystallinity, smooth film surfaces, and uniform mixing of the 3d transition metal cations throughout the B-site sublattice. The magnetic properties are strongly dependent on substrate-induced lattice anisotropy and suggest the presence of long-range magnetic order in these exceptionally disordered materials. The ability to populate multiple elements onto a single sublattice in complex crystal structures opens new possibilities to design functionality in correlated systems and enable novel fundamental studies seeking to understand how diverse local bonding environments can work to generate macroscopic responses, such as those driven by electron-phonon channels and complex exchange interaction pathways.

cond-mat.str-el↗

arXiv2019

Room-temperature insulating ferromagnetic (Ni,Co)1+2xTi1-xO3 thin films

Insulating uniaxial room-temperature ferromagnets are a prerequisite for commonplace spin wave-based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. We show that the uniaxial $A^{1+2x}$Ti$^{4+}$$_{1-x}$O$_3$ (ATO), $A=$Ni$^{2+}$,Co$^{2+}$ and $0.6<x \leq 1$, thin films are electrically insulating ferromagnets already at room-temperature. The octahedra network of the ATO and ilmenite structures are similar yet different octahedra-filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra can continuously be filled up to $x=1$, or vacated $(-0.24<x<0)$ in the ATO structure. TiO-layers, which separate the ferromagnetic (Ni,Co)O-layers and intermediate the antiferromagnetic coupling between the ferromagnetic layers in the NiTiO$_3$ and CoTiO$_3$ ilmenites, can continuously be replaced by (Ni,Co)O-layers to convert the ATO-films to ferromagnetic insulator with abundant direct cation interactions.

cond-mat.mtrl-sci↗

arXiv2019

An Air-Stable and Atomically Thin Graphene/Gallium Superconducting Heterostructure

Two-dimensional layered and atomically thin elemental superconductors may be key ingredients in next-generation quantum technologies, if they can be stabilized and integrated into heterostructured devices under ambient conditions. However, atomically thin elemental superconductors are largely unexplored outside ultra-high vacuum due to rapid oxidation, and even 2D layered superconductors require complex encapsulation strategies to maintain material quality. Here we demonstrate environmentally stable, single-crystal, few-atom-thick superconducting gallium, 2D-Ga, produced by confinement heteroepitaxy (CHet) at the interface of epitaxial graphene (EG) and silicon carbide (SiC). 2D-Ga becomes superconducting at 4 K; this elevation over bulk alpha-Ga (Tc~1 K) is primarily attributed to an increased density of states at the Fermi level as the incipient Ga-Ga dimerization seen in alpha-Ga is suppressed by epitaxy to SiC. We also demonstrate the importance of controlling SiC surface morphology (i.e. step height) and defect-engineering in graphene layers prior to intercalation to achieve large-area uniform 2D-Ga layers with isotropic transport properties. This work demonstrates that unique 2D forms of 3D materials can be stabilized at the EG/SiC interface, which represents a scalable route towards air-stable crystalline 2D superconductors as a potential foundation for next-generation quantum technologies.

cond-mat.supr-con↗

arXiv2019

Lattice Disorder Effect on Magnetic Ordering of Iron Arsenides

This study investigates the changes of magnetic ordering temperature via nano- and mesoscale structural features in an iron arsenide. Although magnetic ground states in quantum materials can be theoretically predicted from known crystal structures and chemical compositions, the ordering temperature is harder to pinpoint due to such local lattice variations. In this work we find surprisingly that a locally disordered material can exhibit a significantly larger Neel temperature (TN) than an ordered material of precisely the same chemical stoichiometry. Here, a EuFe2As2 crystal, which is a 122 parent of iron arsenide superconductors, is found through synthesis to have ordering below TN = 195 K (for the disordered crystal) or TN = 175 K (for the ordered crystal). In the higher TN crystals, there are shorter planar Fe-Fe bonds [2.7692(2) A vs. 2.7745(3) A], a randomized in-plane defect structure, and diffuse scattering along the [00L] crystallographic direction that manifests as a rather broad specific heat peak. For the lower TN crystals, the a-lattice parameter is larger and the in-plane microscopic structure shows defect ordering along the antiphase boundaries, giving a larger TN and a higher superconducting temperature (Tc) upon the application of pressure. First principles calculations find a strong interaction between c-axis strain and interlayer magnetic coupling, but little impact of planar strain on the magnetic order. Neutron single-crystal diffraction shows that the low-temperature magnetic phase transition due to localized Eu moments is not lattice or disorder sensitive, unlike the higher-temperature Fe sublattice ordering. This study demonstrates a higher magnetic ordering point arising from local disorder in 122.

cond-mat.supr-con↗

arXiv2017

Effect of surface morphology and magnetic impurities on the electronic structure in cobalt-doped BaFe2As2 superconductors

Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show that low-temperature-cleaved optimally-doped Ba(Fe1-xCox)2As2 crystals with x=0.06, with Tc = 22 K, have complicated morphologies. Although the cleavage surface and hence the morphologies are variable, the superconducting gap maps show the same gap widths and nanometer size inhomogeneities irrelevant to the morphology. Based on the spectroscopy and LBH maps, the bright patches and dark stripes in the morphologies are identified as Ba and As dominated surface terminations, respectively. Magnetic impurities, possibly due to cobalt or Fe atoms, are believed to create local in-gap state and in addition suppress the superconducting coherence peaks. This study will clarify the confusion on the cleavage surface terminations of the Fe-based superconductors, and its relation with the electronic structures.

cond-mat.supr-con↗

arXiv2015

Strain Doping: Reversible Single-Axis Control of a Complex Oxide Lattice via Helium Implantation

We report on the use of helium ion implantation to independently control the out-of-plane lattice constant in epitaxial La0.7Sr0.3MnO3 thin films without changing the in-plane lattice constants. The process is reversible by a vacuum anneal. Resistance and magnetization measurements show that even a small increase in the out-of-plane lattice constant of less than 1% can shift the metal-insulator transition and Curie temperatures by more than 100 °C. Unlike conventional epitaxy-based strain tuning methods which are constrained not only by the Poisson effect but by the limited set of available substrates, the present study shows that strain can be independently and continuously controlled along a single axis. This permits novel control over orbital populations through Jahn-Teller effects, as shown by Monte Carlo simulations on a double-exchange model. The ability to reversibly control a single lattice parameter substantially broadens the phase space for experimental exploration of predictive models and leads to new possibilities for control over materials' functional properties.

cond-mat.str-el↗

arXiv2014

Active control of magnetoresistance of organic spin valves using ferroelectricity

Organic spintronic devices have been appealing because of the long spin life time of the charge carriers in the organic materials and their low cost, flexibility and chemical diversity. In previous studies, the control of resistance of organic spin valves is generally achieved by the alignment of the magnetization directions of the two ferromagnetic electrodes, generating magnetoresistance.1 Here we employ a new knob to tune the resistance of organic spin valves by adding a thin ferroelectric interfacial layer between the ferromagnetic electrode and the organic spacer. We show that the resistance can be controlled by not only the spin alignment of the two ferromagnetic electrodes, but also by the electric polarization of the interfacial ferroelectric layer: the MR of the spin valve depends strongly on the history of the bias voltage which is correlated with the polarization of the ferroelectric layer; the MR even changes sign when the electric polarization of the ferroelectric layer is reversed. This new tunability can be understood in terms of the change of relative energy level alignment between ferromagnetic electrode and the organic spacer caused by the electric dipole moment of the ferroelectric layer. These findings enable active control of resistance using both electric and magnetic fields, opening up possibility for multi-state organic spin valves and shed light on the mechanism of the spin transport in organic spin valves.

cond-mat.mtrl-sci↗

arXiv2014

Persistent metal-insulator transition at the surface of an oxygen-deficient, epitaxial manganite film

The oxygen stoichiometry has a large influence on the physical and chemical properties of complex oxides. Most of the functionality in e.g. catalysis and electrochemistry depends in particular on control of the oxygen stoichiometry. In order to understand the fundamental properties of intrinsic surfaces of oxygen-deficient complex oxides, we report on in situ temperature dependent scanning tunnelling spectroscopy experiments on pristine oxygen deficient, epitaxial manganite films. Although these films are insulating in subsequent ex situ in-plane electronic transport experiments at all temperatures, in situ scanning tunnelling spectroscopic data reveal that the surface of these films exhibits a metal-insulator transition (MIT) at 120 K, coincident with the onset of ferromagnetic ordering of small clusters in the bulk of the oxygen-deficient film. The surprising proximity of the surface MIT transition temperature of nonstoichiometric films with that of the fully oxygenated bulk suggests that the electronic properties in the surface region are not significantly affected by oxygen deficiency in the bulk. This carries important implications for the understanding and functional design of complex oxides and their interfaces with specific electronic properties for catalysis, oxide electronics and electrochemistry.

cond-mat.mtrl-sci↗

arXiv2013

Electrophoretic-like gating used to control metal-insulator transitions in electronically phase separated manganite wires

Electronically phase separated manganite wires are found to exhibit controllable metal-insulator transitions under local electric fields. The switching characteristics are shown to be fully reversible, polarity independent, and highly resistant to thermal breakdown caused by repeated cycling. It is further demonstrated that multiple discrete resistive states can be accessed in a single wire. The results conform to a phenomenological model in which the inherent nanoscale insulating and metallic domains are rearranged through electrophoretic-like processes to open and close percolation channels.

cond-mat.mes-hall↗

arXiv2013

Growth diagram of La0.7Sr0.3MnO3 thin films using pulsed laser deposition

An experimental study was conducted on controlling the growth mode of La0.7Sr0.3MnO3 thin films on SrTiO3 substrates using pulsed laser deposition (PLD) by tuning growth temperature, pressure and laser fluence. Different thin film morphology, crystallinity and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the adatom nucleation, step advance processes and their relationship to film growth were theoretically analyzed and a growth diagram was constructed. Three boundaries between highly and poorly crystallized growth, 2D and 3D growth, stoichiometric and non-stoichiometric growth were identified in the growth diagram. A good fit of our experimental observation with the growth diagram was found. This case study demonstrates that a more comprehensive understanding of the growth mode in PLD is possible.

cond-mat.mtrl-sci↗

arXiv2013

Room-temperature multiferroic hexagonal LuFeO$_3$ films

The crystal and magnetic structures of single-crystalline hexagonal LuFeO$_3$ films have been studied using x-ray, electron and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO$_3$ films are room-temperature multiferroics.

cond-mat.mtrl-sci↗

arXiv2013

Structural and magnetic properties in the quantum S=1/2 dimer systems Ba3(Cr1-xVx)2O8 with site disorder

We report a comprehensive study of dc susceptibility, specific heat, neutron diffraction, and inelastic neutron scattering measurements on polycrystalline Ba3(Cr1-xVx)2O8 samples, where x=0, 0.06, 0.15, and 0.53. A Jahn-Teller structure transition occurs for x=0, 0.06, and 0.15 samples and the transition temperature is reduced upon vanadium substitution from 70(2) K at x=0 to 60(2) K at x=0.06 and 0.15. The structure becomes less distorted as x increases and such transition disappears at x=0.53. The observed magnetic excitation spectrum indicates that the singlet ground state remains unaltered and spin gap energy Δ=1.3(1) meV is identical within the instrument resolution for all x. In addition, the dispersion bandwidth W decreases with increase of x. At x=0.53, W is reduced to 1.4(1) meV from 2.0(1) meV at x=0.

cond-mat.mtrl-sci↗

arXiv2013

Magnetism and electronic structure of La2ZnIrO6 and La2MgIrO6: candidate Jeff =1/2 Mott insulators

We study experimentally and theoretically the electronic and magnetic properties of two insulating double perovskites that show similar atomic and electronic structure, but different magnetic properties. In magnetization measurements, La2ZnIrO6 displays weak ferromagnetic behavior below 7.5 K whereas La2MgIrO6 shows antiferromagnetic behavior (AFM) below TN = 12 K. Electronic structure calculations find that the weak ferromagnetic behavior observed in La2ZnIrO6 is in fact due to canted antiferromagnetism. The calculations also predict canted antiferromagnetic behavior in La2MgIrO6, but intriguingly this was not observed. Neutron diffraction measurements confirm the essentially antiferromagnetic behavior of both systems, but lack the sensitivity to resolve the small (0.22 μB/Ir) ferromagnetic component in La2ZnIrO6. Overall, the results presented here indicate the crucial role of spin-orbit coupling (SOC) and the on-site Coulomb repulsion on the magnetic, transport, and thermodynamic properties of both compounds. The electronic structure calculations show that both compounds, like Sr2IrO4, are Jeff = 1/2 Mott insulators. Our present findings suggest that La2ZnIrO6 and La2MgIrO6 provide a new playground to study the interplay between SOC and on-site Coulomb repulsion in a 5d transition metal oxide.

cond-mat.str-el↗

arXiv2013

Microscopic and Spectroscopic Evidence for a Slater Metal-Insulator Transition in Sr2IrO4

Layered 5d transition metal oxides (TMOs) have attracted significant interest in recent years because of the rich physical properties induced by the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion. In Sr2IrO4, this interplay opens a gap near the Fermi energy and stabilizes a Jeff=1/2 spin-orbital entangled insulating state at low temperatures. Whether this metal-insulating transition (MIT) is Mott-type (electronic-correlation driven) or Slater-type (magnetic-order driven) has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy (STM/S). The STS results clearly illustrate the opening of the (~150-250 meV) insulating gap at low temperatures, in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the measured temperature dependence of the gap width coupled with our DFT+dynamical mean field theory (DMFT) results strongly support the Slater-type MIT scenario in Sr2IrO4. The STS data further reveal a pseudogap structure above the Neel temperature, presumably related to the presence of antiferromagnetic fluctuations.

cond-mat.str-el↗

arXiv2012

Growth diagram and magnetic properties of hexagonal LuFe$_2$O$_4$ thin films

A growth diagram of Lu-Fe-O compounds on MgO (111) substrates using pulsed laser deposition is constructed based on extensive growth experiments. The LuFe$_2$O$_4$ phase can only be grown in a small range of temperature and O$_2$ pressure conditions. An understanding of the growth mechanism of Lu-Fe-O compound films is offered in terms of the thermochemistry at the surface. Superparamagnetism is observed in LuFe$_2$O$_4$ film and is explained in terms of the effect of the impurity h-LuFeO$_3$ phase and structural defects .

cond-mat.mtrl-sci↗

arXiv2002

Self-assembly of Nanometer-scale Magnetic Dots with Narrow Size Distributions on an Insulating Substrate

The self-assembly of iron dots on the insulating surface of NaCl(001) is investigated experimentally and theoretically. Under proper growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting layer Furthermore, both the vertical and lateral sizes of the dots can be tuned with the iron dosage without introducing apparent size broadening, even though the clustering is clearly in the strong coarsening regime. These observations are interpreted using a phenomenological mean-field theory, in which a coverage-dependent optimal dot size is selected by strain-mediated dot-dot interactions.

cond-mat.mtrl-sci↗