arXiv subjects

Shuai Dong

Publications and source records attributed to Shuai Dong.

At least 217 records · Page 12Linked to original sources

arXiv2015

Electric-Dipole Effect of Defects on Energy Band Alignment of Rutile and Anatase TiO2

Titanium dioxide materials have been studied intensively and extensively due to photocatalytic applications. A long-standing open question is the energy band alignment of rutile and anatase TiO2 phases, which can affect the photocatalytic process in the composite system. There are basically two contradictory viewpoints about the alignment of these two TiO2 phases supported by respective experiments: 1) straddling type and 2) staggered type. In this work, our DFT plus U calculations find that the perfect rutile (110) and anatase (101) surfaces have the straddling type band alignment, whereas the surfaces with defects can turn the band alignment into the staggered type. The electric dipoles induced by defects are responsible for the reversal of band alignment. Thus the defects introduced during preparations and post-treatment processes of materials are probably the answer to above open question regarding the band alignment, which can be considered in real practice to tune the photocatalytic activity of materials.

cond-mat.mtrl-sci↗

arXiv2015

Hydroxylation of Rutile TiO$_2$ (110) Surface Enhancing Its Reducing Power for Photocatalysis

Hydroxylation of the rutile TiO$_2$ (110) surface has attracted much attention as the excess unpaired electrons introduced by hydroxyls play a critical role in surface chemistry and photocatalysis process of this material. In this work, based on density functional theory calculations with the Hubbard U correction, the electronic structures of the hydroxylated TiO$_2$ (110) surfaces have been studied. One interesting effect is found that the hydroxylation can elevate band edges of TiO$_2$, and thus can enhance its reducing power for photocatalysis. The underlying physical mechanism for such shifts of the band edges are associated with the electric dipoles arising from the hydroxyl groups on the surface.

cond-mat.mtrl-sci↗

arXiv2015

Improving photocatalytic activity of TiO$_2$ through reduction

The rutile TiO$_2$ (110) surface reduced by the bridging oxygen vacancy, bridging hydroxyl group or Ti interstitial atom has been investigated by calculating their electronic structures using the density functional theory plus U method. It is found that defect states located in the forbidden band gap can enhance optical absorption. When the surface is highly reduced, the defect states approach the valence band. More importantly, defects induce a substantial up-shift of the conduction band edge, rendering the reduced surface stronger reducibility. The shifts of both conduction and valence band edges are due to the dipole moments created by these defects.

cond-mat.mtrl-sci↗

arXiv2015

Long-distance temporal quantum ghost imaging over optical fibers

Since the first quantum ghost imaging (QGI) experiment in 1995, many QGI schemes have been put forward. However, the position-position or momentum-momentum correlation required in these QGI schemes cannot be distributed over optical fibers, which limits their large geographical applications. In this paper, we propose and demonstrate a scheme for long distance QGI utilizing frequency correlated photon pairs. In this scheme, the frequency correlation is transformed to the correlation between the illuminating position of one photon and the arrival time of the other photon, by which QGI can be realized in the time domain. Since frequency correlation can be preserved when the photon pairs are distributed over optical fibers, this scheme provides a way to realize long-distance QGI over large geographical scale. In the experiment, long distance QGI over 50 km optical fibers has been demonstrated.

quant-ph↗

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↗

arXiv2015

Charge transfer and hybrid ferroelectricity in (YFeO$_{3}$)$_{n}$/(YTiO$_{3}$)$_{n}$ magnetic superlattices

Interfaces in oxide heterostructures always provide a fertile ground for emergent properties. Charge transfer from a high energy band to a low energy opponent is naturally expected, as occurring in semiconductor $p$-$n$ junctions. In this study, several exceptional physical phenomena have been predicted in (YFeO$_3$)$_n$/(YTiO$_3$)$_n$ superlattices. First, the charge transfer between these Mott insulators is in opposite to the intuitive band alignment scenario. Second, hybrid ferroelectricity with a moderate polarization is generated in the $n=2$ magnetic superlattice. Furthermore, the ferroelectric-type distortion can persist even if the ($AB$O$_3$)$_2$/($AB$'O$_3$)$_2$ system turns to be metallic, rending possible metallic ferroelectricity.

cond-mat.str-el↗

arXiv2015

Dual gate control of bulk transport and magnetism in the spin-orbit insulator Sr2IrO4

The 5d iridates have been the subject of much recent attention due to the predictions of a large array of novel electronic phases driven by twisting strong spin-orbit coupling and Hubbard correlation. As a prototype, the single layered perovskite Sr2IrO4 was first revealed to host a Jeff=1/2 Mott insulating state. In this material, the approximate energy scale of a variety of interactions, involving spin-orbit coupling, magnetic exchange interaction, and the Mott gap, allows close coupling among the corresponding physical excitations, opening the possibility of cross control of the physical properties. Here, we experimentally demonstrate the effective gate control of both the transport and magnetism in a Sr2IrO4-based field effect transistor using an ionic liquid dielectric. This approach could go beyond the surface-limited field effect seen in conventional transistors, reflecting the unique aspect of the Jeff=1/2 state. The simultaneous modulation of conduction and magnetism confirms the proposed intimate coupling of charge, orbital, and spin degrees of freedom in this oxide. These phenomena are probably related to an enhanced deviation from the ideal Jeff=1/2 state due to the gate-promoted conduction. The present work would have important implications in modelling the unusual physics enabled by strong spin-orbit coupling, and provides a new route to explore those emergent quantum phases in iridates.

cond-mat.mtrl-sci↗

arXiv2014

Testing the Monte Carlo - Mean Field approximation in the one-band Hubbard model

The canonical one-band Hubbard model is studied using a computational method that mixes the Monte Carlo procedure with the mean field approximation. This technique allows us to incorporate thermal fluctuations and the development of short-range magnetic order above ordering temperatures, contrary to the crude finite-temperature Hartree-Fock approximation, which incorrectly predicts a Néel temperature $T_N$ that grows linearly with the Hubbard $U/t$. The effective model studied here contains quantum and classical degrees of freedom. It thus belongs to the "spin fermion" model family widely employed in other contexts. Using exact diagonalization, supplemented by the traveling cluster approximation, for the fermionic sector, and classical Monte Carlo for the classical fields, the Hubbard $U/t$ vs. temperature $T/t$ phase diagram is studied employing large three and two dimensional clusters. We demonstrate that the method is capable of capturing the formation of local moments in the normal state without long-range order, the non-monotonicity of $T_N$ with increasing $U/t$, the development of gaps and pseudogaps in the density of states, and the two-peak structure in the specific heat. Extensive comparisons with determinant quantum Monte Carlo results suggest that the present approach is qualitatively, and often quantitatively, accurate, particularly at intermediate and high temperatures. Finally, we study the Hubbard model including plaquette diagonal hopping (i.e. the $t-t^\prime$ Hubbard model) in two dimensions and show that our approach allows us to study low temperature properties where determinant quantum Monte Carlo fails due to the fermion sign problem. Future applications of this method include multi-orbital Hubbard models such as those needed for iron-based superconductors.

cond-mat.str-el↗

arXiv2014

Magnetism and electronic structure of ($001$)- and ($111$)-oriented LaTiO$_3$ bilayers sandwiched in LaScO$_3$ barriers

In this study, the magnetism and electronic structure of LaTiO$_3$ bilayers along both the ($001$) and ($111$) orientations are calculated using the density functional theory. The band insulator LaScO$_3$ is chosen as the barrier layer and substrate to obtain the isolating LaTiO$_3$ bilayer. For both the ($001$)- and ($111$)-oriented cases, LaTiO$_3$ demonstrates the G-type antiferromagnetism as the ground state, similar to the bulk material. However, the electronic structure is significantly changed. The occupied bands of Ti are much narrower in the ($111$) case, giving a nearly flat band. As a result, the exchange coupling between nearest-neighbor Ti ions are reformed in these superlattices, which will affect the Néel temperature significantly.

cond-mat.mtrl-sci↗

arXiv2014

Orientation-dependent magnetism and orbital structure of strained YTiO$_3$ films on LaAlO$_3$ substrates

The strain tuned magnetism of YTiO$_3$ film grown on the LaAlO$_3$ ($110$) substrate is studied by the method of the first principles, and compared with that of the ($001$)-oriented one. The obtained magnetism is totally different, which is ferromagnetic for the film on the ($110$) substrate but A-type antiferromagnetic on the ($001$) one. This orientation-dependent magnetism is attributed to the subtle orbital ordering of YTiO$_3$ film. The $d_{xz}$/$d_{yz}$-type orbital ordering is predominant for the ($001$) one, but for the ($110$) case, the $d_{xy}$ orbital is mostly occupied plus a few contribution from the $d_{xz}$/$d_{yz}$ orbital. Moreover, the lattice mismatch is modest for the ($110$) case but more serious for the ($001$) one, which is also responsible for this contrasting magnetism.

cond-mat.mtrl-sci↗

arXiv2014

BaFe$_2$Se$_3$: a high $T_{\rm C}$ magnetic multiferroic with large ferrielectric polarization

The iron-selenides are important because of their superconducting properties. Here, an unexpected phenomenon is predicted to occur in an iron-selenide compound with a quasi-one-dimensional ladder geometry: BaFe$_2$Se$_3$ should be a magnetic ferrielectric system, driven by its magnetic block order via exchange striction. A robust performance (high $T_{\rm C}$ and large polarization) is expected. Different from most multiferroics, BaFe$_2$Se$_3$ is ferrielectric, with a polarization that mostly cancels between ladders. However, its strong magnetostriction still produces a net polarization that is large ($\sim$$0.1$ $μ$C/cm$^2$) as compared with most magnetic multiferroics. Its fully ferroelectric state, with energy only slightly higher than the ferrielectric, has a giant improper polarization $\sim$$2-3$ $μ$C/cm$^2$.

cond-mat.mtrl-sci↗

arXiv2014

Strain driven sequential magnetic transitions in strained GdTiO3 on compressive substrates: a first-principles study

The compressive strain effects on the magnetic ground state and electronic structure of strained GdTiO 3 have been studied by the first-principles method. Different from the congeneric YTiO3 and LaTiO3 cases both of which becomes the A-type antiferromagnetism on the (001) LaAlO3 substrate despite their contrastive magnetism, the ground state of strained GdTiO3 on the LaAlO3 substrate changes from the original ferromagnetism to G-type antiferromagnetim, instead of the A-type one although Gd 3+ is between Y3+ and La3+. Only when the in-plane compressive strain is large enough, e.g. on the (001) YAlO3 substrate, the ground state finally becomes the A-type one. The band structure calculation shows that these compressive strained GdTiO3 remain insulating, although the band gap changes a little in these strained GdTiO3.

cond-mat.mtrl-sci↗

arXiv2014

Ferroelectric Control of Magnetism and Transport in Oxide Heterostructures

Magnetism and transport are two key functional ingredients in modern electronic devices. In oxide heterostructures, ferroelectricity can provide a new route to control these two properties via electrical operations, which is scientifically interesting and technologically important. In this Brief Review, we will introduce recent progresses on this fast developing research field. Several subtopics will be covered. First, the ferroelectric polarization tuning of interfacial magnetism will be introduced, which includes the tuning of magnetization, easy axis, magnetic phases, as well as exchange bias. Second, the ferroelectric polarization tuning of transverse and tunneling transport will be reviewed.

cond-mat.mtrl-sci↗

arXiv2014

Controlled entanglement routing between two virtual pathways

We demonstrate controlled entanglement routing between bunching and antibunching path-entangled two-photon states in an unbalanced Mach-Zehnder interferometer (UMZI), in which the routing process is controlled by the relative phase difference in the UMZI. Regarding bunching and antibunching path-entangled two-photon states as two virtual ports, we can consider the UMZI as a controlled entanglement router, which bases on the coherent manipulation of entanglement. Half of the entanglement within the input two-photon state is coherently routed between the two virtual ports, while the other is lost due to the time distinguishability introduced by the UMZI. Pure bunching or antibunching path entangled two-photon states are obtained based on this controlled entanglement router. The results show that we can employ the UMZI as general entanglement router for practical quantum information application.

quant-ph↗

arXiv2013

First Principles Study of the Magnetic Properties of LaOMnAs

Recent experiments reported giant magnetoresistance at room temperature in LaOMnAs. Here a density functional theory calculation is performed to investigate magnetic properties of LaOMnAs. The ground state is found to be the G-type antiferromagnetic order within the $ab$ plane but coupled ferromagnetically between planes, in agreement with recent neutron investigations. The electronic band structures suggest an insulating state which is driven by the particular G-type magnetic order, while a metallic state accompanies the ferromagnetic order. This relation between magnetism and conductance may be helpful to qualitatively understand the giant magnetoresistance effects.

cond-mat.mtrl-sci↗

arXiv2013

Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic orders of LaTiO$_3$ under epitaxial strain: a first-principles study

Perovskite LaTiO$_3$ bulk is a typical Mott-insulator with G-type antiferromagnetic order. In this work, the biaxial strain effects on the ground magnetic order of LaTiO$_3$ films grown on various substrates have been studied. For the compressive strain, LaTiO$_3$ films grown on LaAlO$_3$, LaGaO$_3$, and SrTiO$_3$ substrates undergo a phase transition from the original G-type antiferromagnet to A-type antiferromagnet. The underlying physical mechanisms are the lattice distortions tunned by strain. While for the tensile strain, the BaTiO$_3$ and LaScO$_3$ substrates have been tested, which show a tendency to transit the LaTiO$_3$ to the C-type antiferromagnet. Furthermore, our calculations find that the magnetic transitions under epitaxial strain do not change the insulating fact of LaTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2013

Full Control of Magnetism in Manganite Bilayer by Ferroelectric Polarization

An oxide heterostructure made of manganite bilayers and ferroelectric perovskites is predicted to lead to the full control of magnetism when switching the ferroelectric polarizations. By using asymmetric polar interfaces in the superlattices, more electrons occupy the Mn layer at the $n$-type interface side than at the $p$-type side. This charge disproportionation can be enhanced or suppressed by the ferroelectric polarization. Quantum model and density functional theory calculations reach the same conclusion: a ferromagnetic-ferrimagnetic phase transition with maximal change $>90%$ of the total magnetization can be achieved by switching the polarization's direction. This function is robust and provides full control of the magnetization's magnitude, not only its direction, via electrical methods.

cond-mat.mtrl-sci↗