arXiv subjects

Ke Yang

Publications and source records attributed to Ke Yang.

At least 181 records · Page 10Linked to original sources

arXiv2018

Can the quantum spin Hall state of silicene be preserved on substrate

The substrate-induced topological phase transition of silience is a formidable obstacle for developing silicene-based materials and devices for compatibility with current electronics by using its topologically protected dissipationless edge states. First-principles calculations indicate that the substrate will result in a phase transition from topological nontrivial phase to trivial phase of silicene, although its Dirac cone is still obvious. The substrate effect (equivalent to an electric field) annihilates its spin-orbit coupling effect, the reason why its quantum spin Hall effect (QSHE) of silicene has not been experimentally observed. Unfortunately, external electric field seems impossible to recover the QSHE due to the screen effect of substrate. We here first propose a viable strategy (constructing inverse symmetrical sandwich structure (protective layer/silicene/substrate)) to preserve quantum spin Hall (QSH) state of silicene, which is demonstrated by using two representatives (CeO2(111)/silicene/CeO2(111) and CaF2(111)/silicene/CaF2(111)) through the calculated edge states and Z2 invariant. This work takes a critical step towards fundamental physics and realistic applications of silicene-based nanoelectronic devices.

physics.comp-ph↗

arXiv2018

Scalar particle production in a simple Horndeski theory

The scalar particle production through a scalar field non-minimally coupled with geometry is investigated in the context of a spatially homogeneous and isotropic universe. In this paper, in order to study the evolution of particle production over time in the case of analytical solutions, we focus on a simple Horndeski theory. We first suppose that the universe is dominated by a scalar field and derive the energy conservation condition. Then from the thermodynamic point of view, the macroscopic non-conservation of the scalar field energy-momentum tensor can be explained as an irreversible production of the scalar particles. Based on the explanation, we obtain a scalar particle production rate and the corresponding entropy. Finally, since the universe, in general, could be regarded as a closed system satisfying the laws of thermodynamics, we naturally impose some thermodynamic constraints on it. The thermodynamic properties of the universe can provide additional constraints on the simple Horndeski theory.

gr-qc↗

arXiv2017

Anomalous connection between antiferromagnetic and superconducting phases in pressurized non-centrosymmetric heavy fermion compound CeRhGe3

Unconventional superconductivity frequently emerges as the transition temperature of a magnetic phase, typically antiferromagnetic, is suppressed continuously toward zero temperature. Here, we report contrary behavior in pressurized CeRhGe3, a non-centrosymmetric heavy fermion compound. We find that its pressure-tuned antiferromagnetic transition temperature (TN) appears to avoid a continuous decrease to zero temperature by terminating abruptly above a dome of pressure-induced superconductivity. Near 21.5 GPa, evidence for TN suddenly vanishes, the electrical resistance becomes linear in temperature and the superconducting transition reaches a maximum. In light of X-ray absorption spectroscopy measurements, these characteristics appear to be related to a pressured-induced Ce valence instability, which reveals as a sharp increase in the rate of change of Ce valence with applied pressure.

cond-mat.supr-con↗

arXiv2017

Pressure-induced melting of magnetic order and emergence of new quantum state in alpha-RuCl3

Here we report the observation of pressure-induced melting of antiferromagnetic (AFM) order and emergence of a new quantum state in the honeycomb-lattice halide alpha-RuCl3, a candidate compound in the proximity of quantum spin liquid state. Our high-pressure heat capacity measurements demonstrate that the AFM order smoothly melts away at a critical pressure (Pc) of 0.7 GPa. Intriguingly, the AFM transition temperature displays an increase upon applying pressure below the Pc, in stark contrast to usual phase diagrams, for example in pressurized parent compounds of unconventional superconductors. Furthermore, in the high-pressure phase an unusual steady of magnetoresistance is observed. These observations suggest that the high-pressure phase is in an exotic gapped quantum state which is robust against pressure up to ~140 GPa.

cond-mat.str-el↗

arXiv2017

Scalar perturbations of Eddington-inspired Born-Infeld braneworld

We consider the scalar perturbations of Eddington-inspired Born-Infeld braneworld models in this paper. The dynamical equation for the physical propagating degree of freedom $ξ(x^μ,y)$ is achieved by using the Arnowitt-Deser-Misner decomposition method: $F_1(y) {\partial_y^2 ξ} + F_2(y){\partial_y ξ} + {\partial^μ\partial_μ}ξ=0$. We conclude that the solution is tachyonic-free and stable under scalar perturbations for $F_1(y)>0$ but unstable for $F_1(y)< 0$. The stability of a known analytic domain wall solution with the warp factor given by $a(y)= \text{sech}^{\frac{3}{4p}}(ky)$ is analyzed and it is shown that only the solution for $0<p<\sqrt{8/3}$ is stable.

hep-th↗

arXiv2017

Zero resistance from one atmosphere to the pressure of earth's outer core in a superconducting high entropy alloy

We report the observation of extraordinarily robust zero-resistance superconductivity in the pressurized (TaNb)0.67(HfZrTi)0.33 high entropy alloy - a new kind of material with a body-centered cubic crystal structure made from five randomly distributed transition metal elements. The transition to superconductivity (TC) increases from an initial temperature of 7.7 K at ambient pressure to 10 K at ~ 60 GPa, and then slowly decreases to 9 K by 190.6 GPa, a pressure that falls within that of the outer core of the earth. We infer that the continuous existence of the zero-resistance superconductivity from one atmosphere up to such a high pressure requires a special combination of electronic and mechanical characteristics. This high entropy alloy superconductor thus may have a bright future for applications under extreme conditions, and also poses a challenge for understanding the underlying quantum physics.

cond-mat.supr-con↗

arXiv2017

Exploring Temporal Preservation Networks for Precise Temporal Action Localization

Temporal action localization is an important task of computer vision. Though a variety of methods have been proposed, it still remains an open question how to predict the temporal boundaries of action segments precisely. Most works use segment-level classifiers to select video segments pre-determined by action proposal or dense sliding windows. However, in order to achieve more precise action boundaries, a temporal localization system should make dense predictions at a fine granularity. A newly proposed work exploits Convolutional-Deconvolutional-Convolutional (CDC) filters to upsample the predictions of 3D ConvNets, making it possible to perform per-frame action predictions and achieving promising performance in terms of temporal action localization. However, CDC network loses temporal information partially due to the temporal downsampling operation. In this paper, we propose an elegant and powerful Temporal Preservation Convolutional (TPC) Network that equips 3D ConvNets with TPC filters. TPC network can fully preserve temporal resolution and downsample the spatial resolution simultaneously, enabling frame-level granularity action localization. TPC network can be trained in an end-to-end manner. Experiment results on public datasets show that TPC network achieves significant improvement on per-frame action prediction and competing results on segment-level temporal action localization.

cs.CV↗

arXiv2017

Dehydrogenation through the pressure-induced polymerization processes of phosphine

PH3 is studied to understand the superconducting transition and responsible stoichiometry under high pressure by means of Raman, IR, and x-ray diffraction (XRD) measurements, and theoretical calculations. It is found PH3 is stable up to about 8 GPa and then starts to dehydrogenate through two dimerization processes at room temperature as pressure up to 25 GPa. Two resulting phosphorus hydrides, P2H4 and P4H6, are verified experimentally and can be recovered to ambient pressure. On further compression above 35 GPa, P4H6 directly decomposes into elemental phosphorus. The superconductivity transition temperatures of P4H6 at 100 and 200 GPa have been predicted to be 13 and 67 K in agreement with reported results, suggesting it might responsible for the superconductivity at higher pressures. Our results clearly show that P2H4 and P4H6 are only stable P-H compounds between PH3 and elemental phosphorus, shedding light on the superconducting mechanism.

cond-mat.mtrl-sci↗

arXiv2017

Superconductivity in pressurized CeRhGe3 and related non-centrosymmetric compounds

We report the discovery of superconductivity in pressurized CeRhGe3, until now the only remaining non-superconducting member of the isostructural family of non-centrosymmetric heavy-fermion compounds CeTX3 (T = Co, Rh, Ir and X = Si, Ge). Superconductivity appears in CeRhGe3 at a pressure of 19.6 GPa and the transition temperature Tc reaches a maximum value of 1.3 K at 21.5 GPa. This finding provides an opportunity to establish systematic correlations between superconductivity and materials properties within this family. Though ambient-pressure unit-cell volumes and critical pressures for superconductivity vary substantially across the series, all family members reach a maximum Tcmax at a common critical cell volume Vcrit, and Tcmax at Vcrit increases with increasing spin-orbit coupling strength of the d-electrons. These correlations show that substantial Kondo hybridization and spin-orbit coupling favor superconductivity in this family, the latter reflecting the role of broken centro-symmetry.

cond-mat.supr-con↗

arXiv2017

Linearization of a warped $f(R)$ theory in the higher-order frame II: the equation of motion approach

Without using conformal transformation, a simple type of five-dimensional $f(R)-$brane model is linearized directly in its higher-order frame. In this paper, the linearization is conducted in the equation of motion approach. We first derive all the linear perturbation equations without specifying a gauge condition. Then by taking the curvature gauge we derive the master equations of the linear perturbations. We show that these equations are equivalent to those obtained in the quadratical action approach [Phys. Rev. D 95 (2017) 104060], except the vector sector, in which a constraint equation can be obtained in the equation of motion approach but absent in the quadratical action approach. Our work sets an example on how to linearize higher-order theories without using conformal transformation, and might be useful for studying more complicated theories.

gr-qc↗

arXiv2017

Cooperative In-Vivo Nano-Network Communication at Terahertz Frequencies

Nano devices have great potential to play a vital role in future medical diagnostics and treatment technologies because of its non-invasive nature and ability to reach delicate body sites easily as compared to conventional devices. In this paper, a novel concept of cooperative communication for in-vivo nano-network is presented to enhance the communication among these devices. The effect on the system outage probability performance is conducted for various parameters including relay lacement, number of relays, transmit power, bandwidth and carrier frequency. Results show approximately a 10-fold increase in the system outage performance whenever an additional relay is included in the cooperative network, hence show a great potential of using cooperative communication to enhance the performance of nano-network at terahertz frequencies.

cs.ET↗

arXiv2016

Pressuring the low-temperature orthorhombic phase with a non-trivial topological state of Ru2Sn3 to room temperature

We report high pressure studies of the structural stability of Ru2Sn3, a new type of three dimensional topological insulator (3D-TI) with unique quasi-one dimensional Dirac electron states throughout the surface Brillouin zone of its one-atmosphere low temperature orthorhombic form. Our in-situ high-pressure synchrotron x-ray diffraction and electrical resistance measurements reveal that upon increasing pressure the tetragonal to orthorhombic shifts to higher temperature. We find that the stability of the orthorhombic phase that hosts the non-trivial topological ground state can be pushed up to room temperature by an applied pressure of ~ 20 GPa. This is in contrast to the commonly known 3D-TIs whose ground state is usually destroyed under pressure. Our results indicate that pressure provides a possible pathway for realizing a room-temperature topological insulating state in Ru2Sn3.

cond-mat.str-el↗

arXiv2016

Discovery of a bi-critical point between antiferromagnetic and superconducting phases in pressurized single crystal Ca0.73La0.27FeAs2

One of the most strikingly universal features of the high temperature superconductors is that the superconducting phase emerges in the close proximity of the antiferromagnetic phase, and the interplay between these two phases poses a long standing challenge. It is commonly believed that,as the antiferromagnetic transition temperature is continuously suppressed to zero, there appears a quantum critical point, around which the existence of antiferromagnetic fluctuation is responsible for the development of the superconductivity. In contrast to this scenario, we report the discovery of a bi-critical point identified at 2.88 GPa and 26.02 K in the pressurized high quality single crystal Ca0.73La0.27FeAs2 by complementary in situ high pressure measurements. At the critical pressure, we find that the antiferromagnetism suddenly disappears and superconductivity simultaneously emerges at almost the same temperature, and that the external magnetic field suppresses the superconducting transition temperature but hardly affects the antiferromagnetic transition temperature.

cond-mat.supr-con↗

arXiv2016

Origin of enhanced visible-light photocatalytic activity of transition metal (Fe, Cr and Co) doped CeO2: Effect of 3d-orbital splitting

Enhanced visible light photocatalytic activity of transition metal-doped ceria (CeO2) nanomaterials have experimentally been demonstrated, whereas there are very few reports mentioning the mechanism of this behavior. Here we use first-principles calculations to explore the origin of enhanced photocatalytic performance of CeO2 doped with transition metal impurities (Fe, Cr and Co). When a transition metal atom substitutes a Ce atom into CeO2, t2g and eg levels of 3d orbits appear in the middle of band gap owing to the effect of cubic ligand field, and the former is higher than latter. Interestingly, t2g subset of FeCe (CoCe and CrCe)-Vo-CeO2 is split into two parts: one merges into the conduction band, the other as well as eg will remain in the gap, because O vacancy defect adjacent to transition metal atom will break the symmetry of cubic ligand field. These eg and t2g levels in the band gap are beneficial for absorbing visible light and enhancing quantum efficiency because of forbidden transition, which is one key factor for enhanced visible light photocatalytic activity. The band gap narrowing also leads to a redshift of optical absorbance and high photoactivity. These findings can rationalize the available experimental results and provide some new insights for designing CeO2-based photocatalysts with high photocatalytic performance.

cond-mat.mtrl-sci↗

arXiv2016

The vital role of hole-carriers for superconductivity in pressurized black phosphorus

The influence of carrier type on superconductivity has been an important issue for understanding both conventional and unconventional superconductors [1-7]. For elements that superconduct, it is known that hole-carriers govern the superconductivity for transition and main group metals [8-10]. The role of hole-carriers in elements that are not normally conducting but can be converted to superconductors, however, remains unclear due to the lack of experimental data. Here we report the first in-situ high pressure Hall effect measurements on single crystal black phosphorus, measured up to ~ 50 GPa, and find a correlation between the Hall coefficient and the superconducting transition temperature (TC). Our results reveal that hole-carriers play a vital role in developing superconductivity and enhancing TC. Importantly, we also find a Lifshitz transition in the high-pressure cubic phase at ~17.2GPa, which uncovers the origin of a puzzling valley in the superconducting TC-pressure phase diagram. These results offer insight into the role of hole-carriers in developing superconductivity in simple semiconducting solids under pressure.

cond-mat.supr-con↗

arXiv2016

Interfacial interaction in monolayer transition metal dichalcogenides (MX2)/metal oxide heterostructures and its effects on electronic and optical properties: The case of MX2/CeO2

Two-dimensional transition metal dichalcogenides (MX2)/metal oxide heterostructures have shown unique physical properties, making them promising materials for various applications ranging from photocatalysis to solar energy conversion. Understanding the interfacial interactions is highly desirable for designing these heterostructures having excellent performance. Here we systematically study the interfacial interaction in monolayer MX2 (M=Mo, W; X=S, Se)/CeO2 heterostructures and its effects on electronic and optical properties by density functional theory. It is found that the interfacial interaction in the MX2/CeO2 depends predominantly on the chalcogen (X) element. Particularly, the band gap variation and important electronic states at conduction band minimum or valence band maximum of the heterostructures are determined by the strength of interfacial interaction. The MX2/CeO2 heterostructures with the same chalcogen (X) element have similar absorption spectra from ultraviolet to near-infrared regions. These results suggest that the chalcogen (X) element is a key factor in tuning the properties of MX2/metal oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2016

Correlation between non-centrosymmetry and superconductivity in quasi-one-dimensional compounds A2Cr3As3 (A=K, Rb)

Non-centrosymmetric superconductors, whose crystal structure is absent of inversion symmetry, have recently received special attentions due to the expectation of unconventional pairings and exotic physics associated with such pairings. The newly discovered superconductors A2Cr3As3 (A=K, Rb), featured by the quasi-one dimensional structure with conducting CrAs chains, belongs to such kind of superconductor. In this study, we are the first to report the finding that the superconductivity of A2Cr3As3 (A=K, Rb) has a positive correlation with the extent of non-centrosymmetry. Our in-situ high pressure ac susceptibility and synchrotron x-ray diffraction measurements reveal that the larger bond angle of As-Cr-As in the CrAs chains can be taken as a key factor controlling superconductivity. While the smaller bond angle and the distance between the CrAs chains also affect the superconductivity due to their structural connections with the angle. We find that the larger value of the difference between the larger and samller angles, which is associated with the extent of the non-centrosymmetry of the lattice structure, is in favor of superconductivity. These results are expected to shed a new light on the underlying mechanism of the superconductivity in these Q1D superconductors and also to provide new perspective in understanding other non-centrosymmetric superconductors.

cond-mat.supr-con↗

arXiv2016

Measuring Fairness in Ranked Outputs

Ranking and scoring are ubiquitous. We consider the setting in which an institution, called a ranker, evaluates a set of individuals based on demographic, behavioral or other characteristics. The final output is a ranking that represents the relative quality of the individuals. While automatic and therefore seemingly objective, rankers can, and often do, discriminate against individuals and systematically disadvantage members of protected groups. This warrants a careful study of the fairness of a ranking scheme. In this paper we propose fairness measures for ranked outputs. We develop a data generation procedure that allows us to systematically control the degree of unfairness in the output, and study the behavior of our measures on these datasets. We then apply our proposed measures to several real datasets, and demonstrate cases of unfairness. Finally, we show preliminary results of incorporating our ranked fairness measures into an optimization framework, and show potential for improving fairness of ranked outputs while maintaining accuracy.

cs.DB↗