arXiv subjects

Sen Yang

Publications and source records attributed to Sen Yang.

At least 235 records · Page 13Linked to original sources

arXiv2016

Effect of interface on epitaxy and magnetism in h-RFeO3/Fe3O4/Al2O3 films (R=Lu, Yb)

We have carried out the growth of h-RFeO3 (001) (R=Lu, Yb) thin films on Fe3O4 (111)/Al2O3 (001) substrates, and studied the effect of the h-RFeO3 (001)/Fe3O4 (111) interfaces on the epitaxy and magnetism. The observed epitaxial relations between h-RFeO3 and Fe3O4 indicates an unusual matching of Fe sub-lattices rather than a matching of O sub-lattices. The out-of-plane direction was found to be the easy magnetic axis for h-YbFeO3 (001) but the hard axis for Fe3O4 (111) in the h-YbFeO3 (001)/Fe3O4 (111)/Al2O3 (001) films, suggesting a perpendicular magnetic alignment at the h-YbFeO3 (001)/Fe3O4 (111) interface. These results indicate that Fe3O4 (111)/Al2O3 (001) could be a promising substrate for epitaxial growth of h-RFeO3 films of well-defined interface and for exploiting their spintronic properties.

cond-mat.mtrl-sci↗

arXiv2016

Structural attributes and photo-dynamics of visible spectrum quantum emitters in hexagonal boron nitride

Newly discovered van der Waals materials like MoS$_2$, WSe$_2$, hexagonal boron nitride (h-BN), and recently $\mathrm{C}_2\mathrm{N}$ have sparked intensive research to unveil the quantum behavior associated with their 2D structure. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band gap of 5.95 eV, has been shown to host single quantum emitters which are stable at room temperature in the UV and visible spectral range. In this paper we investigate correlations between h-BN structural features and emitter location from bulk down to the monolayer at room temperature. We demonstrate that chemical etching and ion irradiation can generate emitters in h-BN. We analyze the emitters' spectral features and show that they are dominated by the interaction of their electronic transition with a single Raman active mode of h-BN. Photodynamics analysis reveals diverse rates between the electronic states of the emitter. The emitters show excellent photo stability even under ambient conditions and in monolayers. Comparing the excitation polarization between different emitters unveils a connection between defect orientation and the h-BN hexagonal structure. The sharp spectral features, color diversity, room-temperature stability, long-lived metastable states, ease of fabrication, proximity of the emitters to the environment, outstanding chemical stability, and biocompatibility of h-BN provide a completely new class of systems that can be used for sensing and quantum photonics applications.

cond-mat.mes-hall↗

arXiv2016

Tangent spaces to motivic cohomology groups

BY using Green-Griffiths' results on tangent spaces to algebraic cycles [4], we study the tangent space to $CH^{2}(X,1)$, where $X$ is a nonsingular projective curve over a field $k$ of characteristic $0$.

math.AG↗

arXiv2016

Dissipative entanglement of solid-state spins in diamond

Generating robust entanglement among solid-state spins is key for applications in quantum information processing and precision sensing. We show here a dissipative approach to generate such entanglement among the hyperfine coupled electron nuclear spins using the rapid optical decay of electronic excited states. The combined dark state interference effects of the optical and microwave driving fields in the presence of spontaneous emission from the short-lived excited state leads to a dissipative formation of an entangled steady state. We show that the dissipative entanglement is generated for any initial state conditions of the spins and is resilient to external field fluctuations. We analyze the scheme both for continuous and pulsed driving fields in the presence of realistic noise sources.

quant-ph↗

arXiv2016

Kinetics and Intermediate Phases in Epitaxial Growth of Fe3O4 Films from Deposition and Thermal Reduction

We have studied the growth of Fe3O4 (111) epitaxial films on Al2O3 (001) substrates using a pulsed laser deposition / thermal reduction cycle using an α-Fe2O3 target. While direct deposition onto the Al2O3 (001) substrates results in an α-Fe2O3 epilayer, deposition on the Fe3O4 (111) surface results in a γ-Fe2O3 epilayer. The kinetics of the transitions between Fe2O3 and Fe3O4 were studied by measuring the time constants of the transitions. The transition from α-Fe2O3 to Fe3O4 via thermal reduction turns out to be very slow, due to the high activation energy. Despite the significant grain boundaries due to the mismatch between the unit cells of the film and the substrate, the Fe3O4 (111) films grown from deposition/thermal reduction show high crystallinity.

cond-mat.mtrl-sci↗

arXiv2016

Development of a high quality thin diamond membrane with embedded nitrogen-vacancy centers for hybrid spin-mechanical quantum systems

Hybrid quantum systems (HQSs) have attracted several research interests in the last years. In this Letter, we report on the design, fabrication, and characterization of a novel diamond architecture for HQSs that consists of a high quality thin circular diamond membrane with embedded near-surface nitrogen-vacancy centers (NVCs). To demonstrate this architecture, we employed the NVCs by means of their optical and spin interfaces as nanosensors of the motion of the membrane under static pressure and in-resonance vibration, as well as the residual stress of the membrane. Driving the membrane at its fundamental resonance mode, we observed coupling of this vibrational mode to the spin of the NVCs by Hahn echo signal. Our realization of this architecture will enable futuristic HQS-based applications in diamond piezometry and vibrometry, as well as spin-mechanical and mechanically mediated spin-spin coupling in quantum information science.

cond-mat.mes-hall↗

arXiv2016

K-theoretic Chow groups of derived categories of schemes-on a question by Green-Griffiths

Based on Balmer's tensor triangular Chow group, we propose K-theoretic Chow groups of derived categories of noetherian schemes and their Milnor variants for regular schemes and their thickenings. We discuss functoriality and show that our Chow groups agree with the classical ones for regular schemes. Moreover, we also define tangent spaces to our Chow groups as usually and identify them with cohomology groups of differentials. As an application, we extend Bloch-Quillen identification and soule's variant from regular schemes to their thickenings. This gives a positive answer to a question by Green-Griffiths.

math.AG↗

arXiv2016

High fidelity transfer and storage of photon states in a single nuclear spin

Building a quantum repeater network for long distance quantum communication requires photons and quantum registers that comprise qubits for interaction with light, good memory capabilities and processing qubits for storage and manipulation of photons. Here we demonstrate a key step, the coherent transfer of a photon in a single solid-state nuclear spin qubit with an average fidelity of 98% and storage over 10 seconds. The storage process is achieved by coherently transferring a photon to an entangled electron-nuclear spin state of a nitrogen vacancy centre in diamond, confirmed by heralding through high fidelity single-shot readout of the electronic spin states. Stored photon states are robust against repetitive optical writing operations, required for repeater nodes. The photon-electron spin interface and the nuclear spin memory demonstrated here constitutes a major step towards practical quantum networks, and surprisingly also paves the way towards a novel entangled photon source for photonic quantum computing.

cond-mat.mes-hall↗

arXiv2015

Proposal of room-temperature diamond maser

Lasers have revolutionized optical science and technology, but their microwave counterpart, maser, has not realized its great potential due to its demanding work conditions (high-vacuum for gas maser and liquid-helium temperature for solid-state maser). Room-temperature solid-state maser is highly desirable, but under such conditions the lifetimes of emitters (usually electron spins) are usually too short (~ns) for population inversion. The only room-temperature solid-state maser is based on a pentacene-doped p-terphenyl crystal, which has long spin lifetime (~0.1 ms). This maser, however, operates only in the pulse mode and the material is unstable. Here we propose room-temperature maser based on nitrogen-vacancy (NV) centres in diamond, which feature long spin lifetimes at room temperature (~10 ms), high optical pump efficiency, and material stability. We demonstrate that under readily accessible conditions, room-temperature diamond maser is feasible. Room-temperature diamond maser may facilitate a broad range of microwave technologies.

quant-ph↗

arXiv2015

All-optical preparation of coherent dark states of a single rare earth ion spin in a crystal

All-optical addressing and control of single solid-state based qubits allows for scalable architectures of quantum devices such as quantum networks and quantum simulators. So far, all-optical addressing of qubits was demonstrated only for color centers in diamond and quantum dots. Here, we demonstrate generation of coherent dark state of a single rare earth ion in a solid, namely a cerium ion in yttrium aluminum garnet (YAG). The dark state was formed under the condition of coherent population trapping. Furthermore, high-resolution spectroscopic studies of native and implanted single Ce ions have been performed. They revealed narrow and spectrally stable optical transitions between the spin sublevels of the ground and excited optical states, indicating the feasibility of interfacing single photons with a single electron spin of a cerium ion.

physics.optics↗

arXiv2015

Generation of entangled photon strings using NV centers in diamond

We present a scheme to generate entangled photons using the NV centers in diamond. We show how the long-lived nuclear spin in diamond can mediate entanglement between multiple photons thereby increasing the length of entangled photon string. With the proposed scheme one could generate both n-photon GHZ and cluster states. We present an experimental scheme realizing the same and estimating the rate of entanglement generation both in the presence and absence of a cavity.

quant-ph↗

arXiv2015

Fluctuation and Commensurability Effect of Exciton Density Wave

At low temperatures, indirect excitons formed at the in-plane electron-hole interface in a coupled quantum well structure undergo a spontaneous transition into a spatially modulated state. We report on the control of the instability wavelength, measurement of the dynamics of the exciton emission pattern, and observation of the fluctuation and commensurability effect of the exciton density wave. We found that fluctuations are strongly suppressed when the instability wavelength is commensurate with defect separation along the exciton density wave. The commensurability effect is also found in numerical simulations within the model describing the exciton density wave in terms of an instability due to stimulated processes.

cond-mat.mes-hall↗

arXiv2015

Infinitesimal Theory of Chow Groups via K-theory, Cyclic Homology and the Relative Chern Character

We examine the tangent groups at the identity, and more generally the formal completions at the identity, of the Chow groups of algebraic cycles on a nonsingular quasiprojective algebraic variety over a field of characteristic zero. We settle a question recently raised by Mark Green and Phillip Griffiths concerning the existence of Bloch-Gersten-Quillen-type resolutions of algebraic K-theory sheaves on infinitesimal thickenings of nonsingular varieties, and the relationships between these sequences and their "tangent sequences," expressed in terms of absolute Kähler differentials. More generally, we place Green and Griffiths' concrete geometric approach to the infinitesimal theory of Chow groups in a natural and formally rigorous structural context, expressed in terms of nonconnective K-theory, negative cyclic homology, and the relative algebraic Chern character

math.AG↗

arXiv2014

Towards barrier free contact to MoS2 using graphene electrodes

The two-dimensional (2D) layered semiconductors such as MoS2 have attracted tremendous interest as a new class of electronic materials. However, there is considerable challenge in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as back electrodes to achieve Ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily modified by gate potential to ensure a nearly perfect band alignment with MoS2. We demonstrate, for the first time, a transparent contact can be made to MoS2 with essentially zero contact barrier and linear output behaviour at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator-transition (MIT) can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barrier and only seen in four-terminal devices in conventional metal-contacted MoS2 system. With further passivation y born nitride encapsulation, we demonstrate a record high extrinsic (two-terminal) field effect mobility over 1300 cm2/Vs in MoS2.

cond-mat.mtrl-sci↗

arXiv2014

Nano-engineered Diamond Waveguide as a Robust Bright Platform for Nanomagnetometry Using Shallow Nitrogen Vacancy Centers

Photonic structures in diamond are key to most of its application in quantum technology. Here, we demonstrate tapered nano-waveguides structured directly onto the diamond substrate hosting shallow-implanted nitrogen vacancy (NV) centers. By optimization based on simulations and precise experimental control of the geometry of these pillar-shaped nano-waveguides, we achieve a net photon flux up to ~ $1.7 \cdot 10^6 /s$. This presents the brightest monolithic bulk diamond structure based on single NV centers so far. We observe no impact on excited state lifetime and electronic spin dephasing time ($T_2$) due to the nanofabrication process. Possessing such high brightness with low background in addition to preserved spin quality, this geometry can improve the current nanomagnetometry sensitivity ~ 5 times. In addition, it facilitates a wide range of diamond defects-based magnetometry applications. As a demonstration, we measure the temperature dependency of $T_1$ relaxation time of a single shallow NV center electronic spin. We observe the two-phonon Raman process to be negligible in comparison to the dominant two-phonon Orbach process.

cond-mat.mes-hall↗

arXiv2014

Coherent control of single spins in silicon carbide at room temperature

Spins in solids are cornerstone elements of quantum spintronics. Leading contenders such as defects in diamond, or individual phosphorous dopants in silicon have shown spectacular progress but either miss established nanotechnology or an efficient spin-photon interface. Silicon carbide (SiC) combines the strength of both systems: It has a large bandgap with deep defects and benefits from mature fabrication techniques. Here we report the characterization of photoluminescence and optical spin polarization from single silicon vacancies in SiC, and demonstrate that single spins can be addressed at room temperature. We show coherent control of a single defect spin and find long spin coherence time under ambient conditions. Our study provides evidence that SiC is a promising system for atomic-scale spintronics and quantum technology.

cond-mat.mes-hall↗

arXiv2014

Macroscopic quantum superposition of spin ensembles with ultra-long coherence times via superradiant masing

Macroscopic quantum phenomena such as lasers, Bose-Einstein condensates, superfluids, and superconductors are of great importance in foundations and applications of quantum mechanics. In particular, quantum superposition of a large number of spins in solids is highly desirable for both quantum information processing and ultrasensitive magnetometry. Spin ensembles in solids, however, have rather short collective coherence time (typically less than microseconds). Here we demonstrate that under realistic conditions it is possible to maintain macroscopic quantum superposition of a large spin ensemble (such as about ~10^{14} nitrogen-vacancy center electron spins in diamond) with an extremely long coherence time ~10^8 sec under readily accessible conditions. The scheme, following the mechanism of superradiant lasers, is based on superradiant masing due to coherent coupling between collective spin excitations (magnons) and microwave cavity photons. The coherence time of the macroscopic quantum superposition is the sum of the magnon life time and the cavity lifetime, further elongated by the total number of coherent magnons and photons, which have macroscopic values when masing occurs. The macroscopic quantum coherence of spin ensembles can be exploited for magnetometry with sensitivity ~10 fT/(Hz)^{1/2}. The long-living collective states of spin ensembles in solids will provide a new platform for studying macroscopic quantum phenomena and are useful for quantum technologies.

quant-ph↗

arXiv2014

Quantum Simulation of Helium Hydride in a Solid-State Spin Register

\emph{Ab initio} computation of molecular properties is one of the most promising applications of quantum computing. While this problem is widely believed to be intractable for classical computers, efficient quantum algorithms exist which have the potential to vastly accelerate research throughput in fields ranging from material science to drug discovery. Using a solid-state quantum register realized in a nitrogen-vacancy (NV) defect in diamond, we compute the bond dissociation curve of the minimal basis helium hydride cation, HeH$^+$. Moreover, we report an energy uncertainty (given our model basis) of the order of $10^{-14}$ Hartree, which is ten orders of magnitude below desired chemical precision. As NV centers in diamond provide a robust and straightforward platform for quantum information processing, our work provides several important steps towards a fully scalable solid state implementation of a quantum chemistry simulator.

quant-ph↗