arXiv subjects

Zhenyu Wang

Publications and source records attributed to Zhenyu Wang.

216 records · Page 12Linked to original sources

arXiv2018

Pulsed polarisation for robust DNP

Dynamical nuclear polarisation (DNP) is an important technique that uses polarisation transfer from electron to nuclear spins to achieve nuclear hyperpolarisation. As the electron spin of the nitrogen vacancy (NV) centres in diamond can be optically initialised nearly perfectly even at room temperature and ambient conditions, new opportunities become possible by the combination of efficient DNP with optically polarised NV centres. Among such applications are nanoscale nuclear magnetic resonance spectroscopy of liquids, hyperpolarised nanodiamonds as MRI contrast agents as well as the initialisation of nuclear spin based diamond quantum simulators. Current realisations of DNP perform the polarisation transfer by achieving energetic resonance between electrons and nuclei via carefully tuned microwave fields or by using quasi-adiabatic sweep-based schemes across resonance points. The former limits robustness against control errors while the latter limits polarisation rates, making the realisation of the applications extremely challenging. Here we introduce the concept of Hamiltonian engineering by pulse sequences and use it for the systematic design of polarisation sequences that are simultaneously robust and fast. We derive sequences theoretically and demonstrate experimentally that they are capable of efficient polarisation transfer from an optically polarised nitrogen-vacancy centre in diamond to the surrounding $^{13}$C nuclear spin bath even in the presence of control errors, making it an ideal tool for the realisation of the above NV centre based applications.

quant-ph↗

arXiv2018

Eavesdropping on spin waves inside the domain-wall nanochannel via three-magnon processes

One recent breakthrough in the field of magnonics is the experimental realization of reconfigurable spin-wave nanochannels formed by magnetic domain wall with a width of $10-100$ nm [Wagner \emph{et al}., Nat. Nano. \textbf{11}, 432 (2016)]. This remarkable progress enables an energy-efficient spin-wave propagation with a well-defined wave vector along its propagating path inside the wall. In the mentioned experiment, a micro-focus Brillouin light scattering spectroscopy was taken in a line-scans manner to measure the frequency of the bounded spin wave. Due to their localization nature, the confined spin waves can hardly be detected from outside the wall channel, which guarantees the information security to some extent. In this work, we theoretically propose a scheme to detect/eavesdrop on the spin waves inside the domain-wall nanochannel via nonlinear three-magnon processes. We send a spin wave in one magnetic domain to interact with the bounded mode in the wall. Two kinds of three-magnon processes, i.e., confluence and splitting, are expected to occur. The confluence process is conventional. We predict a stimulated three-magnon splitting (or "magnon laser") effect: the presence of a bound magnon propagating along the domain wall channel assists the splitting of the incident wave into two modes, one of which is identical to the bound mode in the channel. Micromagnetic simulations confirm our theoretical analysis. These results demonstrate that one is able to uniquely infer the spectrum of the spin-wave in the domain-wall nanochannel once we know both the injection and the transmitted waves.

cond-mat.mes-hall↗

arXiv2017

Large deformation and instability of soft hollow cylinder with surface effects

Surface stress, which is always neglected in classical elastic theories, has recently emerged as a key role in the mechanics of highly deformable soft solids. In this paper, the effect of surface stress on the deformation and instability of soft hollow cylinder are analyzed. By incorporating surface energy density function into the constitutive model of a hyper-elastic theory, explicit solutions are obtained for the deformation of soft hollow cylinder under the conditions of uniform pressure loading and geometric everting. It is found that surface tension evidently alters the deformation of the soft cylinder. Specifically, the surface stiffness resists the deformation, but the residual surface stress is inclined to larger deformation. Effects of surface stress on the instability of the soft hollow cylinder is also explored. For both the pressure loading and geometric everting conditions, significant changes in critical condition of the creases are found by varying the surface parameter. The results in this work reveal that surface energy obviously influences both the deformation and the instability of soft hollow cylinder at finite deformation. The obtained results will be helpful for understanding and predicting the mechanical behavior of soft structures accurately.

cond-mat.soft↗

arXiv2017

Defect Role in the Carrier Tunable Topological Insulator (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ Thin Films

Alloys of Bi$_2$Te$_3$ and Sb$_2$Te$_3$ ((Bi$_{1-x}$Sb$_x$)$_2$Te$_3$) have played an essential role in the exploration of topological surface states, allowing us to study phenomena that would otherwise be obscured by bulk contributions to conductivity. Thin films of these alloys have been particularly important for tuning the energy of the Fermi level, a key step in observing spin-polarized surface currents and the quantum anomalous Hall effect. Previous studies reported the chemical tuning of the Fermi level to the Dirac point by controlling the Sb:Bi composition ratio, but the optimum ratio varies widely across various studies with no consensus. In this work, we use scanning tunneling microscopy and Landau level spectroscopy, in combination with X-ray photoemission spectroscopy to isolate the effects of growth factors such as temperature and composition, and to provide a microscopic picture of the role that disorder and composition play in determining the carrier density of epitaxially grown (Bi,Sb)$_2$Te$_3$ thin films. Using Landau level spectroscopy, we determine that the ideal Sb concentration to place the Fermi energy to within a few meV of the Dirac point is $x\sim 0.7$. However, we find that the post- growth annealing temperature can have a drastic impact on microscopic structure as well as carrier density. In particular, we find that when films are post-growth annealed at high temperature, better crystallinity and surface roughness are achieved; but this also produces a larger Te defect density, adding n-type carriers. This work provides key information necessary for optimizing thin film quality in this fundamentally and technologically important class of materials.

cond-mat.mtrl-sci↗

arXiv2017

Quasiparticle Interference and Strong Electron-Mode Coupling in the Quasi-One-Dimensional Bands of Sr$_2$RuO$_4$

The single-layered ruthenate Sr$_2$RuO$_4$ has attracted a great deal of interest as a spin-triplet superconductor with an order parameter that may potentially break time reversal invariance and host half-quantized vortices with Majorana zero modes. While the actual nature of the superconducting state is still a matter of controversy, it has long been believed that it condenses from a metallic state that is well described by a conventional Fermi liquid. In this work we use a combination of Fourier transform scanning tunneling spectroscopy (FT-STS) and momentum resolved electron energy loss spectroscopy (M-EELS) to probe interaction effects in the normal state of Sr$_2$RuO$_4$. Our high-resolution FT-STS data show signatures of the β-band with a distinctly quasi-one-dimensional (1D) character. The band dispersion reveals surprisingly strong interaction effects that dramatically renormalize the Fermi velocity, suggesting that the normal state of Sr$_2$RuO$_4$ is that of a 'correlated metal' where correlations are strengthened by the quasi 1D nature of the bands. In addition, kinks at energies of approximately 10meV, 38meV and 70meV are observed. By comparing STM and M-EELS data we show that the two higher energy features arise from coupling with collective modes. The strong correlation effects and the kinks in the quasi 1D bands may provide important information for understanding the superconducting state. This work opens up a unique approach to revealing the superconducting order parameter in this compound.

cond-mat.str-el↗

arXiv2016

Resonance Beyond Frequency-Matching

Resonance, defined as the oscillation of a system when the temporal frequency of an external stimulus matches a natural frequency of the system, is important in both fundamental physics and applied disciplines. However, the spatial character of oscillation is not considered in the definition of resonance. In this work, we reveal the creation of spatial resonance when the stimulus matches the space pattern of a normal mode in an oscillating system. The complete resonance, which we call multidimensional resonance, is a combination of both the spatial and the conventionally defined (temporal) resonance and can be several orders of magnitude stronger than the temporal resonance alone. We further elucidate that the spin wave produced by multidimensional resonance drives considerably faster reversal of the vortex core in a magnetic nanodisk. Our findings provide insight into the nature of wave dynamics and open the door to novel applications.

cond-mat.mes-hall↗

arXiv2015

Observation of a van Hove singularity and implication for strong coupling induced Cooper pairing in KFe2As2

Scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) have been investigated on single crystal samples of KFe2As2. A van Hove singularity (vHs) has been directly observed just a few meV below the Fermi level E_F of superconducting KFe2As2, which locates in the middle of the principle axes of the first Brillouin zone. The majority of the density-of-states at E_F, mainly contributed by the proximity effect of the saddle point to E_F, is non-gapped in the superconducting state. Our observation of nodal behavior of the momentum area close to the vHs points, while providing consistent explanations to many exotic behaviours previously observed in this material, suggests Cooper pairing induced by a strong coupling mechanism.

cond-mat.supr-con↗

arXiv2014

Anisotropic Superconducting Gap and Elongated Vortices with Caroli-De-Gennes-Matricon States in the New Superconductor Ta4Pd3Te16

The superconducting state is formed by the condensation of a large number of Cooper pairs. The normal state electronic properties can give significant influence on the superconducting state. For usual type-II superconductors, the vortices are cylinder like with a round cross-section. For many two dimensional superconductors, such as Cuprates, 2H-NbSe$_2$ etc., albeit the in-plane anisotropy, the vortices generally have a round shape. In this paper we report results based on the scanning tunneling microscopy/spectroscopy measurements on a newly discovered superconductor Ta$_4$Pd$_3$Te$_{16}$. The chain like conducting channels of PdTe$_2$ in Ta$_4$Pd$_3$Te$_{16}$ make a significant anisotropy of the in-plane Fermi velocity. We suggest at least one anisotropic superconducting gap with gap minima or possible node exists in this multiband system. In addition, elongated vortices are observed with an anisotropy of $ξ_{\parallel b}/ξ_{\perp b}\approx 2.5$. Clear Caroli-de-Gennes-Matricon states are also observed within the vortex cores. Our results will initiate the study on the elongated vortices and superconducting mechanism in the new superconductor Ta$_4$Pd$_3$Te$_{16}$.

cond-mat.supr-con↗

arXiv2014

The spectroscopic signature of Kondo screening on single adatoms in Na(Fe0.96Co0.03Mn0.01)As

The electronic states of surface adatoms in Na(Fe0.96Co0.03Mn0.01)As have been studied by low temperature scanning tunneling spectroscopy. The spectra recorded on the adatoms display both superconducting coherence peaks and an asymmetric resonance in a larger energy scale. The Fano-type line shape of the spectra points towards a possible Kondo effect at play. The apparent energy position of the resonance peak shifts about 5 meV to the Fermi level when measured across the critical temperature, supporting that the Bogoliubov quasiparticle is responsible for the Kondo screening in the superconducting state. The tunneling spectra do not show the subgap bound states, which is explained as the weak pair breaking effect given by the weak and broad scattering potential after the Kondo screening.

cond-mat.supr-con↗

arXiv2014

Experimental observation of Dirac-like surface states and topological phase transition in Pb$_{1-x}$Sn$_x$Te(111) films

The surface of a topological crystalline insulator (TCI) carries an even number of Dirac cones protected by crystalline symmetry. We epitaxially grew high quality Pb$_{1-x}$Sn$_x$Te(111) films and investigated the TCI phase by in-situ angle-resolved photoemission spectroscopy. Pb$_{1-x}$Sn$_x$Te(111) films undergo a topological phase transition from trivial insulator to TCI via increasing the Sn/Pb ratio, accompanied by a crossover from n-type to p-type doping. In addition, a hybridization gap is opened in the surface states when the thickness of film is reduced to the two-dimensional limit. The work demonstrates an approach to manipulating the topological properties of TCI, which is of importance for future fundamental research and applications based on TCI.

cond-mat.mtrl-sci↗

arXiv2013

In-Gap Quasiparticle Excitations Induced by Non-Magnetic Cu Impurities in Na(Fe0.96Co0.03Cu0.01)As Revealed by Scanning Tunneling Spectroscopy

The pairing mechanism in the iron pnictides remains unresolved yet. One of the central issues is the structure of the superconducting order parameter which classifies the community into two different and highly disputed camps. On one hand the picture of pairing based on the magnetic origin predicts a sign reversal gap on the electron and hole Fermi pockets, leading to the S+- pairing. On the other hand, a more conventional S++ pairing gap was suggested based on the phonon or orbital fluctuation mediated pairing. In the superconducting state, the impurities may generate a unique pattern of local density of states in space and energy, which are regarded as the fingerprints for checking the structure of the pairing gap. In this study, we successfully identified the non-magnetic and magnetic impurities in Na(Fe0.97-xCo0.03Tx)As (T=Cu, Mn) and investigated the spatial resolved scanning tunneling spectroscopy. We present clear evidence of the in-gap quasiparticle states induced by the nonmagnetic Cu impurities, giving decisive evidence of the S+- pairing. This is corroborated by the consistency between the experimental data and the first-principles calculations based on the S+- pairing gap with a scalar scattering potential.

cond-mat.supr-con↗

arXiv2013

Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As

Since the discovery of high temperature superconductivity in the iron pnictides and chalcogenides in early 2008, a central issue has been the microscopic origin of the superconducting pairing. Although previous experiments suggest that the pairing may be induced by exchanging the antiferromagnetic spin fluctuations and the superconducting order parameter has opposite signs in the electron and hole pockets as predicted by the S+- pairing model, it remains unclear whether there is a bosonic mode from the tunneling spectrum which has a close and universal relationship with superconductivity as well as the spin excitation. In this paper, based on the measurements of scanning tunneling spectroscopy, we show the clear evidence of a bosonic mode with the energy identical to that of the neutron spin resonance in two completely different systems Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As with different superconducting transition temperatures. In both samples, the superconducting coherence peaks and the mode feature vanish simultaneously inside the vortex core or above Tc, indicating a close relationship between superconductivity and the bosonic mode. Our data also demonstrate a universal ratio between the mode energy and superconducting transition temperature, that is [mode energy]/kBTc ~ 4.3, which underlines the unconventional mechanism of superconductivity in the iron pnictide superconductors.

cond-mat.supr-con↗

arXiv2013

Unexpected Giant Superconducting Fluctuation and Anomalous Semiconducting Normal State in NdO1-xFxBi1-yS2 Single Crystals

The BiS2-based superconductors were discovered recently. The superconductivity has been proved by many other groups. Since the previous experiments were all done on polycrystalline samples, therefore there remains a concern whether the superconductivity is really derived from the materials intrinsically or from some secondary phases. Experiments on single crystals are highly desired. In this paper, we report the successful growth of the NdO1-xFxBi1-yS2 single crystals. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at about 5 K, while an unexpected giant superconducting fluctuation appears at temperatures as high as 2-4 kBTC. Analysis based on the anisotropic Ginzbaug-Landau theory gives an anisotropy of 30-45. Two gap features with magnitudes of about 3.5+-0.3 meV and 7.5+-1 meV were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition at about 5 K yielding a huge ratio 2Delta_s/kBTc =16.8, the larger gap remains up to about 26 K. The normal state recovered by applying a high magnetic field shows an anomalous semiconducting behavior. All these suggest that the superconductivity in this newly discovered superconductor cannot be formatted into the BCS theory.

cond-mat.supr-con↗

arXiv2012

Unexpected weak spatial variation of local density of sates induced by individual Co impurity atoms in Na(Fe{1-x}Cox)As as revealed by scanning tunneling spectroscopy

We use spatially resolved scanning tunneling spectroscopy in Na(Fe{1-x}Cox)As to investigate the impurity effect induced by Co dopants. The Co impurities are successfully identified, and the spatial distributions of local density of state at different energies around these impurities are investigated. It is found that the spectrum shows negligible spatial variation at different positions near the Co impurity, although there is a continuum of the in-gap states which lifts the zero-bias conductance to a finite value. Our results put constraints on the S+- and S++ models and sharpen the debate on the role of scattering potentials induced by the Co dopants.

cond-mat.supr-con↗

arXiv2012

Multi-Band Exotic Superconductivity in the New Superconductor Bi4O4S3

Resistivity, Hall effect and magnetization have been investigated on the new superconductor Bi4O4S3. A weak insulating behavior has been induced in the normal state when the superconductivity is suppressed. Hall effect measurements illustrate clearly a multiband feature dominated by electron charge carriers, which is further supported by the magnetoresistance data. Interestingly, a kink appears on the temperature dependence of resistivity at about 4 K at all high magnetic fields when the bulk superconductivity is completely suppressed. This kink can be well traced back to the upper critical field Hc2(T) in the low field region, and is explained as the possible evidence of residual Cooper pairs on the one dimensional chains.

cond-mat.supr-con↗

arXiv2012

Vortex images on Ba{1-x}KxFe2As2 observed directly by the magnetic force microscopy

The vortex states on optimally doped Ba0.6K0.4Fe2As2 and underdoped Ba0.77K0.23Fe2As2 single crystals are imaged by magnetic force microscopy at various magnetic fields below 100 Oe. Local triangular vortex clusters are observed in optimally doped samples. The vortices are more ordered than those in Ba(Fe{1-x}Co{x})2As2, and the calculated pinning force per unit length is about 1 order of magnitude weaker than that in optimally Co-doped 122 at the same magnetic field, indicating that the Co doping at the Fe sites induces stronger pinning. The proportion of six-neighbored vortices to the total amount increases quickly with increasing magnetic field, and the estimated value reaches 100% at several tesla. Vortex chains are also found in some local regions, which enhance the pinning force as well as the critical current density. Lines of vortex chains are observed in underdoped samples, and they may have originated from the strong pinning near the twin boundaries arising from the structural transition.

cond-mat.supr-con↗

arXiv2011

Theory of cooperation in a micro-organismal snow-drift game

We present a mean field model for the phase diagram of a community of micro-organisms, interacting through their metabolism so that they are, in effect, engaging in a cooperative social game. We show that as a function of the concentration of the nutrients glucose and histidine, the community undergoes a phase transition separating a state in which one strain is dominant to a state which is characterized by coexisting populations. Our results are in good agreement with recent experimental results, correctly predicting quantitative trends and the phase diagram.

physics.bio-ph↗

arXiv2010

Fixed points and limit cycles in the population dynamics of lysogenic viruses and their hosts

Starting with stochastic rate equations for the fundamental interactions between microbes and their viruses, we derive a mean field theory for the population dynamics of microbe-virus systems, including the effects of lysogeny. In the absence of lysogeny, our model is a generalization of that proposed phenomenologically by Weitz and Dushoff. In the presence of lysogeny, we analyze the possible states of the system, identifying a novel limit cycle, which we interpret physically. To test the robustness of our mean field calculations to demographic fluctuations, we have compared our results with stochastic simulations using the Gillespie algorithm. Finally, we estimate the range of parameters that delineate the various steady states of our model.

q-bio.PE↗