arXiv subjects

H. Xiao

Publications and source records attributed to H. Xiao.

At least 37 records · Page 2Linked to original sources

arXiv2016

Temperature-tunable Fano resonance induced by strong coupling between Weyl fermions and phonons in TaAs

Strong coupling between discrete phonon and continuous electron-hole pair excitations can give rise to a pronounced asymmetry in the phonon line shape, known as the Fano resonance. This effect has been observed in a variety of systems, such as stripe-phase nickelates, graphene and high-$T_{c}$ superconductors. Here, we reveal explicit evidence for strong coupling between an infrared-active $A_1$ phonon and electronic transitions near the Weyl points (Weyl fermions) through the observation of a Fano resonance in the recently discovered Weyl semimetal TaAs. The resultant asymmetry in the phonon line shape, conspicuous at low temperatures, diminishes continuously as the temperature increases. This anomalous behavior originates from the suppression of the electronic transitions near the Weyl points due to the decreasing occupation of electronic states below the Fermi level ($E_{F}$) with increasing temperature, as well as Pauli blocking caused by thermally excited electrons above $E_{F}$. Our findings not only elucidate the underlying mechanism governing the tunable Fano resonance, but also open a new route for exploring exotic physical phenomena through the properties of phonons in Weyl semimetals.

cond-mat.str-el↗

arXiv2016

Optical observation of spin-density-wave fluctuations in Ba122 iron-based superconductors

In iron-based superconductors, a spin-density-wave (SDW) magnetic order is suppressed with doping and unconventional superconductivity appears in close proximity to the SDW instability. The optical response of the SDW order shows clear gap features: substantial suppression in the low-frequency optical conductivity, alongside a spectral weight transfer from low to high frequencies. Here, we study the detailed temperature dependence of the optical response in three different series of the Ba122 system [Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ and BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$]. Intriguingly, we found that the suppression of the low-frequency optical conductivity and spectral weight transfer appear at a temperature $T^{\ast}$ much higher than the SDW transition temperature $T_{SDW}$. Since this behavior has the same optical feature and energy scale as the SDW order, we attribute it to SDW fluctuations. Furthermore, $T^{\ast}$ is suppressed with doping, closely following the doping dependence of the nematic fluctuations detected by other techniques. These results suggest that the magnetic and nematic orders have an intimate relationship, in favor of the magnetic-fluctuation-driven nematicity scenario in iron-based superconductors.

cond-mat.supr-con↗

arXiv2015

A Bayesian Calibration-Prediction Method for Reducing Model-Form Uncertainties with Application in RANS Simulations

Model-form uncertainties in complex mechanics systems are a major obstacle for predictive simulations. Reducing these uncertainties is critical for stake-holders to make risk-informed decisions based on numerical simulations. For example, Reynolds-Averaged Navier-Stokes (RANS) simulations are increasingly used in mission-critical systems involving turbulent flows. However, for many practical flows the RANS predictions have large model-form uncertainties originating from the uncertainty in the modeled Reynolds stresses. Recently, a physics-informed Bayesian framework has been proposed to quantify and reduce model-form uncertainties in RANS simulations by utilizing sparse observation data. However, in the design stage of engineering systems, measurement data are usually not available. In the present work we extend the original framework to scenarios where there are no available data on the flow to be predicted. In the proposed method, we first calibrate the model discrepancy on a related flow with available data, leading to a statistical model for the uncertainty distribution of the Reynolds stress discrepancy. The obtained distribution is then sampled to correct the RANS-modeled Reynolds stresses for the flow to be predicted. The extended framework is a Bayesian calibration-prediction method. The merits of the proposed method are demonstrated on two flows that are challenging to standard RANS models. By not requiring observation data on the flow to be predicted, the present calibration-prediction method will gain wider acceptance in practical engineering application. While RANS modeling is chosen to demonstrate the merits of the proposed framework, the methodology is generally applicable to other complex mechanics models involving solids, fluids flows, or the coupling between the two, where model-form uncertainties are present in the constitutive relations.

physics.flu-dyn↗

arXiv2015

Spin-flop transition and magnetic phase diagram in CaCo$_{2}$As$_{2}$ revealed by torque measurements

The magnetic properties of CaCo$_{2}$As$_{2}$ single crystal was systematically studied by using dc magnetization and magnetic torque measurements. A paramagnetic to antiferromagnetic transition occurs at $T_N$ = 74 K with Co spins being aligned parallel to the c axis. For $H \parallel c$, a field-induced spin-flop transition was observed below $T_N$ and a magnetic transition from antiferromagnetic to paramagnetic was inferred from the detailed analysis of magnetization and magnetic torque. Finally, we summarize the magnetic phase diagram of CaCo$_{2}$As$_{2}$ based on our results in the \emph{H-T} plane.

cond-mat.supr-con↗

arXiv2015

Optical Signatures of Weyl Points in TaAs

We present a systematic study of both the temperature and frequency dependence of the optical response in TaAs, a material that has recently been realized to host the Weyl semimetal state. Our study reveals that the optical conductivity of TaAs features a narrow Drude response alongside a conspicuous linear dependence on frequency. The width of the Drude peak decreases upon cooling, following a $T^{2}$ temperature dependence which is expected for Weyl semimetals. Two linear components with distinct slopes dominate the 5-K optical conductivity. A comparison between our experimental results and theoretical calculations suggests that the linear conductivity below $\sim$230~cm$^{-1}$ is a clear signature of the Weyl points lying in very close proximity to the Fermi energy.

cond-mat.mtrl-sci↗

arXiv2015

Spin-Fluctuation-Induced Non-Fermi-Liquid Behavior with suppressed superconductivity in LiFe$_{1-x}$Co$_{x}$As

A series of LiFe$_{1-x}$Co$_{x}$As compounds with different Co concentrations have been studied by transport, optical spectroscopy, angle-resolved photoemission spectroscopy and nuclear magnetic resonance. We observed a Fermi liquid to non-Fermi liquid to Fermi liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-NFL-FL crossover, we found that both the low-energy spin fluctuations and Fermi surface nesting are enhanced and then diminished, strongly suggesting that the NFL behavior in LiFe$_{1-x}$Co$_{x}$As is induced by low-energy spin fluctuations which are very likely tuned by Fermi surface nesting. Our study reveals a unique phase diagram of LiFe$_{1-x}$Co$_{x}$As where the region of NFL is moved to the boundary of the superconducting phase, implying that they are probably governed by different mechanisms.

cond-mat.supr-con↗

arXiv2015

Anomalous phonon redshift in K-doped BaFe2As2 iron pnictides

The effect of K, Co and P dopings on the lattice dynamics in the BaFe$_2$As$_2$ system is studied by infrared spectroscopy. We focus on the phonon at $\sim$ 253 cm$^{-1}$, the highest energy in-plane infrared-active Fe-As mode in BaFe$_2$As$_2$. Our studies show that the Co and P dopings lead to a blue shift of this phonon in frequency, which can be simply interpreted by the change of lattice parameters induced by doping. In sharp contrast, an unusual red shift of the same mode was observed in the K-doped compound, at odds with the above explanation. This anomalous behavior in K-doped BaFe$_2$As$_2$ is more likely associated with the coupling between lattice vibrations and other channels, such as charge or spin. This coupling scenario is also supported by the asymmetric line shape and intensity growth of the phonon in the K-doped compound.

cond-mat.supr-con↗

arXiv2014

Electron-phonon coupling in the superconducting single crystal Rb$_{0.8}$Fe$_{1.68}$Se$_2$

The optical properties of the superconducting single crystal Rb$_{0.8}$Fe$_{1.68}$Se$_2$ with $T_{c}$ $\simeq$ 31 K have been measured over a wide frequency range in the $ab$ plane. We found that the optical conductivity is dominated by a series of infrared-active phonon modes at low-frequency region as well as several other high-frequency bound excitations. The low-frequency optical conductivity has rather low value and shows quite small Drude-like response, indicating low carriers density in this material. Furthermore, the phonon modes increase continuously in frequency with decreasing temperature; specifically, the phonon mode around 200 cm$^{-1}$ shows an enhanced asymmetry effect at low temperatures, suggesting an increasing electron-phonon coupling in this system.

cond-mat.supr-con↗

arXiv2013

Hidden $T$-Linear Scattering Rate in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Revealed by Optical Spectroscopy

The optical properties of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ have been determined in the normal state for a number of temperatures over a wide frequency range. Two Drude terms, representing two groups of carriers with different scattering rates ($1/τ$), well describe the real part of the optical conductivity, $σ_{1}(ω)$. A "broad" Drude component results in an incoherent background with a $T$-independent $1/τ_b$, while a "narrow" Drude component reveals a $T$-linear $1/τ_n$ resulting in a resistivity $ρ_n \equiv 1/σ_{1n}(ω\rightarrow 0)$ also linear in temperature. An arctan($T$) low-frequency spectral weight is also a strong evidence for a $T$-linear 1/$τ$. Comparison to other materials with similar behavior suggests that the $T$-linear $1/τ_n$ and $ρ_n$ in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.

cond-mat.supr-con↗

arXiv2012

Filamentary superconductivity across the phase diagram of Ba(Fe,Co)$_2$As$_2$

We show magnetotransport results on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.0 \leq x \leq 0.13$) single crystals. We identify the low temperature resistance step at 23 K in the parent compound with the onset of filamentary superconductivity (FLSC), which is suppressed by an applied magnetic field in a similar manner to the suppression of bulk superconductivity (SC) in doped samples. FLSC is found to persist across the phase diagram until the long range antiferromagnetic order is completely suppressed. A significant suppression of FLSC occurs for $0.02<x<0.04$, the doping concentration where bulk SC emerges. Based on these results and the recent report of an electronic anisotropy maximum for 0.02 $\leq x \leq$ 0.04 [Science 329, 824 (2010)], we speculate that, besides spin fluctuations, orbital fluctuations may also play an important role in the emergence of SC in iron-based superconductors.

cond-mat.supr-con↗

arXiv2012

Transport Measurements on Nano-engineered Two Dimensional Superconducting Wire Networks

Superconducting triangular Nb wire networks with high normal-state resistance are fabricated by using a negative tone hydrogen silsesquioxane (HSQ) resist. Robust magnetoresistance oscillations are observed up to high magnetic fields and maintained at low temperatures, due to the eective reduction of wire dimensions. Well-defined dips appear at integral and rational values (1/2, 1/3, 1/4) of the reduced flux f = Phi/Phi_0, which is the first observation in the triangular wire networks. These results are well consistent with theoretical calculations for the reduced critical temperature as a function of f.

cond-mat.mes-hall↗

arXiv2012

Edge superconducting state in Nb thin film with rectangular arrays of antidots

Superconducting Nb thin films with rectangular arrays of submicron antidots have been systemically investigated by transport measurements. In low fields, the magnetoresistance curves demonstrate well-defined dips at integral and rational numbers of flux quanta per unit cell, which corresponds to a superconducting wire network-like regime. When the magnetic field is higher than a saturation field, interstitial vortices interrupt the collective oscillation in low fields and form vortex sublattice, where a larger magnetic field interval is observed. In higher fields, a crossover behavior from the interstitial sublattice state to a single-loop-like state is observed, characterized by oscillations with a period of $Φ_0/πr_{eff}^2$, originating from the existence of edge superconducting states with a size $r_{eff}$ around the antidots.

cond-mat.mes-hall↗

arXiv2012

Wire network behavior in superconducting Nb films with diluted triangular arrays of holes

We present transport measurement results on superconducting Nb films with diluted triangular arrays (honeycomb and kagomé) of holes. The patterned films have large disk-shaped interstitial regions even when the edge-to-edge separations between nearest neighboring holes are comparable to the coherence length. Changes in the field interval of two consecutive minima in the field dependent resistance $R(H)$ curves are observed. In the low field region, fine structures in the $R(H)$ and $T_c(H)$ curves are identified in both arrays. Comparison of experimental data with calculation results shows that these structures observed in honeycomb and kagomé hole arrays resemble those in wire networks with triangular and $T_3$ symmetries, respectively. Our findings suggest that even in these specified periodic hole arrays with very large interstitial regions, the low field fine structures are determined by the connectivity of the arrays

cond-mat.supr-con↗

arXiv2012

Evidence for filamentary superconductivity nucleated at antiphase domain walls in antiferromagnetic CaFe$_2$As$_2$

Resistivity, magnetization and microscopic $^{75}$As nuclear magnetic resonance (NMR) measurements in the antiferromagnetically ordered state of the iron-based superconductor parent material CaFe$_2$As$_2$ exhibit anomalous features that are consistent with the collective freezing of domain walls. Below $T^*\approx 10$ K, the resistivity exhibits a peak and downturn, the bulk magnetization exhibits a sharp increase, and $^{75}$As NMR measurements reveal the presence of slow fluctuations of the hyperfine field. These features in both the charge and spin response are strongly field dependent, are fully suppressed by $H^*\approx 15$ T, and suggest the presence of filamentary superconductivity nucleated at the antiphase domain walls in this material.

cond-mat.supr-con↗

arXiv2011

Strong magnetic fluctuations in superconducting state of CeCoIn$_5$

We show results on the vortex core dissipation through current-voltage measurements under applied pressure and magnetic field in the superconducting phase of CeCoIn$_5$. We find that as soon as the system becomes superconducting, the vortex core resistivity increases sharply as the temperature and magnetic field decrease. The sharp increase in flux flow resistivity is due to quasiparticle scattering on critical antiferromagnetic fluctuations. The strength of magnetic fluctuations below the superconducting transition suggests that magnetism is complimentary to superconductivity and therefore must be considered in order to fully account for the low-temperature properties of CeCoIn$_5$.

cond-mat.supr-con↗

arXiv2011

Stray field and superconducting surface spin valve effect in La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-δ}$ bilayers

Electronic transport and magnetization measurements were performed on La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-δ}$ (LCMO/YBCO) bilayers below the superconducting transition temperature in order to study the interaction between magnetism and superconductivity. This study shows that a substantial number of weakly pinned vortices are induced in the YBCO layer by the large out-of-plane stray field in the domain walls. Their motion gives rise to large dissipation peaks at the coercive field. The angular dependent magnetoresistance (MR) data reveal the interaction between the stripe domain structure present in the LCMO layer and the vortices and anti-vortices induced in the YBCO layer by the out-of-plane stray field. In addition, this study shows that a superconducting surface spin valve effect is present in these bilayers as a result of the relative orientation between the magnetization at the LCMO/YBCO interface and the magnetization in the interior of the LCMO layer that can be tuned by the rotation of a small $H$. This latter finding will facilitate the development of superconductive magnetoresistive memory devices. These low-magnetic field MR data, furthermore, suggest that triplet superconductivity is induced in the LCMO layer, which is consistent with recent reports of triplet superconductivity in LCMO/YBCO/LCMO trilayers and LCMO/YBCO bilayers.

cond-mat.supr-con↗

arXiv2009

A Simple and Efficient Regularization Method for 3D BEM: Application to Frequency-Domain Elastodynamics

An efficient and easy-to-implement method is proposed to regularize integral equations in the 3D boundary element method (BEM). The method takes advantage of an assumed three-noded triangle discretization of the boundary surfaces. The method is based on the derivation of analytical expressions of singular integrals. To demonstrate the accuracy of the method, three elastodynamic problems are numerically worked out in the frequency domain: a cavity under harmonic pressure, diffraction of a plane wave by a spherical cavity, and amplification of seismic waves in a semispherical alluvial basin (the second one is also investigated in the time domain). The numerical results are compared to semi-analytical solutions; a close agreement is found for all problems, showing the accuracy of the proposed method.

physics.class-ph↗

arXiv2008

Anomalous Paramagnetic Magnetization in Mixed State of CeCoIn$_5$ single crystals

Magnetization and torque measurements were performed on CeCoIn$_5$ single crystals to study the mixed-state thermodynamics. These measurements allow the determination of both paramagnetic and vortex responses in the mixed-state magnetization. The paramagnetic magnetization is suppressed in the mixed state with the spin susceptibility increasing with increasing magnetic field. The dependence of spin susceptibility on magnetic field is due to the fact that heavy electrons contribute both to superconductivity and paramagnetism and a large Zeeman effect exists in this system. No anomaly in the vortex response was found within the investigated temperature and field range.

cond-mat.str-el↗