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T. Zhou

Publications and source records attributed to T. Zhou.

At least 19 recordsLinked to original sources

On-chip Bragg peak extraction from MHz frame rate X-ray detectors using a cellular automaton architecture

High-frame rate pixel detectors can produce data volumes that exceed available off-chip bandwidth, yet in many applications only a sparse subset of each frame carries relevant information. Spatially localized events, including diffraction peaks in crystallography, particle hits in tracking detectors, fluorescence spots in biological imaging, and other applications, all require that clusters of above-threshold pixels be identified and extracted from an otherwise featureless background. Conventionally this is performed in software algorithms such as connected-component labeling on full frames, but at MHz frame rates the resulting throughput becomes prohibitive. We present a lightweight hardware architecture that performs peak localization and patch extraction directly in the sensor silicon, transmitting only small pixel patches rather than complete frames. FPGA-based testing on an AMD Alveo V80 validated the synthesizability and timing closure of the peak-finding module in real hardware. The design replaces global connected-component labeling with a cellular automaton that uses purely local, fixed-iteration neighborhood operations, eliminating the label storage and equivalence-resolution logic that make conventional approaches impractical on-chip. We validate the architecture on X-ray Bragg peak detection for far-field high-energy diffraction microscopy and show that every peak found by a software reference is recovered, with equivalent downstream reconstructions. The architecture sustains several-hundred-kHz frame rates in 130nm CMOS and exceeds 1MHz in 28nm.

physics.ins-det

Two-Dimensional Far-Field Correlations of X-ray Photon Pairs

We directly observe far-field correlations of x-ray photon pairs generated by spontaneous parametric down-conversion (SPDC). Using an energy-resolved, two-dimensional photon counting detector we record the full ring-shaped emission of both photons across a broad bandwidth and extract pair correlations directly from raw events without imposing angular constraints. The ring radii scale with photon energy, in quantitative agreement with transverse phase matching, providing a stringent momentum-space validation of x-ray SPDC. These observations open a route to leveraging quantum correlations in x-ray imaging and metrology, including correlation-enhanced magnification and reduced blurring.

quant-ph

Extending the Takagi-Taupin equations for x-ray nanobeam Bragg coherent diffraction

We present a new approach for simulating x-ray nanobeam Bragg coherent diffraction patterns based on the Takagi-Taupin equations. Compared to conventional methods, the current approach can be universally applied to any weakly strained system including semi-infinite crystals that diffract dynamically. It addresses issues such as the curved wavefront and re-divergence of the focused incident beam. We show excellent agreement against experimental data on a strained La0.7Sr0.3MnO3 thin film on SrTiO3 substrate, and a path to extracting physical information using automatic differentiation.

cond-mat.mes-hall

Neural Network Methods for Radiation Detectors and Imaging

Recent advances in image data processing through machine learning and especially deep neural networks (DNNs) allow for new optimization and performance-enhancement schemes for radiation detectors and imaging hardware through data-endowed artificial intelligence. We give an overview of data generation at photon sources, deep learning-based methods for image processing tasks, and hardware solutions for deep learning acceleration. Most existing deep learning approaches are trained offline, typically using large amounts of computational resources. However, once trained, DNNs can achieve fast inference speeds and can be deployed to edge devices. A new trend is edge computing with less energy consumption (hundreds of watts or less) and real-time analysis potential. While popularly used for edge computing, electronic-based hardware accelerators ranging from general purpose processors such as central processing units (CPUs) to application-specific integrated circuits (ASICs) are constantly reaching performance limits in latency, energy consumption, and other physical constraints. These limits give rise to next-generation analog neuromorhpic hardware platforms, such as optical neural networks (ONNs), for high parallel, low latency, and low energy computing to boost deep learning acceleration.

physics.ins-det

Absence of $3a_0$ Charge Density Wave Order in the Infinite Layer Nickelates

A hallmark of many unconventional superconductors is the presence of many-body interactions which give rise to broken symmetry states intertwined with superconductivity. Recent resonant soft x-ray scattering experiments report commensurate $3a_0$ charge density wave order in the infinite layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both the cuprates and nickelates. Here, we present x-ray scattering and spectroscopy measurements on a series of NdNiO$_{2+x}$ samples which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO$_2$. The $3a_0$ superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with 3 unit cell periodicity. The absence of any observable charge density wave order in NdNiO$_2$ highlights a crucial difference between the phase diagrams of the cuprate and nickelate superconductors.

cond-mat.supr-con

Quantum phase diagram of the $S=1/2$ triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$

The magnetic phases of the ideal spin-1/2 triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$ are identified and studied using $^{135,137}$Ba nuclear magnetic resonance (NMR) spectroscopy in magnetic fields ranging to 30T, oriented parallel and near perpendicular to the crystallographic $ab$-plane. For both directions, the saturation field is approximately 33T. Notably, the NMR spectra provide microscopic evidence for the stabilization of an up-up-down spin configuration for in-plane fields, giving rise to an one-third magnetization plateau ($M_\text{sat}/3$), as well as for a higher field phase transition near to $\sim (3/5)M_\text{sat}$ for both field orientations. Phase transitions are signaled by the evolution of the NMR spectra, and in some cases through spin-lattice relaxation measurements. The results are compared with expectations obtained from a semi-classical energy density modeling, in which quantum effects are incorporated by effective interactions extracted from the spin-wave analysis of the two-dimensional model. The interlayer coupling also plays a significant role in the outcome. Good agreement between the model and the experimental results is achieved, except for the case of fields approaching the saturation value applied along the c-axis.

cond-mat.str-el

Antiferromagnetic order in Ca$_{10}$(Pt$_3$As$_8$)(Fe$_2$As$_2$)$_5$ observed by $^{75}$As NMR

$^{75}$As nuclear magnetic resonance (NMR) measurements carried out on underdoped, non-superconducting Ca$_{10}$(Pt$_3$As$_8$)(Fe$_2$As$_2$)$_5$ reveal physical properties that are similar but not identical to the 122 superconductor parent compounds such as BaFe$_2$As$_2$. Results from the single crystal study indicate a phase transition to an antiferromagnetic (AF) state on cooling through T~100K, albeit nonuniformly. Specifically, the NMR lineshape reflects the presence of staggered hyperfine fields on the As sites associated with a striped AF order. The variation of the internal hyperfine field with temperature suggests that the phase transition to the AF state is discontinuous, and therefore likely coincident with the structural transition inferred from transport experiments.

cond-mat.str-el

Mueller matrices for anisotropic metamaterials generated using 4x4 matrix formalism

Forward models for the Mueller Matrix (MM) components of materials with relative magnetic permeability tensor μ \neq 1 are studied. 4x4 matrix formalism is employed to produce general solutions for the complex reflection coefficients and MMs of dielectric-magnetic materials having arbitrary crystal symmetry and arbitrary laboratory orientation. For certain orientations of materials with simultaneously diagonalizable ε and μ tensors (with coincident principal axes), analytic solutions to the Berreman equation are available. For the single layer thin film configuration of these materials, analytic formulas for the complex reflection and transmission coefficients are derived for orthorhombic or higher crystal symmetry. The separation of the magnetic and dielectric contributions to the optical properties of a material are demonstrated using measurements of the MM at varying angles of incidence.

physics.optics

The Layer 0 Inner Silicon Detector of the D0 Experiment

This paper describes the design, fabrication, installation and performance of the new inner layer called Layer 0 (L0) that was inserted in the existing Run IIa Silicon Micro-Strip Tracker (SMT) of the D0 experiment at the Fermilab Tevatron collider. L0 provides tracking information from two layers of sensors, which are mounted with center lines at a radial distance of 16.1 mm and 17.6 mm respectively from the beam axis. The sensors and readout electronics are mounted on a specially designed and fabricated carbon fiber structure that includes cooling for sensor and readout electronics. The structure has a thin polyimide circuit bonded to it so that the circuit couples electrically to the carbon fiber allowing the support structure to be used both for detector grounding and a low impedance connection between the remotely mounted hybrids and the sensors.

physics.ins-det

Temperature Dependent Local Structure of LaFeAsO_{1-x}F_x: Probing the Bond Correlations

The local structure of the parent and doped LaFeAsO1-xFx (pnictide) compounds were studied by x-ray absorption spectroscopy. In the doped system, the Fe-As and Fe-Fe correlations are well modeled by an Einstein model with no low temperature anomalies. For the Fe-As bonds, the Einstein temperatures are identical for the doped (11%) and undoped samples, but the doped sample is found to have a lower level of static disorder. For the Fe-Fe correlation, doping enhances the effective Einstein temperature of Fe-Fe atom correlation. The results suggest that the onset of superconductivity in the F doped system may be related to enhanced magnetic correlations. Density functional calculations of the charge density reveal strong bonding between neighboring As ions but metal-like behavior in the Fe layers.

cond-mat.supr-con

Phenomenological theory of spin excitations in La- and Y-based cuprates

Motivated by recent inelastic neutron scattering (INS) experiments on La-based cuprates and based on the fermiology theories, we study the spin susceptibility for La-based (e.g., La$_{2-x}$Sr$_x$CuO$_4$) and Y-based (e.g., YBa$_2$Cu$_3$O$_y$) cuprates, respectively. The spin excitation in YBa$_2$Cu$_3$O$_y$ is dominated by a sharp resonance peak at the frequency 40 meV in the superconducting state. Below and above the resonance frequency, the incommensurate (IC) peaks develop and the intensity of the peaks decreases dramatically. In the normal state, the resonant excitation does not occur and the IC peaks are merged into commensurate ones. The spin excitation of La$_{2-x}$Sr$_x$CuO$_4$ is significantly different from that of Y-based ones, namely, the resonance peak does not exist due to the decreasing of the superconducting gap and the presence of the possible spin-stripe order. The spectra are only enhanced at the expected resonance frequency (about 18 meV) while it is still incommensurate. On the other hand, another frequency scale at the frequency 55 meV is also revealed, namely the spectra are commensurate and local maximum at this frequency. We elaborate all the results based on the Fermi surface topology and the d-wave superconductivity, and suggest that the spin-stripe order be also important in determining the spin excitation of La-based cuprates. A coherent picture for the spin excitations is presented for Y-based and La-based cuprates.

cond-mat.supr-con

Is the dispersion anomaly of the electron spectrum induced by the charge-density-wave in high-$T_c$ superconductors?

We propose that the presence of a rotationally symmetric charge density wave (CDW) with the modulation of 4-lattice constant in high-$T_c$ superconductors is essentially responsible for an anomalous quasiparticle dispersion revealed recently by angle-resolved photoemission spectroscopy (ARPES) experiments. We elaborate clearly that the nodal quasiparticle is well-defined at low energies, while the dispersion breaks up at the energy $E_1$ and reappears at the energy $E_2$. Our results are in good agreement with both the ARPES and scanning tunneling microscopy experiments.

cond-mat.supr-con

Doping dependance of the spin resonance peak in bilayer high-$T_c$ superconductors

Motivated by a recent experiment on the bilayer Y$_{1-x}$Ca$_{x}$Ba$_2$Cu$_3$O$_y$ superconductor and based on a bilayer $t-J$ model, we calculate the spin susceptibility at different doping densities in the even and odd channels in a bilayer system. It is found that the intensity of the resonance peak in the even channel is much weaker than that in the odd one, with the resonance position being at a higher frequency. While this difference decreases as the doping increases, and both the position and amplitude of the resonance peaks in the two channels are very similar in the deeply overdoped sample. Moreover, the resonance frequency in the odd channel is found to be linear with the critical temperature $T_c$, while the resonance frequency increases as doping decreases in the even channel and tends to saturate at the underdoped sample. We elaborate the results based on the Fermi surface topology and the d-wave superconductivity.

cond-mat.supr-con

Theory of optical conductivity in detwinned YBa$_2$Cu$_3$O$_{y}$

We study theoretically the chain and in-plane optical conductivities of detwinned YBa$_2$Cu$_3$O$_{y}$. We elaborate that the chain is superconducting for $y>6.67$ while insulating for $y<6.67$ due to the competition between the plane-chain coupling and the antiferromagnetic order, corresponding to a superconductor-insulator transition. Stemming also from the coupling between the plane and chain, a new peak emerges at a low frequency in the in-plane spectra in the superconducting state, while it disappears in the normal state. Our scenario accounts satisfactorily for very recent experiments.

cond-mat.supr-con

Anisotropy of incommensurate spin and charge fluctuations in detwinned YBa$_2$Cu$_3$O$_{6+δ}$

Motivated by a recent neutron scattering experiment on detwinned YBa$_2$Cu$_3$O$_{6+δ}$ superconductor, we examine the frequency and doping dependence of the anisotropy in the spin and charge fluctuation arising from the coupling between the plane and the chain. Starting from the two-dimensional $t$-$t^{'}$-$J$ model and using the random-phase approximation (RPA), we find a pronounced anisotropy of the incommensurate (IC) peaks in the spin channel, namely the peak intensity at the $(π\pmδ,π)$ direction is stronger than that at the $(π,π\pmδ)$ direction in a wide frequency range from $ω=0.2J$ to the resonance frequency $ω_r=0.35J$. Above the resonance frequency, the IC peaks reemerge. Their intensities shift to the diagonal direction and no anisotropy exists along the two diagonal directions. We find that this anisotropy is robust with respect to the possible variation of the RPA correction factor and to the dopings. The charge fluctuation is also found to be incommensurate for all energies considered and peak at $(0,δ)$ and $(δ,0)$. An anisotropy in its IC peak intensity along the $k_x$ and the $k_y$ direction exists, but in sharp contrast to the spin channel, the maximum intensity of the IC peak is along the $k_y$ direction. Moreover, the IC peak in the charge channel exhibits an upward dispersion, in contrast to the downward dispersion below the spin resonance frequency for the spin IC peak. We explain these features based on the effect of the plane-chain coupling on the topology of the Fermi surface.

cond-mat.supr-con

Theoretical understanding of the quasiparticle dispersion in bilayer high-$T_{c}$ superconductors

The renormalization of quasiparticle (QP) dispersion in bilayer high-$T_{c}$ cuprates is investigated theoretically by examining respectively the interactions of the QP with spin fluctuations (SF) and phonons. It is illustrated that both interactions are able to give rise to a kink in the dispersion around the antinodes (near $(π,0)$). However, remarkable differences between the two cases are found for the peak/dip/hump structure in the lineshape, the QP weight, and the interlayer coupling effect on the kink, which are suggested to serve as a discriminance to single out the dominant interaction in the superconducting state. A comparison to recent photoemission experiments shows clearly that the coupling to the spin resonance is dominant for the QP around antinodes in bilayer systems.

cond-mat.supr-con

Partial quantum statistics and its implications for narrow band materials

Based upon the newly proposed partial quantum statistics [T. Zhou, Solid State Commun. 115, 185 (2000)], some canonical physical properties of partially localized electron systems have been calculated. The calculated transport and superconducting properties of such systems are very different from those of Landau Fermi liquids, but display some striking similarities to the properties of high temperature superconductors and some other narrow band materials.

cond-mat.str-el