In-plane Fluctuations Research Articles

Dynamic frequency shift in NV− center in diamond induced by anisotropic in-plane vacuum fluctuation

Sensor applications of a negatively charged nitrogen vacancy (NV−) center in diamond are now in practical use, yet for finer sensitivity, a comprehensive understanding of various kinds of sources that cause detrimental relaxation and damping is still required. During the course of theoretical study regarding this, we found that Gaussian white noise with the zero mean has a substantial effect, which manifests itself in a period of free induction decay (FID) oscillation. This effect is experimentally detectable through comparison with zero-field splitting fixed by, e.g., optically detected magnetic resonance. The result is corroborated by a different analytical framework, the Lindblad master equation. Our finding in the FID oscillation period, or an equivalent energy shift, is concluded to fall into a class of dynamic frequency shift.

Mid-infrared modulated photoluminescence mapping to investigate in-plane distributions of bandedge transitions in As-doped HgCdTe

In-plane distribution of band edge electronic structure of the narrow-gap arsenic (As)-doped HgCdTe is a crucial topic fundamentally and technically for mid-infrared detector array but hard to be investigated for long experimentally. This work reports a demonstration of the modulated photoluminescence (PL) mapping carried on As-doped HgCdTe, with the spatial resolution of a typical array pixel scale and with the elaborate band edge transitions being distinguished. Curve fittings related to the bandgap and to the impurities of Hg vacancy (VHg), As donor (AsHg), and AsHg–VHg pair are confirmed for PL mapping assisted by a temperature-dependent PL analysis. The spatial non-uniformity of Cd composition and the distributions of impurity relative concentrations are derived. Correlation analysis suggests that the As atom directly replacing the Hg atom in the complete lattice and the As atom occupying the already-existing VHg are the two approaches for the AsHg formation during As doping. The non-uniform band edge properties may lead to the in-plane conductivity fluctuation detrimental to the array performance. The results indicate the significance of probing the band edge impurity uniformity of As-doped HgCdTe for arrays fabrication, and the modulated PL mapping serving as a desirable and effective tool.

Effects of the wall heat flux on the flow characteristics of large-scale coherent structures in a pipe with enhanced heat transfer

In this study, we use stereoscopic particle image velocimetry (SPIV) to measure the internal instantaneous flow field of a typical pipe with enhanced heat transfer (a single-start spirally corrugated pipe with a pitch-to-diameter ratio S/D = 2.22) with and without a wall heat flux for Reynolds number ReD = 10000 and 25000. Proper orthogonal decomposition (POD) is used to extract the large-scale structures of the internal flow field. The POD results for the flow in the spirally corrugated pipe are employed to quantitatively analyze the turbulent kinetic energy (TKE) and Reynolds shear stress (RSS) using the corresponding results for a straight circular pipe as a benchmark. The results reveal the flow patterns of the large-scale motions in the flow field of spirally corrugated pipes are not sensitive to the wall heat flux and Reynolds number. Nevertheless, convective heat transfer can boost the energy content of large-scale coherent structures in a spiral pipe because of an increase in in-plane velocity fluctuations. Furthermore, the large-scale coherent structures of the swirling flow are reflected in multiple Q2 and Q4 events, which contribute positively to the net RSS. However, convective heat transfer reduces the contribution of these structures to the total RSS through the various parameters associated with these events.

Lower bound on the spread of valley splitting in Si/SiGe quantum wells induced by atomic rearrangement at the interface

The achievement of universal quantum computing critically relies on scalability. However, ensuring the necessary uniformity for scalable silicon electron spin qubits poses a significant challenge due to the considerable fluctuations in valley splitting energy (E VS) across quantum dot arrays, which impede the initialization of qubit systems comprising multiple spins and give rise to spin–valley entanglement resulting in the loss of spin information. These E VS fluctuations have been attributed to variations in the in-plane averaged alloy concentration along the confinement direction of Si/SiGe quantum wells. In this study, employing atomistic pseudopotential calculations, we unveil a significant spectrum of E VS even in the absence of such concentration fluctuations. This spectrum represents the lower limit of the wide range of E VS observed in numerous Si/SiGe quantum devices. By constructing simplified interface atomic step models, we analytically demonstrate that the lower bound of the E VS spread originates from the in-plane random distribution of Si and Ge atoms within SiGe barriers — an inherent characteristic that has been previously overlooked. Additionally, we propose an interface engineering approach to mitigate the in-plane randomness-induced fluctuations in E VS by inserting a few monolayers of pure Ge barrier at the Si/SiGe interface. Our findings provide valuable insights into the critical role of in-plane randomness in determining E VS in Si/SiGe quantum devices and offer reliable methods to enhance the feasibility of scalable Si-based spin qubits.

Canted ferromagnetic order in nonsuperconducting Eu(Fe1−xNix)2As2

The magnetic order in a series of nonsuperconducting $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals is investigated in detail by polarized neutron diffraction. The Fe and Eu magnetic sublattices are found to be almost magnetically decoupled in this system. With the Ni doping, the Eu sublattice shows a dramatic change in its magnetic ground state, from the A-type antiferromagnetic order for $\mathit{x}\ensuremath{\le}0.02$, in which the spins lie in the $\mathit{ab}$ plane, to a pure canted ferromagnetic order for $0.04\ensuremath{\le}\mathit{x}\ensuremath{\le}0.20$, in which the spins are canted with a small angle off the $\mathit{c}$ axis. By comparison with the $\mathit{c}$-axis perfectly aligned ferromagnetic structure widely observed in other superconducting compounds, the lack of superconductivity in $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{As}}_{2}$ might be associated with the formation of in-plane ferromagnetism. The doping-induced change of the lattice constants and accordingly the variation of the strength of indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments is speculated to be responsible for the dramatic change of the Eu spin structure. In addition, a spin reorientation of the ${\mathrm{Eu}}^{2+}$ moments in an intermediate temperature range is observed for $\mathit{x}=0.20$, in which some in-plane spin fluctuations persists above the long-range ordering temperature ${T}_{\mathrm{Eu}}$.

The observation of quantum fluctuations in a kagome Heisenberg antiferromagnet

The search for the experimental evidence of quantum spin liquid (QSL) states is critical but extremely challenging, as the quenched interaction randomness introduced by structural imperfection is usually inevitable in real materials. YCu3(OH)6.5Br2.5 (YCOB) is a spin-1/2 kagome Heisenberg antiferromagnet (KHA) with strong coupling of 〈J1〉 ~ 51 K but without conventional magnetic freezing down to 50 mK ~ 0.001〈J1〉. Here, we report a Br nuclear magnetic resonance (NMR) study of the local spin susceptibility and dynamics on the single crystal of YCOB. The temperature dependence of NMR main-line shifts and broadening can be well understood within the frame of the KHA model with randomly distributed hexagons of alternate exchanges, compatible with the formation of a randomness-induced QSL state at low temperatures. The in-plane spin fluctuations as measured by the spin-lattice relaxation rates (1/T1) exhibit a weak temperature dependence down to T ~ 0.03〈J1〉. Our results demonstrate that the majority of spins remain highly fluctuating at low temperatures despite the quenched disorder in YCOB.

Influence of Filament Winding Tension on the Deformation of Composite Flywheel Rotors with H-Shaped Hubs.

The residual stress plays an important role in composite flywheel rotors composed of filament windings. The fiber tension during high-prestressed winding is the main source of the rotor deformation and residual stress of composite layers. In this study, the effect of the winding tension gradient on deformation was monitored in real-time. Two types of in-plane winding tension fluctuation methods were developed to investigate the effect of tension on deformation. Online and offline measurements were performed for the strain acquisition. A wireless strain instrument was used for online deformation monitoring and a laser scanner was used for the offline surface reconstruction. Additionally, different filament winding strategies were carried out to improve the efficiency of the winding tension by finite element analysis. The results indicated that the deviation between numerical and experimental results was within 8%. Based on the proposed numerical method, the influence of the in-plane and out-of-plane winding tension gradient distributions on the rotation process of the H-shaped rotor was analyzed. An in-plane winding strategy with variable tension was developed, which increased the initial failure speed by 160%.

Emerging Long‐Range Order from a Freeform Disordered Metasurface

Recently, disordered metasurfaces have attracted considerable interest due to their potential applications in imaging, holography, and wavefront shaping. However, how to emerge long-range ordered phase distribution in disordered metasurfaces remains an outstanding problem. Here, a general framework is proposed to generate a spatially homogeneous in-plane phase distribution from a disordered metasurface, by engineering disorder parameters together with topology optimization. As a proof-of-concept demonstration, an all-dielectric disordered supercell metasurface with relatively homogeneous in-plane phase fluctuation is designed by disorder parameter engineering, manifesting as polarization conversion-dependent random scattering or unidirectional transmission. Then, a topology optimization approach is utilized to overcome the lattice coupling effect and to further improve the homogeneity of complex electric field fluctuation. In comparison with the initial supercell metasurface, both the phase fluctuation range and the relative efficiency of the topology-optimized freeform metasurface are significantly improved, leading to a long-range ordered electric field distribution. Moreover, three experimental realizations are performed, all of which agree well with the theoretical results. This methodology may inspire more exotic optical phenomena and find more promising applications in disordered metasurfaces and disorderedoptics.

Diffuse scattering from lamellar structures.

Lamellar structures are formed in a variety of soft materials including lipids, surfactants, block polymers, clays, colloids, semicrystalline polymers and others. Lamellar phases are characterized by scattering patterns containing pseudo-Bragg peaks from the layer ordering. However, fluctuations of the lamellae give rise to diffuse scattering in addition. This diffuse scattering can provide valuable information on the elastic properties of lamellae which control their fluctuations. A number of models to account for this are described in this Tutorial Review, along with examples from the literature. In addition, diffuse scattering from in-plane fluctuations or structures such as perforations or patterned nanoparticles is considered. This type of diffuse scattering can give unique information on the nature of, and positional (and bond orientational) ordering within, correlated structures within the lamellar plane. Anisotropic diffuse scattering features from thermotropic smectic phases is also briefly discussed.

Spin and charge order in doped spin-orbit coupled Mott insulators

We study a two-dimensional single band Hubbard Hamiltonian with antisymmetric spin-orbit coupling. We argue that this is the minimal model to understand the electronic properties of locally non-centrosymmetric transition-metal (TM) oxides such as Sr$_2$IrO$_4$. Based on exact diagonalizations of small clusters and the random phase approximation, we investigate the correlation effects on charge and magnetic order as a function of doping and of the TM-oxygen-TM bond angle $\theta$. For small doping and $\theta$ $\lesssim$ $15^\circ$ we find dominant commensurate in-plane antiferromagnetic fluctuations while ferromagnetic fluctuations dominate for $\theta$ $\gtrsim$ $25^\circ$. Moderately strong nearest-neighbor Hubbard interactions can also stabilize a charge density wave order. Furthermore, we compare the dispersion of magnetic excitations for the hole-doped case to resonant inelastic X-ray scattering data and find good qualitative agreement.

Dynamical spin correlations of the kagome antiferromagnet

Temperature-dependent dynamical spin correlations, which can be readily accessed via a variety of experimental techniques, hold the potential of offering a unique fingerprint of quantum spin liquids and other intriguing dynamical states. In this work we present an in-depth study of the temperature-dependent dynamical spin structure factor $S({\bf q}, \omega)$ of the antiferromagnetic (AFM) Heisenberg spin-1/2 model on the kagome lattice with additional Dzyaloshinskii--Moriya (DM) interactions. Using the finite-temperature Lanczos method on lattices with up to $N = 30$ sites we find that even without DM interactions, chiral low-energy spin fluctuations of the $120^\circ$ AFM order parameter dominate the dynamical response. This leads to a nontrivial frequency dependence of $S({\bf q}, \omega)$ and the appearance of a pronounced low-frequency mode at the M point of the extended Brillouin zone. Adding an out-of-plane DM interactions $D^z$ gives rise to an anisotropic dynamical response, a softening of in-plane spin fluctuations, and, ultimately, the onset of a coplanar AFM ground-state order at $D^z > 0.1 J$. Our results are in very good agreement with existing inelastic neutron scattering and temperature-dependent NMR spin-lattice relaxation rate ($1/T_1$) data on the paradigmatic kagome AFM herbertsmithite, where the effect of its small $D^z$ on the dynamical spin correlations is shown to be rather small, as well as with $1/T_1$ data on the novel kagome AFM YCu$_3$(OH)$_6$Cl$_3$, where its substantial $D^z \approx 0.25 J$ interaction is found to strongly affect the spin dynamics.

Broadening of the drumhead-mode spectrum due to in-plane thermal fluctuations of two-dimensional trapped ion crystals in a Penning trap

Two-dimensional crystals of ions stored in Penning traps are a leading platform for quantum simulation and sensing experiments. For small amplitudes, the out-of-plane motion of such crystals can be described by a discrete set of normal modes called the drumhead modes, which can be used to implement a range of quantum information protocols. However, experimental observations of crystals with Doppler-cooled and even near-ground-state-cooled drumhead modes reveal an unresolved drumhead-mode spectrum. In this work, we establish in-plane thermal fluctuations in ion positions as a major contributor to the broadening of the drumhead-mode spectrum. In the process, we demonstrate how the confining magnetic field leads to unconventional in-plane normal modes, whose average potential and kinetic energies are not equal. This property, in turn, has implications for the sampling procedure required to choose the in-plane initial conditions for molecular-dynamics simulations. For current operating conditions of the NIST Penning trap, our study suggests that the two-dimensional crystals produced in this trap undergo in-plane potential-energy fluctuations of the order of 10mK. Our study therefore motivates the need for designing improved techniques to cool the in-plane degrees of freedom.

Self-Propelled Janus Colloids in Shear Flow.

To fully harness the potential of artificial active colloids, investigation of their response to various external stimuli including external flow is of great interest. Therefore, in this study, we perform experiments on SiO2-Pt Janus particles suspended in an aqueous medium in a capillary subjected to different shear flow rates. Particles were propelled using varied H2O2 (fuel) concentrations. For a particular propulsion speed, with increasing shear flow, a continuous transition in the motion of active Janus particles (JPs) from the usual random active motion to preferential movement along the vorticity direction and then finally to migration along the flow was observed. This transition was accompanied by a significant decline in in-plane fluctuations of the particle trajectories. Another key observation is that the activity of JPs produces a delay in shear-induced rolling, which at moderate flow, allows the JPs to adopt a specific orientation, facilitating their migration along the vorticity direction. At higher flow rates, once shear flow overcomes the activity-induced resistance and initiates rolling, the probability of JPs adopting such preferred orientations reduces. Our analysis further revealed that these transitions are governed by a nondimensional quantity λ, which compares the relative strength of the shear-induced particle flow to the propulsion speed.

Magnetic oscillation modes in square-lattice artificial spin ice

Small amplitude dipolar oscillations are considered in artificial spin ice on a square lattice in two dimensions. The net magnetic moment of each elongated magnetic island in the spin ice is assumed to have Heisenberg-like dynamics. Each island's magnetic moment is assumed to be influenced by shape anisotropies and by the dipolar interactions with its nearest neighbors. The magnetic dynamics is linearized around one of the ground states, leading to an $8\times 8$ matrix to be diagonalized for the magnetic spin wave modes. Analytic solutions are found and classified as antisymmetric and symmetric with regard to their in-plane dynamic fluctuations. Although only the leading dipolar interactions are included, modes similar to these may be observable experimentally.

Lifshitz transition and frustration of magnetic moments in infinite-layer NdNiO2 upon hole doping

Motivated by the recent discovery of superconductivity in the infinite-layer (Sr,Nd)NiO$_2$ films with Sr content $x \simeq0.2$ [Li et al., Nature (London) \textbf{572}, 624 (2019)], we examine the effects of electron correlations and Sr-doping on the electronic structure, Fermi surface topology, and magnetic correlations in (Nd,Sr)NiO$_2$ using a combination of dynamical mean-field theory of correlated electrons and band-structure methods. Our results reveal a remarkable orbital selective renormalization of the Ni $3d$ bands, with $m$*/$m\sim 3$ and 1.3 for the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals, respectively, that suggests orbital-dependent localization of the Ni $3d$ states. We find that upon hole doping (Nd,Sr)NiO$_2$ undergoes a Lifshitz transition of the Fermi surface which is accompanied by a change of magnetic correlations from the three-dimensional (3D) N\'eel $G$-type (111) to the quasi-2D $C$-type (110). We show that magnetic interactions in (Nd,Sr)NiO$_2$ demonstrate an unanticipated frustration, which suppresses magnetic order, implying the importance of in-plane spin fluctuations to explain its superconductivity. Our results suggest that frustration is maximal for Sr-doping $x \simeq 0.1$--0.2, which is in agreement with an experimentally observed doping value Sr $x \simeq 0.2$ of superconducting (Nd,Sr)NiO$_2$.

19F NMR Study of the Bilayer Iron-Based Superconductor KCa2Fe4As4F2**Supported by the National Key Research and Development Program of China under Grant No 2017YFA0302901, the National Basic Research Program of China under Grant No 2015CB921304, the National Science Foundation of China under Grant Nos 11674375 and 11634015, and the Strategic Priority Research Program and Key Research Program of Frontier Sciences of

We report a 19F nuclear magnetic resonance (NMR) study on single-crystal KCa2Fe4As4F2 (Tc ∼ 33.3 K). The 19F NMR spectral shape of KCa2Fe4As4F2 is weakly dependent on temperature and the Knight shift is small, which implies weak coupling between the CaF layer and the FeAs layer. The temperature dependence of 1/19T1 shows a hump below Tc, however the 1/75T1 decreases just below Tc, which implies that there are strong in-plane magnetic fluctuations in the CaF layers than in the FeAs layers. This may be caused by the motion of vortices. The absence of the coherence peak suggests unconventional superconductivity in KCa2Fe4As4F2.

Stabilization of the boundary layer by streamwise control

In this paper, the effect of a special active control on the Blasius boundary layer is investigated with the Orr-Sommerfeld and the Reynolds-Orr energy equations. The control moves the wall in the streamwise direction proportional to the fluctuating wall shear stress which was proposed by Józsa et al. [“Active and passive in-plane wall fluctuations in turbulent channel flows,” J. Fluid Mech. 866, 689–720 (2019)]. Our results showed that the negative proportional parameters destabilize the flow, while the positive values lead to a contradictory outcome. According to the Orr-Sommerfeld equation, with the right choice of the parameter, the critical Reynolds number can be significantly increased. At the same time, the Reynolds-Orr equation predicts that any streamwise movement of the wall proportional to the wall shear stress slightly destabilizes the flow.

AlGaN multiple quantum wells by PA-MBE for deep UV emission: Effect of growth interruptions

Growth of AlGaN multiple quantum wells by Plasma Assisted-Molecular Beam Epitaxy has been optimized for use in deep ultraviolet light emitting diodes. A series of samples were grown on c-plane sapphire using an AlN buffer layer, an ~1 μm AlGaN layer, followed by 40-pairs of wells and barriers. The III/V flux ratio was varied from near stoichiometric to excess group III and a plasma exposure step was introduced for some samples after the deposition of wells and barriers. The quality of the well-barrier interface was estimated by the number and intensity of X-Ray Diffraction (XRD) superlattice peaks. Our results indicate that interface abruptness is greatly enhanced by the addition of this process step. While the room temperature photoluminescence (PL) intensity increases significantly upon introduction of the interruption after the barrier layer, this effect is reversed with additional interruption after the well layers. Temperature dependent PL measurements show an anomalous variation with increasing temperature with a maxima around 110°-150°C. This is due to photogeneration of e-h pairs in the underlying AlGaN layer and its subsequent recombination in the quantum wells by diffusion through percolative barrier layers with in-plane potential fluctuations. This is arrested at lowest temperatures causing a reduction in PL intensity. The PL intensity ratio I300K/I4K is highest for samples grown under excess group III, but reduces with the addition of the growth interruption step after the barrier layer. Growth under stoichiometric conditions together with dual growth interruptions was found to be optimal, with improved PL properties while maintaining high interface quality.

Thermochromic luminescence and dielectric response of copper(I) iodide based MOFs as luminescent thermometer

A copper(I) iodide based MOFs crystal with the formula [Cu4I4(Pz)2]n (Pz = [1,4-bis(4-pyridylmethyl)piperazine]) (1) was synthesized using a solvothermal method. The compound 1 crystallized in the monoclinic space group C2/c at room temperature and consisted of Cu4I4 clusters. Along the b-axis, an irregular window with a size of 7.8 × 31.6 Å was formed. The neighboring rotators (piperazine ring) had a long distance of 12.3 Å between their rotational axes (CH2-), making it easy to rotate. From the view of the crystal structure, 1 can serve as an ideal amphidynamic model, and an in-plane oscillatory fluctuation of the piperazine ring results from the dielectric relaxation. In addition, this compound also showed luminescence thermochromic properties, and a good linear relationship between the luminescence intensity ration of the three emission bands and the temperature was observed. Thus, this compound can be excellent as a luminescent thermometer. These findings provide a new strategy for constructing electrical and optical properties of amphidynamic MOFs.

Active and passive in-plane wall fluctuations in turbulent channel flows

Compliant walls offer the tantalising possibility of passive flow control. This paper examines the mechanics of compliant surfaces driven by wall shear stresses, with solely in-plane velocity response. We present direct numerical simulations of turbulent channel flows at low ($Re_{\unicode[STIX]{x1D70F}}\approx 180$) and intermediate ($Re_{\unicode[STIX]{x1D70F}}\approx 1000$) Reynolds numbers. In-plane spanwise and streamwise active controls proposed by Choiet al. (J. Fluid Mech., vol. 262, 1994, pp. 75–110) are revisited in order to characterise beneficial wall fluctuations. An analytical framework is then used to map the parameter space of the proposed compliant surfaces. The direct numerical simulations show that large-scale passive streamwise wall fluctuations can reduce friction drag by at least$3.7\pm 1\,\%$, whereas even small-scale passive spanwise wall motions lead to considerable drag penalty. It is found that a well-designed compliant wall can theoretically exploit the drag-reduction mechanism of an active control; this may help advance the development of practical active and passive control strategies for turbulent friction drag reduction.