Bath Spin Research Articles

Dephasing dynamics of a central spin interacting with a classical Ising spin bath

We study the effect of interaction between bath spins on central spin dynamics. For this end, we consider various types of interactions among the bath fluctuators. We give an analytical relation for the dephasing dynamics of a central spin homogeneously interacting with a bath of classically interacting spins in thermal equilibrium. The obtained exact relation for the central spin dynamics depends on the average of the number of bath spins N in the canonical ensemble. We find the relation of dephasing time with bath's temperature and strength of interaction between bath spins. We show that if the classically interacting bath is large enough, off-diagonal coherence will decay into Gaussian form. This Gaussian decay is valid for temperatures higher than critical temperature. Moreover, we analytically prove the equivalence of dephasing dynamics for central spin models with classical Ising spin bath and quantum flip-flop spin bath interactions. Therefore, the dephasing dynamics for central spin models with quantum spin bath interactions can be reproduced with much less expensive classical spin bath interaction model instead. Our results can be applied in a number of quantum systems used in quantum sensing such as color centers in solids or quantum dots manuscript.

Spin current driven by ultrafast magnetization of FeRh

Laser-induced ultrafast demagnetization is an important phenomenon that probes arguably the ultimate limits of the angular momentum dynamics in solid. Unfortunately, many aspects of the dynamics remain unclear except that the demagnetization transfers the angular momentum eventually to the lattice. In particular, the role and origin of electron-carried spin currents in the demagnetization process are debated. Here we experimentally probe the spin current in the opposite phenomenon, i.e., laser-induced ultrafast magnetization of FeRh, where the laser pump pulse initiates the angular momentum build-up rather than its dissipation. Using the time-resolved magneto-optical Kerr effect, we directly measure the ultrafast-magnetization-driven spin current in a FeRh/Cu heterostructure. A strong correlation between the spin current and the magnetization dynamics of FeRh is found even though the spin filter effect is negligible in this opposite process. This result implies that the angular momentum build-up is achieved by an angular momentum transfer from the electron bath (supplier) to the magnon bath (receiver) and followed by the spatial transport of angular momentum (spin current) and dissipation of angular momentum to the phonon bath (spin relaxation).

Spatial spin-spin correlations of the single-impurity Anderson model with a ferromagnetic bath

We investigate the interplay between the Kondo effect and the ferromagnetism by an one dimension Anderson impurity model with a spin partially polarized bath, using the projective truncation approximation under Lacroix basis.The equal-time spatial spin-spin correlation function (SSCF) is calculated. For the case of spin-unpolarized conduction electrons, it agrees qualitatively with the results from density matrix renormalization group (DMRG). For system with partially spin-polarized conduction electrons, an oscillation in the envelope of SSCF emerges due to the beating of two Friedel oscillations associated to two spin-split Fermi surfaces of conduction electrons. The period is proportional to the inverse of magnetic field $h$. A fitting formula is proposed to perfectly fits the numerical results of SSCF in both the short- and long-range regions. For large enough bath spin polarization, a bump appears in the curve of the integrated SSCF. It marks the boundary between the suppressed Kondo cloud and the polarized bath sites.

Lower and upper bounds of quantum battery power in multiple central spin systems

We study the energy transfer process in quantum battery systems consisting of multiple central spins and bath spins. Here with "quantum battery" we refer to the central spins, whereas the bath serves as the "charger". For the single central-spin battery, we analytically derive the time evolutions of the energy transfer and the charging power with arbitrary number of bath spins. For the case of multiple central spins in the battery, we find the scaling-law relation between the maximum power $P_{max}$ and the number of central spins $N_B$. It approximately satisfies a scaling law relation $P_{max}\propto N_{B}^{\alpha}$, where scaling exponent $\alpha$ varies with the bath spin number $N$ from the lower bound $\alpha =1/2$ to the upper bound $\alpha =3/2$. The lower and upper bounds correspond to the limits $N\to 1$ and $N\gg N_B$, respectively. In thermodynamic limit, by applying the Holstein-Primakoff (H-P) transformation, we rigorously prove that the upper bound is $P_{max}=0.72 B A \sqrt{N} N_{B}^{3/2}$, which shows the same advantage in scaling of a recent charging protocol based on the Tavis-Cummins model. Here $B$ and $A $ are the external magnetic field and coupling constant between the battery and the charger.

Impact on Development of ZnS Nanoparticles Thin Film Deposited by Chemical Bath Deposition and Spin Coating

Research comparing the advantages of spin coating and chemical bath deposition is going, and there are varied views on these methods. Here, we used spin coating and chemical bath deposition to prepare thin films of ZnS nanoparticles. The film was analysed by photoluminescence (PL) spectrophotometry, field emission scanning electron microscopy (FE-SEM), ultraviolet spectroscopy (UV-Vis), and energy-dispersive X-ray (EDX) spectroscopy. The UV-Vis spectra revealed that the wavelength of ZnS is between 220 nm - 320 nm while the PL spectra showed a peak centred in the blue region. Both spin coating and chemical bath deposition rendered spherical nanoparticles but of different sizes 17.9 nm and 21.2 -25.7 nm, respectively. It was concluded that each method has its potential. This work can help researchers choose a suitable method for fabricating thin films, depending on the aims and objectives of their work.

Influence of deposition techniques on quality and photodetection properties of tin disulfide (SnS2) thin films

Tin disulfide (SnS2) is one of the potential candidates for optoelectronics because of its chemical and environmental stability accompanied by favourable characteristics. Herein, the SnS2 thin films are deposited by different techniques; chemical bath deposition (CBD), dip coating (DC) and spin coating (SC). The energy dispersive analysis of x-ray confirms the stoichiometry of the films deposited by these techniques. The x-ray diffraction showed hexagonal lattice structure of thin films. The morphology is studied by transmission electron, scanning electron and optical microscopy. The selected area electron diffraction exhibited ring patterns, confirming the polycrystalline nature of the deposited thin films. The atomic force microscopy showed presence of globular grains, hills and valleys on surfaces of thin films. The absorption spectra analysis showed the thin film possess direct optical bandgap of 2.39 eV for CBD, 2.50 eV for DC and 2.75 eV for SC. Raman spectra of the as-deposited SnS2 thin films showed occurrence of A1g phonon mode at 314 cm−1. The electrical transport properties assessment depicts the n-type semiconducting nature of deposited thin films. The as-deposited thin films showed good pulsed photoresponse for white light illumination intensities of 80 mW/cm2 and 120 mW/cm2.

Fourth-order spin correlation function in the extended central spin model

Spin-noise spectroscopy has developed into a very powerful tool to access the electron spin dynamics. While the spin-noise power spectrum in an ensemble of quantum dots in a magnetic field is essentially understood, we argue that the investigation of the higher-order cumulants promises to provide additional information not accessible by the conventional power-noise spectrum. We present a quantum-mechanical approach to the correlation function of the spin-noise power operators at two different frequencies for small spin bath sizes and compare the results with a simulation obtained from the classical spin dynamics for large number of nuclear spins. This bispectrum is defined as a two-dimensional frequency cut in the parameter space of the fourth-order spin correlation function. It reveals information on the influence of the nuclear-electric quadrupolar interactions on the long-time electron spin dynamics dominated by a magnetic field. For large bath sizes and spin lengths the quantum-mechanical spectra converge to those of the classical simulations. The broadening of the bispectrum across the diagonal in the frequency space is a direct measure of the quadrupolar interaction strength. A narrowing is found with increasing magnetic field indicating a suppression of the influence of quadrupolar interactions in favor of the nuclear Zeeman effect.

N-N type core-shell heterojunction engineering with MoO3 over ZnO nanorod cores for enhanced solar energy harvesting application in a photoelectrochemical cell

Herein, we report an easy and scalable chemical bath deposition and spin coating route of n-n hetero-architecture engineering to fabricate MoO3-ZnO core-shell nanorods (NRs) based photoanode, which is indeed the first time demonstration of this particular nano-heterojunction for solar energy conversion and hydrogen energy generation in a photoelectrochemical (PEC) cell. We further tune the MoO3 shell thickness by varying spin coated layer thickness. An average thickness ∼100 nm of MoO3over 450 nm ZnO NRs significantly improves the photocurrent from 3.3 to 27.6μAcm−2. A 7.5 fold increase in applied bias photon to current conversion efficiency (ABPE) value is achieved upon visible light illumination (Visible light, 10 mWcm−2) with a maximum value of 0.15% than bare ZnO NRs (0.02%). In addition, hydrogen gas (5 μmolcm−2) is evolved even at no external potential applied to the PEC cell with this MoO3-ZnO. The physical insight of the enhanced PEC performance is also elucidated. We find the n-n hetero-architecture to provide a suitable solution to maximize solar light absorption along with enhanced charge carrier separation which also boosts the charge transportation and mobility in the junction region due to suitable core-shell interfacial band alignment and modulation of interfacial electronic structure. Mainly, the MoO3 shell provides a potential solution to get more catalytically active sites and the outer Mo-O dipole layer transfers holes to the electrolyte for easy oxidation of water.

A unified stochastic formulation of dissipative quantum dynamics. I. Generalized hierarchical equations.

We extend a standard stochastic theory to study open quantum systems coupled to a generic quantum environment. We exemplify the general framework by studying a two-level quantum system coupled bilinearly to the three fundamental classes of non-interacting particles: bosons, fermions, and spins. In this unified stochastic approach, the generalized stochastic Liouville equation (SLE) formally captures the exact quantum dissipations when noise variables with appropriate statistics for different bath models are applied. Anharmonic effects of a non-Gaussian bath are precisely encoded in the bath multi-time correlation functions that noise variables have to satisfy. Starting from the SLE, we devise a family of generalized hierarchical equations by averaging out the noise variables and expand bath multi-time correlation functions in a complete basis of orthonormal functions. The general hierarchical equations constitute systems of linear equations that provide numerically exact simulations of quantum dynamics. For bosonic bath models, our general hierarchical equation of motion reduces exactly to an extended version of hierarchical equation of motion which allows efficient simulation for arbitrary spectral densities and temperature regimes. Similar efficiency and flexibility can be achieved for the fermionic bath models within our formalism. The spin bath models can be simulated with two complementary approaches in the present formalism. (I) They can be viewed as an example of non-Gaussian bath models and be directly handled with the general hierarchical equation approach given their multi-time correlation functions. (II) Alternatively, each bath spin can be first mapped onto a pair of fermions and be treated as fermionic environments within the present formalism.

Nanostructured metal-oxide thin film towards the electronic device applications

Today, most of the low voltage electronics and opto-electronic devices are based on silicon technology. Metal oxide nanostructured semiconducting materials could be one of the alternatives for such devices. Metal oxide semiconductors are economical and can be easily synthesised by wet chemical processes. The formation of pn-junction is an important task for converting any two semiconducting materials interface to suitable application like rectifiers, photodetectors, light emitting diodes (LED), photovoltaic devices and lasers etc. The low cost semiconductor metal oxide materials like NiO, CuO, ZnO, TiO2, SnO2 are useful to develop pn-junction for such application. These metal oxides are easily synthesised by wet chemical process like chemical bath deposition (CBD), hydrothermal deposition (HTD), and spin coating techniques. In this present work, the ITO/NiO and NiO/ZnO two different junctions have been prepared using wet chemical processes. The morphological studies are performed by using scanning electron microscopy (SEM) and I-V characterisation studied using Keithley source meter. I-V characterisation confirmed the formation of pn-junction with rectification ratio of 22.4 and 15.7 at 2 V for NiO/ITO and NiO/ZnO junction respectively. The high ideality factor 2.06 and 2.2 depicts the high recombination of hole-electron carrier at diffusion region.

Surfactant and template free synthesis of porous ZnS nanoparticles

ZnS thin films composed of porous nanoparticles have been deposited on to glass substrates by combining three simple synthesis methodologies i.e. chemical bath deposition, co-precipitation and spin coating. The XRD results reveal the cubic phase of ZnS thin films crystallized at nano scale. The crystallite size estimated by Scherrer formula was 3.4 nm. The morphology of the samples was analyzed through scanning electron microscopy (SEM) and is evident that thin films are composed of porous nanoparticles with an average size of 150 nm and pores of 40 nm on almost every grain. Crystallinity, phase and morphology were further confirmed via transmission electron microscopy (TEM). The stoichiometry and phase purity of thin films were determined by energy dispersive X-ray (EDX) spectrum and X-ray photoelectron spectroscopy (XPS) analysis, respectively. The surface topography and homogeneity of thin films were analyzed by atomic force microscopy (AFM) and obtained root mean square roughness (4.0326 nm) reveals the morphologically homogeneous growth of ZnS on glass substrates. The UV–Vis spectroscopy and photoluminescence (PL) were carried out to estimate the band gap and observe the emission spectra in order to speculate the viability of ZnS porous nanoparticles in optoelectronic devices and sensors.

A Comparative Study on Structural and Optical Properties of ZnO Micro-Nanorod Arrays Grown on Seed Layers Using Chemical Bath Deposition and Spin Coating Methods

In this study, Zinc Oxide (ZnO) seed layers were prepared on Indium Tin Oxide (ITO) substrates by using Chemical Bath Deposition (CBD) method and Sol-gel Spin Coating (SC) method. ZnO micro-nanorod arrays were grown on ZnO seed layers by using Hydrothermal Synthesis method. Seed layer effects of structural and optical properties of ZnO arrays were characterized. X-ray diffractometer (XRD), Scanning Electron Microscopy (SEM) and Ultraviolet Visible (UV-Vis) Spectrometer were used for analyses. ZnO micro-nanorod arrays consisted of a single crystalline wurtzite ZnO structure for each seed layer. Besides, ZnO rod arrays were grown smoothly and vertically on SC seed layer, while ZnO rod arrays were grown randomly and flower like structures on CBD seed layer. The optical absorbance peaks found at 422 nm wavelength in the visible region for both ZnO arrays. Optical bandgap values were determined by using UV-Vis measurements at 3.12 and 3.15 eV for ZnO micro-nanorod arrays on CBD seed layer and for ZnO micro-nanorod arrays on SC-seed layer respectively. DOI: http://dx.doi.org/10.5755/j01.ms.22.4.13443

Theory of open quantum systems with bath of electrons and phonons and spins: Many-dissipaton density matrixes approach

This work establishes a strongly correlated system-and-bath dynamics theory, the many-dissipaton density operators formalism. It puts forward a quasi-particle picture for environmental influences. This picture unifies the physical descriptions and algebraic treatments on three distinct classes of quantum environments, electron bath, phonon bath, and two-level spin or exciton bath, as their participating in quantum dissipation processes. Dynamical variables for theoretical description are no longer just the reduced density matrix for system, but remarkably also those for quasi-particles of bath. The present theoretical formalism offers efficient and accurate means for the study of steady-state (nonequilibrium and equilibrium) and real-time dynamical properties of both systems and hybridizing environments. It further provides universal evaluations, exact in principle, on various correlation functions, including even those of environmental degrees of freedom in coupling with systems. Induced environmental dynamics could be reflected directly in experimentally measurable quantities, such as Fano resonances and quantum transport current shot noise statistics.

Early stage disentanglement and non-Markovianity

We have studied an early stage disentanglement by solving an exactly solvable spin bath model in the Markovian and non-Markovian regimes. In the Markovian regime, the central two qubit spins are non-interacting in which entanglement strongly depends on mean number of photon. In the non-Markovian regime, we solved by considering and ignoring the interaction between central two spins and observe that the entanglement strongly depends on the spin bath interaction, qubit spin interaction, bath spin interaction, thermal temperature and the initial state. Further, the concurrence of an initially unentangled state mixed with an entangled state also depends on the initial state.

Entanglement Dynamics of Two Coupled Spins in a Spin Star Environment

We theoretically study the entanglement dynamics of two coupled spins in a spin star environment, whose elements are coupled to local bosonic baths. It is shown that the dynamics of the entanglement depends on the initial state of the system and the coupling strength between the two coupled central spins, the interactions between the central system and the environment, as well as interactions between the bath spin and the reservoir. We also investigate the effect of non-Markovian dynamics in contrast with the Markovian case. It is found that the non-Markovian dynamics has a significant effect on the disentanglement between the two central spins.

Dynamics of Quantum Dissonance of Two Qubit XXZ Model with Dzyloshinsky-Moriya Interaction Coupled to Non-Markovian Environment

We consider the dynamics of quantum correlations of two coupled spin qubits with Dzyaloshinsky-Moriya (DM) interaction influenced by a local external magnetic field along the z-direction and coupled to bath spin-\(\frac{1}{2}\) particles as independent non-Markovian environment. For this model, we calculate the entanglement measure of concurrence, quantum discord and quantum dissonance and find effects of DM interaction, bath-system coupling constant and temperature on the dynamics of quantum correlation. At last, we obtain the teleportation for this model by using fidelity and observe changes of DM interaction, bath-system coupling constant, temperature and magnetic field on fidelity.

Quantum decoherence of the central spin in a sparse system of dipolar coupled spins

The central spin decoherence problem has been researched for over 50 years in the context of both nuclear magnetic resonance and electron spin resonance. Until recently, theoretical models have employed phenomenological stochastic descriptions of the bath-induced noise. During the last few years, cluster expansion methods have provided a microscopic, quantum theory to study the spectral diffusion of a central spin. These methods have proven to be very accurate and efficient for problems of nuclear-induced electron spin decoherence in which hyperfine interactions with the central electron spin are much stronger than dipolar interactions among the nuclei. We provide an in-depth study of central spin decoherence for a canonical scale-invariant all-dipolar spin system. We show how cluster methods may be adapted to treat this problem in which central and bath spin interactions are of comparable strength. Our extensive numerical work shows that a properly modified cluster theory is convergent for this problem even as simple perturbative arguments begin to break down. By treating clusters in the presence of energy detunings due to the long-range (diagonal) dipolar interactions of the surrounding environment and carefully averaging the effects over different spin states, we find that the nontrivial flip-flop dynamics among the spins becomes effectively localized by disorder in the energy splittings of the spins. This localization effect allows for a robust calculation of the spin echo signal in a dipolarly coupled bath of spins of the same kind, while considering clusters of no more than 6 spins. We connect these microscopic calculation results to the existing stochastic models. We, furthermore, present calculations for a series of related problems of interest for candidate solid state quantum bits including donors and quantum dots in silicon as well as nitrogen-vacancy centers in diamond.

Dynamics of a two-level system coupled to a bath of spins

The dynamics of a two-level system coupled to a spin bath is investigated via the numerically exact multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) theory. Consistent with the previous work on linear response approximation [N. Makri, J. Phys. Chem. B 103, 2823 (1999)], it is demonstrated numerically that this spin-spin-bath model can be mapped onto the well-known spin-boson model if the system-bath coupling strength obeys an appropriate scaling behavior. This linear response mapping, however, may require many bath spin degrees of freedom to represent the practical continuum limit. To clarify the discrepancies resulted from different approximate treatments of this model, the population dynamics of the central two-level system has been investigated near the transition boundary between the coherent and incoherent motions via the ML-MCTDH method. It is found that increasing temperature favors quantum coherence in the nonadiabatic limit of this model, which corroborates the prediction in the previous work [J. Shao and P. Hanggi, Phys. Rev. Lett. 81, 5710 (1998)] based on the non-interacting blip approximation (NIBA). However, the coherent-incoherent boundary obtained by the exact ML-MCTDH simulation is slightly different from the approximate NIBA results. Quantum dynamics in other physical regimes are also discussed.

Quantum many-body theory of qubit decoherence in a finite-size spin bath

Decoherence of a center spin or qubit in a spin bath is essentially determined by the many-body bath evolution. We develop a cluster-correlation expansion (CCE) theory for the spin-bath dynamics relevant to the qubit decoherence problem. A cluster-correlation term is recursively defined as the evolution of a group of bath spins divided by the cluster correlations of all the subgroups. This correlation accounts for the authentic (nonfactorizable) collective excitations within a given group. The bath propagator is the product of all possible cluster correlation terms. For a finite-time evolution as in the qubit decoherence problem, a convergent result can be obtained by truncating the expansion up to a certain cluster size. The two-spin cluster truncation of the CCE corresponds to the pair-correlation approximation developed previously [W. Yao et al., Phys. Rev. B 74, 195301 (2006)]. In terms of the standard linked cluster expansion, a cluster-correlation term is the infinite summation of all the connected diagrams with all and only the spins in the group flip-flopped, and thus the expansion is exact whenever convergence occurs. When the individual contribution of each higher-order correlation term to the decoherence is small (while all the terms combined in product could still contribute substantially), as the usual case for relatively large baths, where the decoherence could complete well within the bath spin flip-flop time, the CCE coincides with the cluster expansion [W. M. Witzel and S. Das Sarma, Phys. Rev. B 74, 035322 (2006)]. For small baths, however, the qubit decoherence may not complete within the bath spin flip-flop time scale and thus individual higher-order cluster correlations could become significant. In such cases, only the CCE converges to the exact coherent dynamics of multispin clusters. We check the accuracy of the CCE in an exactly solvable spin-chain model.

Spin and entanglement dynamics in the central-spin model with homogeneouscouplings

We calculate exactly the time-dependent reduced density matrix for the central spin in thecentral-spin model with homogeneous Heisenberg couplings. Therefrom, the dynamicsand the entanglement entropy of the central spin are obtained. A rich variety ofbehaviours are found, depending on the initial state of the bath spins. For an initiallyunpolarized unentangled bath, the polarization of the central spin decays to zero in thethermodynamic limit, while its entanglement entropy becomes maximal. On theother hand, if the unpolarized environment is initially in an eigenstate of thetotal bath spin, the central spin and the entanglement entropy exhibit persistentmonochromatic large-amplitude oscillations. This raises the question of to whatextent entanglement of the bath spins prevents decoherence of the central spin.