arXiv subjects

Y. G. Semenov

Publications and source records attributed to Y. G. Semenov.

At least 19 recordsLinked to original sources

arXiv2021

Molecular field approximation in theory of ferromagnetic phase transition in diluted magnetic semiconductors

In this pedagogical paper, the comparative analysis of two common approaches describing the ferromagnetic phase transition in diluted magnetic semiconductors (DMS) is expounded in terms of Weiss field approximation. Assuming a finite spin polarization of the magnetic ions, the treatment of carrier-ion exchange interaction in first order evokes the homogeneous Weiss molecular field that polarizes the spins of free carriers. In turn, this spin polarization of the free carriers exerts the effective field that may stabilize DMS spin polarization below a critical temperature Tc. The treatment of such self-consistent spontaneous DMS magnetization can be done in terms of spin-spin interaction independent on inter-ion distance and infinitesimal in thermodynamic limit. On the other hand, the taking additionally into account the second order effects of carrier-ion exchange interaction treat a Weiss field in term of the RKKY indirect spin-spin interaction, which oscillates and not disappears at finite inter-ion distance in case of finite carriers concentration. These both approaches result in same Curie temperature Tc provided non-correlated homogeneous random distribution of the localized spin moments over sample volume. We discuss the origin of such coincidence and show when this is not a case in other more realistic models of the conducting DMSs.

cond-mat.mtrl-sci↗

arXiv2011

Unusual magnetoresistance in a topological insulator with a single ferromagnetic barrier

Tunneling surface current through a thin ferromagnetic barrier in a three-dimensional topological insulator is shown to possess an extraordinary response to the orientation of barrier magnetization. In contrast to conventional magnetoresistance devices that are sensitive to the relative alignment of two magnetic layers, a drastic change in the transmission current is achieved by a single layer when its magnetization rotates by 90 degrees. Numerical estimations predict a giant magnetoresistance as large as 800 % at room temperature and the proximate exchange interaction of 40 meV in the barrier. When coupled with electrical control of magnetization direction, this phenomenon may be used to enhance the gating function with potentially sharp turn-on/off for low power applications.

cond-mat.str-el↗

arXiv2011

Weak ferromagnetism of antiferromagnetic domains in graphene with defects

Magnetic properties of graphene with randomly distributed magnetic defects/vacancies are studied in terms of the Kondo Hamiltonian in the mean field approximation. It has been shown that graphene with defects undergoes a magnetic phase transition from a paramagnetic to a antiferromagnetic (AFM) phase once the temperature reaches the critical point $T_{N}$. The defect straggling is taken into account as an assignable cause of multiple nucleation into AFM domains. Since each domain is characterized by partial compensating magnetization of the defects associated with different sublattices, together they reveal a super-paramagnetic behavior in a magnetic field. Theory qualitatively describe the experimental data provided the temperature dependence of the AFM domain structure.

cond-mat.mtrl-sci↗

arXiv2010

Electron-Phonon Interactions in Bilayer Graphene: A First Principles Approach

Density functional perturbation theory is used to analyze electron-phonon interaction in bilayer graphene. The results show that phonon scattering in bilayer graphene bears more resemblance with bulk graphite than monolayer graphene. In particular, electron-phonon scattering in the lowest conduction band is dominated by six lowest (acoustic and acoustic-like) phonon branches with only minor contributions from optical modes. The total scattering rate at low/moderate electron energies can be described by a simple two-phonon model in the deformation potential approximation with effective constants Dac $\approx$ 15 eV and Dop $\approx 2.8 \times 108$ eV/cm for acoustic and optical phonons, respectively. With much enhanced acoustic phonon scattering, the low field mobility of bilayer graphene is expected to be significantly smaller than that of monolayer graphene.

cond-mat.mes-hall↗

arXiv2010

Graphene spin capacitor for magnetic field sensing

An analysis of a novel magnetic field sensor based on a graphene spin capacitor is presented. The proposed device consists of graphene nanoribbons on top of an insulator material connected to a ferromagnetic source/drain. The time evolution of spin polarized electrons injected into the capacitor can be used for an accurate determination at room temperature of external magnetic fields. Assuming a spin relaxation time of 100 ns, magnetic fields on the order of $\sim 10$ mOe may be detected at room temperature. The observational accuracy of this device depends on the density of magnetic defects and spin relaxation time that can be achieved.

cond-mat.mes-hall↗

arXiv2010

Electron spin relaxation in carbon nanotubes

The long standing problem of inexplicably short spin relaxation in carbon nanotubes (CNTs) is examined. The curvature-mediated spin-orbital interaction is shown to induce fluctuating electron spin precession causing efficient relaxation in a manner analogous to the Dyakonov-Perel mechanism. Our calculation estimates longitudinal (spin-flip) and transversal (decoherence) relaxation times as short as 150 ps and 110 ps at room temperature, respectively, along with a pronounced anisotropic dependence. Interference of electrons originating from different valleys can lead to even faster dephasing. The results can help clarify the measured data, resolving discrepancies in the literature.

cond-mat.mtrl-sci↗

arXiv2009

First Principles Analysis of Electron-Phonon Interaction in Graphene

The electron-phonon interaction in monolayer graphene is investigated by using density functional perturbation theory. The results indicate that the electron-phonon interaction strength is of comparable magnitude for all four in-plane phonon branches and must be considered simultaneously. Moreover, the calculated scattering rates suggest an acoustic phonon contribution that is much weaker than previously thought, revealing the role of optical phonons even at low energies. Accordingly it is predicted, in good agreement with a recent measurement, that the intrinsic mobility of graphene may be more than an order of magnitude larger than the high values reported in suspended samples.

cond-mat.mes-hall↗

arXiv2009

Electrically Controlled Magnetic Memory and Programmable Logic based on Graphene/Ferromagnet Hybrid Structures

It has been shown that the combining of the electrical effect on the exchange bias field with giant magneto-resistance effect of the graphene/ferromagnet hybrid structures reveals a new non-volatile magnetic random access memory device conception. In such device an electric bias realizes the writing bits instead a magnetic field of remote word line with high energy consumption. Interplay of two graphene mediated exchange bias fields applied to different sides of free ferromagnet results in programable logic operations that depends on specific realization of the structure.

cond-mat.mtrl-sci↗

arXiv2008

Electrical Control of Exchange Bias Mediated by Graphene

The role of graphene in mediating the exchange interaction is theoretically investigated when it is placed between two ferromagnetic dielectric materials. The calculation based on a tight-binding model illustrates that the magnetic interactions at the interfaces affect not only the graphene band structure but also the thermodynamic potential of the system. This induces an indirect exchange interaction between the magnetic layers that can be considered in term of an effective exchange bias field. The analysis clearly indicates a strong dependence of the effective exchange bias on the properties of the mediating layer, revealing an effective mechanism of electrical control even at room temperature. This dependence also results in qualitatively different characteristics for the cases involving mono- and bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2008

Ferromagnet proximity effects and magnetoresistance of bilayer graphene

A drastic modification of electronic band structure is predicted in bilayer graphene when it is placed between two ferromagnetic insulators. Due to the exchange interaction with the proximate ferromagnet, the electronic energy dispersion in the graphene channel strongly depends on the magnetization orientation of two ferromagnetic layers, $\mathbf{M_{1}}$ and $\mathbf{M_{2}} $. While the parallel configuration $\mathbf{M_{1}}= \mathbf{M_{2}}$ leads to simple spin splitting of both conduction and valence bands, an energy gap is induced as soon as the angle $θ$ between $\mathbf{M_{1}}$ and $% \mathbf{M_{2}}$ becomes non-zero with the maximum achieved at $θ=π$ (i.e., antiparallel alignment). Consequently, bilayer graphene may exhibit a sizable magnetoresistive effect in the current-in-plane configuration. A rough estimate suggests the resistance changes on the order of tens of percent at room temperature. This effect is expected to become more pronounced as the temperatures decreases.

cond-mat.mtrl-sci↗

arXiv2007

Electron spin relaxation via flexural phonon modes in semiconducting carbon nanotubes

This work considers the g-tensor anisotropy induced by the flexural thermal vibrations in one-dimensional structures and its role in electron spin relaxation. In particular, the mechanism of spin-lattice relaxation via flexural modes is studied theoretically for localized and delocalized electronic states in semiconducting carbon nanotubes in the presence of magnetic field. The calculation of one-phonon spin-flip process predicts distinctive dependencies of the relaxation rate on temperature, magnetic field and nanotube diameter. Comparison with the spin relaxation caused by the hyperfine interaction clearly suggests the relative efficiency of the proposed mechanism at sufficiently high temperatures. Specifically, the longitudinal spin relaxation time in the semiconducting carbon nanotubes is estimated to be as short as 30 microseconds at room temperature.

cond-mat.mtrl-sci↗

arXiv2007

Graphene based spin field effect transistor

A spin field effect transistor (FET) is proposed by utilizing a graphene nanoribbon as the channel. Similar to the conventional spin FETs, the device involves ferromagnetic metals as a source and drain; they, in turn, are connected to the graphene channel. Due to the negligible spin-orbital coupling in the carbon based materials, the bias can accomplishes spin manipulation by means of electrical control of electron exchange interaction with a ferromagnetic dielectric attached to the nanoribbon between source and drain. The numerical estimations show the feasibility of graphene-based spin FET if a bias varies exchange interaction on the amount around 5 meV. It was shown that the device stability to the thermal dispersion can provide the armchair nanoribbons of specific width that keeps the Dirac point in electron dispersion law.

cond-mat.mtrl-sci↗

arXiv2006

Elastic Spin Relaxation Processes in Semiconductor Quantum Dots

Electron spin decoherence caused by elastic spin-phonon processes is investigated comprehensively in a zero-dimensional environment. Specifically, a theoretical treatment is developed for the processes associated with the fluctuations in the phonon potential as well as in the electron procession frequency through the spin-orbit and hyperfine interactions in the semiconductor quantum dots. The analysis identifies the conditions (magnetic field, temperature, etc.) in which the elastic spin-phonon processes can dominate over the inelastic counterparts with the electron spin-flip transitions. Particularly, the calculation results illustrate the potential significance of an elastic decoherence mechanism originating from the intervalley transitions in semiconductor quantum dots with multiple equivalent energy minima (e.g., the X valleys in SiGe). The role of lattice anharmonicity and phonon decay in spin relaxation is also examined along with that of the local effective field fluctuations caused by the stochastic electronic transitions between the orbital states. Numerical estimations are provided for typical GaAs and Si-based quantum dots.

cond-mat.other↗

arXiv2006

Electron spin relaxation in semiconducting carbon nanotubes: the role of hyperfine interaction

A theory of electron spin relaxation in semiconducting carbon nanotubes is developed based on the hyperfine interaction with disordered nuclei spins I=1/2 of $^{13}$C isotopes. It is shown that strong radial confinement of electrons enhances the electron-nuclear overlap and subsequently electron spin relaxation (via the hyperfine interaction) in the carbon nanotubes. The analysis also reveals an unusual temperature dependence of longitudinal (spin-flip) and transversal (dephasing) relaxation times: the relaxation becomes weaker with the increasing temperature as a consequence of the particularities in the electron density of states inherent in one-dimensional structures. Numerical estimations indicate relatively high efficiency of this relaxation mechanism compared to the similar processes in bulk diamond. However, the anticipated spin relaxation time of the order of 1 s in CNTs is still much longer than those found in conventional semiconductor structures.

cond-mat.mtrl-sci↗

arXiv2005

Non-volatile bistability effect based on electrically controlled phase transition in scaled magnetic semiconductor nanostructures

We explore the bistability effect in a dimensionally scaled semiconductor nanostruncture consisting of a diluted magnetic semiconductor quantum dot (QD) and a reservoir of itinerant holes separated by a barrier. The bistability stems from the magnetic phase transition in the QD mediated by the changes in the hole population. Our calculation shows that when properly designed, the thermodynamic equilibrium of the scaled structure can be achieved at two different configurations; i.e., the one with the QD in a ferromagnetic state with a sufficient number of holes and the other with the depopulated QD in a paramagnetic state. Subsequently, the parameter window suitable for this bistability formation is discussed along with the the conditions for the maximum robustness/non-volatility. To examine the issue of scaling, an estimation of the bistabiity lifetime is made by considering the thermal fluctuation in the QD hole population via the spontaneous transitions. A numerical evaluation is carried out for a typical carrier-mediated magnetic semiconductor (e.g., GaMnAs) as well as for a hypothetical case of high Curie temperature for potential room temperature operation.

cond-mat.other↗

arXiv2004

Bistability in a magnetic and nonmagnetic double-quantum-well structure mediated by the magnetic phase transition

The hole distribution in a double quantum well (QW) structure consisting of a magnetic and a nonmagnetic semiconductor QW is investigated as a function of temperature, the energy shift between the QWs, and other relevant parameters. When the itinerant holes mediate the ferromagnetic ordering, it is shown that a bistable state can be formed through hole redistribution, resulting in a significant change in the properties of the constituting magnetic QW (i.e., the paramagnetic-ferromagnetic transition). The model calculation also indicates a large window in the system parameter space where the bistability is possible. Hence, this structure could form the basis of a stable memory element that may be scaled down to a few hole regime.

cond-mat.other↗

arXiv2004

Correlation of phonon decay with spin phase diffusion of localized electrons

A spin decoherence mechanism is proposed for localized electrons. The irregular phonon phase disturbances originated from phonon relaxation can influence electron spin precession with a net effect of spin phase decay. A quantitative analysis demonstrates relatively high efficiency of this mechanism in the low temperature and low magnetic field regime compared to the spin-flip processes.

cond-mat.mtrl-sci↗

arXiv2003

Phonon-mediated electron spin phase diffusion in a quantum dot

An effective spin relaxation mechanism that leads to electron spin decoherence in a quantum dot is proposed. In contrast to the common calculations of spin-flip transitions between the Kramers doublets, we take into account a process of phonon-mediated fluctuation in the electron spin precession and subsequent spin phase diffusion. Specifically, we consider modulations in the longitudinal g-factor and hyperfine interaction induced by the phonon-assisted transitions between the lowest electronic states. Prominent differences in the temperature and magnetic field dependence between the proposed mechanisms and the spin-flip transitions are expected to facilitate its experimental verification. Numerical estimation demonstrates highly efficient spin relaxation in typical semiconductor quantum dots.

cond-mat.mtrl-sci↗