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Showing papers on "Exchange interaction published in 2015"


Journal ArticleDOI
TL;DR: In this article, single-layer chromium trihalides (SLCTs) were shown to constitute a series of stable 2D intrinsic ferromagnetic (FM) semiconductors with indirect gaps and their valence and conduction bands are fully spin-polarized in the same spin direction.
Abstract: Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial to develop low-dimensional spintronic devices. Using density functional theory, we show that single-layer chromium trihalides (SLCTs) (CrX3, X = F, Cl, Br and I) constitute a series of stable 2D intrinsic FM semiconductors. A free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low cleavage energy and a high in-plane stiffness. Electronic structure calculations using the HSE06 functional indicate that both bulk and single-layer CrX3 are half semiconductors with indirect gaps and their valence and conduction bands are fully spin-polarized in the same spin direction. The energy gaps and absorption edges of CrBr3 and CrI3 are found to be in the visible frequency range, which implies possible opto-electronic applications. Furthermore, SLCTs are found to possess a large magnetic moment of 3 μB per formula unit and a sizable magnetic anisotropy energy. The magnetic exchange constants of SLCTs are then extracted using the Heisenberg spin Hamiltonian and the microscopic origins of the various exchange interactions are analyzed. A competition between a near 90° FM superexchange and a direct antiferromagnetic (AFM) exchange results in a FM nearest-neighbour exchange interaction. The next and third nearest-neighbour exchange interactions are found to be FM and AFM, respectively, and this can be understood by the angle-dependent extended Cr–X–X–Cr superexchange interaction. Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo simulations and the values can be further increased by applying a biaxial tensile strain. The unique combination of robust intrinsic ferromagnetism, half semiconductivity and large magnetic anisotropy energies renders the SLCTs as promising candidates for next-generation semiconductor spintronic applications.

563 citations


Journal ArticleDOI
TL;DR: In this paper, a family of Heusler alloys with a compensated ferrimagnetic state was designed and a giant exchange bias of more than 3 T and a large coercivity were established.
Abstract: Rational material design can accelerate the discovery of materials with improved functionalities. This approach can be implemented in Heusler compounds with tunable magnetic sublattices to demonstrate unprecedented magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. Our design approach is also demonstrated on a second material with a magnetic transition above room temperature, Mn-Fe-Ga, exemplifying the universality of the concept and the feasibility of room-temperature applications. These findings may lead to the development of magneto-electronic devices and rare-earth-free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.

235 citations


Journal ArticleDOI
TL;DR: A family of Heusler alloys with a compensated ferrimagnetic state is designed, exemplifying the universality of the concept and the feasibility of room-temperature applications and may lead to the development of magneto-electronic devices and rare-earth-free exchange-biased hard magnets.
Abstract: The discovery of materials with improved functionality can be accelerated by rational material design. Heusler compounds with tunable magnetic sublattices allow to implement this concept to achieve novel magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a similarly large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. We demonstrate the applicability of our design concept on a second material, Mn-Fe-Ga, with a magnetic transition above room temperature, exemplifying the universality of the concept and the feasibility of room-temperature applications. Our study points to a new direction for novel magneto-electronic devices. At the same time it suggests a new route for realizing rare-earth free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.

203 citations


Journal ArticleDOI
TL;DR: It is demonstrated that time-periodic modulation of the electronic structure by electric fields can be used to reversibly control Jex on ultrafast timescales in extended antiferromagnetic Mott insulators.
Abstract: Electronic interactions underlie the exchange interaction responsible for the magnetic ordering and dynamics of magnetic materials. Here, Mentink et al. theoretically demonstrate the ultrafast and reversible tuning of the exchange interaction in Mott insulators driven by a time-periodic electric field.

198 citations


Journal ArticleDOI
TL;DR: This work proposes a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction and demonstrates that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry.
Abstract: Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.

154 citations


Journal ArticleDOI
TL;DR: In this article, the long-range exchange interaction between an electron and a hole in the exciton is demonstrated to be an efficient mechanism for rapid mixing between bright excitons made of electron-hole pairs in different valleys.
Abstract: Monolayers of transition metal dichalcogenides, namely, molybdenum and tungsten disulfides and diselenides demonstrate unusual optical properties related to the spin–valley locking effect. Particularly, excitation of monolayers by circularly polarized light selectively creates electron–hole pairs or excitons in non‐equivalent valleys in momentum space, depending on the light helicity. This allows studying the inter‐valley dynamics of charge carriers and Coulomb complexes by means of optical spectroscopy. Here, we present a concise review of the neutral exciton fine structure and its spin and valley dynamics in monolayers of transition metal dichalcogenides. It is demonstrated that the long‐range exchange interaction between an electron and a hole in the exciton is an efficient mechanism for rapid mixing between bright excitons made of electron–hole pairs in different valleys. We discuss the physical origin of the long‐range exchange interaction and outline its derivation in both the electrodynamical and urn:x-wiley:15213951:media:pssb201552211:pssb201552211-math-0001 approaches. We further present a model of bright exciton spin dynamics driven by an interplay between the long‐range exchange interaction and scattering. Finally, we discuss the application of the model to describe recent experimental data obtained by time‐resolved photoluminescence and Kerr rotation techniques.

105 citations


Journal ArticleDOI
TL;DR: In this paper, the Dzyaloshinskii-Moriya interaction was shown to give rise to topological behavior in the archetypal quantum magnet strontium copper borate.
Abstract: The spins in quantum magnets couple to each other through an exchange interaction. Here, the authors show that a weak coupling between neighbouring spins called the Dzyaloshinskii–Moriya interaction can give rise to topological behaviour in the archetypal quantum magnet strontium copper borate.

103 citations


Journal ArticleDOI
TL;DR: In this paper, the long-range exchange interaction between an electron and a hole in the exciton is demonstrated to be an efficient mechanism for rapid mixing between bright excitons made of electron-hole pairs in different valleys.
Abstract: Monolayers of transition metal dichalcogenides, namely, molybdenum and tungsten disulfides and diselenides demonstrate unusual optical properties related to the spin-valley locking effect. Particularly, excitation of monolayers by circularly polarized light selectively creates electron-hole pairs or excitons in non-equivalent valleys in momentum space, depending on the light helicity. This allows studying the inter-valley dynamics of charge carriers and Coulomb complexes by means of optical spectroscopy. Here we present a concise review of the neutral exciton fine structure and its spin and valley dynamics in monolayers of transition metal dichalcogenides. It is demonstrated that the long-range exchange interaction between an electron and a hole in the exciton is an efficient mechanism for rapid mixing between bright excitons made of electron-hole pairs in different valleys. We discuss the physical origin of the long-range exchange interaction and outline its derivation in both the electrodynamical and $\mathbf k \cdot \mathbf p$ approaches. We further present a model of bright exciton spin dynamics driven by an interplay between the long-range exchange interaction and scattering. Finally, we discuss the application of the model to describe recent experimental data obtained by time-resolved photoluminescence and Kerr rotation techniques.

96 citations


Journal ArticleDOI
TL;DR: This work considers 1D lattices described by Hubbard or Bose-Hubbard models, in the presence of periodic high-frequency perturbations, such as uniform ac force or modulation of hopping coefficients, to find explicitly additional corrections to the effective Hamiltonians due to interactions, corresponding to nontrivial processes.
Abstract: We consider 1D lattices described by Hubbard or Bose-Hubbard models, in the presence of periodic high-frequency perturbations, such as uniform ac force or modulation of hopping coefficients. Effective Hamiltonians for interacting particles are derived using an averaging method resembling classical canonical perturbation theory. As is known, a high-frequency force may renormalize hopping coefficients, causing interesting phenomena such as coherent destruction of tunneling and creation of artificial gauge fields. We find explicitly additional corrections to the effective Hamiltonians due to interactions, corresponding to nontrivial processes such as single-particle density-dependent tunneling, correlated pair hoppings, nearest neighbor interactions, etc. Some of these processes arise also in multiband lattice models, and are capable of giving rise to a rich variety of quantum phases. The apparent contradiction with other methods, e.g., Floquet-Magnus expansion, is explained. The results may be useful for designing effective Hamiltonian models in experiments with ultracold atoms, as well as in the field of ultrafast nonequilibrium magnetism. An example of manipulating exchange interaction in a Mott-Hubbard insulator is considered, where our corrections play an essential role.

89 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate that the monolayer CrSiTe 3 is an intrinsic ferromagnetic semiconductor and that it can be enhanced significantly by applying an elastic tensile stain, implying their potential applications in spintronic devices at room temperature.

87 citations


Journal ArticleDOI
TL;DR: In this article, the magnetocaloric effect in double perovskite Gd2NiMnO6 (GNMO) and GCMO samples was investigated by magnetic and heat capacity measurements.
Abstract: We have investigated the magnetocaloric effect in double perovskite Gd2NiMnO6 (GNMO) and Gd2CoMnO6 (GCMO) samples by magnetic and heat capacity measurements. Ferromagnetic ordering is observed at ~130 K (~ 112 K) in GNMO (GCMO), while the Gd exchange interactions seem to dominate for T < 20 K. In GCMO, below 50 K, antiferromagnetic behaviour due to the 3d–4f negative exchange interaction is observed. A maximum entropy (−ΔS M) and adiabatic temperature change of ~35.5 J Kg−1 K−1 (~ 24 J Kg−1 K−1) and 10.5 K (6.5 K) is observed in GNMO (GCMO) for a magnetic field change of 7 T at low temperatures. Absence of magnetic and thermal hysteresis and their insulating nature make them promising for low temperature magnetic refrigeration.

Journal ArticleDOI
TL;DR: A highly corrugated hexagonal boron nitride monolayer is utilized to mediate the coupling between a cobalt spin in CoHx (x=1,2) complexes and the metal contact to quantitatively demonstrate how the Kondo exchange interaction mimics chemical tailoring and changes the magnetic anisotropy.
Abstract: Single molecule magnets and single spin centres can be individually addressed when coupled to contacts forming an electrical junction. To control and engineer the magnetism of quantum devices, it is necessary to quantify how the structural and chemical environment of the junction affects the spin centre. Metrics such as coordination number or symmetry provide a simple method to quantify the local environment, but neglect the many-body interactions of an impurity spin coupled to contacts. Here, we utilize a highly corrugated hexagonal boron nitride monolayer to mediate the coupling between a cobalt spin in CoHx (x=1,2) complexes and the metal contact. While hydrogen controls the total effective spin, the corrugation smoothly tunes the Kondo exchange interaction between the spin and the underlying metal. Using scanning tunnelling microscopy and spectroscopy together with numerical simulations, we quantitatively demonstrate how the Kondo exchange interaction mimics chemical tailoring and changes the magnetic anisotropy.

Journal ArticleDOI
TL;DR: In this paper, the authors show irreversibility in the FCC-ZFC curve due to antiferromagnetism and ferromagnetic exchange interaction in LaFeO3.

Journal ArticleDOI
TL;DR: In this paper, the authors explain the strategy behind QCTFF, the current name for a novel atomistic protein force field, which is constructed using Quantum Chemical Topology (QCT).
Abstract: In this perspective, we explain the strategy behind QCTFF, the current name for a novel atomistic protein force field. The atoms are constructed using Quantum Chemical Topology (QCT). These topological atoms determine how a system's energy is partitioned. We give a brief account of the results hitherto obtained, and a glimpse of unpublished results. Combining this QCT partitioning with the universal quantum expression of energy, leads to four types of fundamental energy contributions. The first of these is intra-atomic and the remaining three interatomic: (i) atomic self-energy, (ii) Coulomb energy, (iii) exchange energy, and (iv) correlation energy. All structural and dynamic effects emerge from the interplay of these contributions. The machine learning method kriging captures well how they change in response to a change in nuclear configuration. Second, the Coulomb energy is represented by a converging multipolar series expansion when the nuclei are sufficiently far apart. © 2015 The Authors International Journal of Quantum Chemistry Published by Wiley Periodicals, Inc.

Journal ArticleDOI
TL;DR: It is demonstrated that isolated quantum dots are a possible route to increase the number of coherently coupled quantum dots and couple coherently two electron spins in an efficient manner following a scheme initially proposed by Loss and DiVincenzo.
Abstract: We studied experimentally the dynamics of the exchange interaction between two antiparallel electron spins in an isolated double quantum dot where coupling to the electron reservoirs can be ignored. We demonstrate that the level of control of such a double dot is higher than in conventional double dots. In particular, it allows us to couple coherently two electron spins in an efficient manner following a scheme initially proposed by Loss and DiVincenzo [Phys. Rev. A 57, 120 (1998)]. The present study demonstrates that isolated quantum dots are a possible route to increase the number of coherently coupled quantum dots.

Journal ArticleDOI
TL;DR: Radio frequency reflectometry is used to measure singlet–triplet states of a few-donor Si:P double quantum dot and demonstrate that the exchange energy can be tuned by at least two orders of magnitude, from 20 μeV to 8 meV.
Abstract: Spin states of the electrons and nuclei of phosphorus donors in silicon are strong candidates for quantum information processing applications given their excellent coherence times. Designing a scalable donor-based quantum computer will require both knowledge of the relationship between device geometry and electron tunnel couplings, and a spin readout strategy that uses minimal physical space in the device. Here we use radio frequency reflectometry to measure singlet–triplet states of a few-donor Si:P double quantum dot and demonstrate that the exchange energy can be tuned by at least two orders of magnitude, from 20 μeV to 8 meV. We measure dot–lead tunnel rates by analysis of the reflected signal and show that they change from 100 MHz to 22 GHz as the number of electrons on a quantum dot is increased from 1 to 4. These techniques present an approach for characterizing, operating and engineering scalable qubit devices based on donors in silicon.

Journal ArticleDOI
TL;DR: This work predicts the existence of a 2D structure, a monolayer of rocksalt-structured CrN (100) surface, which is both ferromagnetic and biocompatible and demonstrates that the displayed ferromagnetism is robust against thermal and mechanical perturbations.
Abstract: Design and synthesis of two-dimensional (2D) materials with robust ferromagnetism and biocompatibility is highly desirable due to their potential applications in spintronics and biodevices. However, the hotly pursued 2D sheets including pristine graphene, monolayer BN, and layered transition metal dichalcogenides are nonmagnetic or weakly magnetic. Using biomimetic particle swarm optimization (PSO) technique combined with ab initio calculations we predict the existence of a 2D structure, a monolayer of rocksalt-structured CrN (100) surface, which is both ferromagnetic and biocompatible. Its dynamic, thermal and magnetic stabilities are confirmed by carrying out a variety of state-of-the-art theoretical calculations. Analyses of its band structure and density of states reveal that this material is half-metallic, and the origin of the ferromagnetism is due to p-d exchange interaction between the Cr and N atoms. We demonstrate that the displayed ferromagnetism is robust against thermal and mechanical perturbations. The corresponding Curie temperature is about 675 K which is higher than that of most previously studied 2D monolayers.

Journal ArticleDOI
TL;DR: In this paper, the spin-orbit spin-transfer torque is decomposed into field-like and damping-like torques for two-dimensional free-electron and tight-binding models with Rashba spinorbit coupling.
Abstract: In ferromagnet/heavy-metal bilayers, an in-plane current gives rise to spin-orbit spin-transfer torque, which is usually decomposed into fieldlike and dampinglike torques. For two-dimensional free-electron and tight-binding models with Rashba spin-orbit coupling, the fieldlike torque acquires nontrivial dependence on the magnetization direction when the Rashba spin-orbit coupling becomes comparable to the exchange interaction. This nontrivial angular dependence of the fieldlike torque is related to the Fermi surface distortion, determined by the ratio of the Rashba spin-orbit coupling to the exchange interaction. On the other hand, the dampinglike torque acquires nontrivial angular dependence when the Rashba spin-orbit coupling is comparable to or stronger than the exchange interaction. It is related to the combined effects of the Fermi surface distortion and the Fermi sea contribution. The angular dependence is consistent with experimental observations and can be important to understand magnetization dynamics induced by spin-orbit spin-transfer torques.

Journal ArticleDOI
TL;DR: In this article, a digital quantum simulation of the paradigmatic Heisenberg and Ising interacting spin models using a two transmon-qubit circuit quantum electrodynamics setup is presented.
Abstract: Systems of interacting quantum spins show a rich spectrum of quantum phases and display interesting many-body dynamics. Computing characteristics of even small systems on conventional computers poses significant challenges. A quantum simulator has the potential to outperform standard computers in calculating the evolution of complex quantum systems. Here, we perform a digital quantum simulation of the paradigmatic Heisenberg and Ising interacting spin models using a two transmon-qubit circuit quantum electrodynamics setup. We make use of the exchange interaction naturally present in the simulator to construct a digital decomposition of the model-specific evolution and extract its full dynamics. This approach is universal and efficient, employing only resources which are polynomial in the number of spins and indicates a path towards the controlled simulation of general spin dynamics in superconducting qubit platforms.

Journal ArticleDOI
TL;DR: In this article, femtosecond stimulated Raman scattering was used to unravel the ultrafast photo-induced dynamics of magnetic excitations at the edge of the Brillouin zone.
Abstract: Femtosecond stimulated Raman experiments on the antiferromagnetic system KNiF3 are implemented to understand how the exchange interaction — a crucial interaction that rules magnetic phenomena — is influenced by ultrafast optical excitation. Manipulating the macroscopic phases of solids using ultrashort light pulses has resulted in spectacular phenomena, including metal–insulator transitions1,2,3, superconductivity4 and subpicosecond modification of magnetic order5. The development of this research area strongly depends on the understanding and optical control of fundamental interactions in condensed matter, in particular the exchange interaction. However, disentangling the timescales relevant for the contributions of the exchange interaction and spin dynamics to the exchange energy, Eex, is a challenge. Here, we introduce femtosecond stimulated Raman scattering to unravel the ultrafast photo-induced dynamics of magnetic excitations at the edge of the Brillouin zone. We find that femtosecond laser excitation of the antiferromagnet KNiF3 triggers a spectral shift of the two-magnon line, the energy of which is proportional to Eex. By unravelling the photo-induced modification of the two-magnon line frequency from a dominating nonlinear optical effect, we find that Eex is increased by the electromagnetic stimulus.

Journal ArticleDOI
TL;DR: An approach for entangling electron spin qubits localized on spatially separated impurity atoms or quantum dots via a multielectron, two-level quantum dot is presented, yielding an experimentally accessible method of coupling donor electron spins in silicon via a hybrid impurity-dot system.
Abstract: We present an approach for entangling electron spin qubits localized on spatially separated impurity atoms or quantum dots via a multielectron, two-level quantum dot. The effective exchange interaction mediated by the dot can be understood as the simplest manifestation of Ruderman-Kittel-Kasuya-Yosida exchange, and can be manipulated through gate voltage control of level splittings and tunneling amplitudes within the system. This provides both a high degree of tunability and a means for realizing high-fidelity two-qubit gates between spatially separated spins, yielding an experimentally accessible method of coupling donor electron spins in silicon via a hybrid impurity-dot system.

Journal ArticleDOI
TL;DR: In this article, the authors investigate the electrical control of the exchange coupling between donor bound electrons in silicon with a detuning gate bias, crucial for the implementation of the two-qubit gate in a silicon quantum computer.
Abstract: We investigate the electrical control of the exchange coupling (J) between donor bound electrons in silicon with a detuning gate bias, crucial for the implementation of the two-qubit gate in a silicon quantum computer. We find the asymmetric 2P-1P system provides a highly tunable exchange-curve with mitigated J-oscillation, in which 5 orders of magnitude change in the exchange energy can be achieved using a modest range of electric field for 15 nm qubit separation. Compared to the barrier gate control of exchange in the Kane qubit, the detuning gate design reduces the demanding constraints of precise donor separation, gate width, density and location, as a range of J spanning over a few orders of magnitude can be engineered for various donor separations. We have combined a large-scale full band atomistic tight-binding method with a full configuration interaction technique to capture the full two-electron spectrum of gated donors, providing state-of-the-art calculations of exchange energy in 1P-1P and 2P-1P qubits.

Journal ArticleDOI
TL;DR: A quantum heat engine with a working substance of two particles, one with a spin-1/2 and the other with an arbitrary spin, coupled by Heisenberg exchange interaction, and subject to an external magnetic field is investigated.
Abstract: We investigate a quantum heat engine with a working substance of two particles, one with a spin-1/2 and the other with an arbitrary spin (spin s), coupled by Heisenberg exchange interaction, and subject to an external magnetic field. The engine operates in a quantum Otto cycle. Work harvested in the cycle and its efficiency are calculated using quantum thermodynamical definitions. It is found that the engine has higher efficiencies at higher spins and can harvest work at higher exchange interaction strengths. The role of exchange coupling and spin s on the work output and the thermal efficiency is studied in detail. In addition, the engine operation is analyzed from the perspective of local work and efficiency. We develop a general formalism to explore local thermodynamics applicable to any coupled bipartite system. Our general framework allows for examination of local thermodynamics even when global parameters of the system are varied in thermodynamic cycles. The generalized definitions of local and cooperative work are introduced by using mean field Hamiltonians. The general conditions for which the global work is not equal to the sum of the local works are given in terms of the covariance of the subsystems. Our coupled spin quantum Otto engine is used as an example of the general formalism.

Journal ArticleDOI
TL;DR: The in situ tuning of the resonance frequency, as large as 10 GHz, is demonstrated in a spintronics microwave device through manipulating the interface exchange interaction.
Abstract: Exchange interaction at the interface between magnetic layers exhibits significant contribution to the magnetic resonance frequency The in situ tuning of the resonance frequency, as large as 10 GHz, is demonstrated in a spintronics microwave device through manipulating the interface exchange interaction

Journal ArticleDOI
TL;DR: In this paper, the Coulomb exchange interaction is considered as the interaction between the spin-down electrons being in the quantum states occupied by one electron, giving main contribution in the equilibrium.
Abstract: Separate spin evolution quantum hydrodynamics is generalized to include the Coulomb exchange interaction. The Coulomb exchange interaction is considered as the interaction between the spin-down electrons being in the quantum states occupied by one electron, giving main contribution in the equilibrium. The generalized model is applied to study the non-linear spin-electron acoustic waves. Existence of the spin-electron acoustic soliton is demonstrated. Contributions of the concentration, spin polarization, and exchange interaction in the properties of the spin electron acoustic soliton are studied.

Journal ArticleDOI
TL;DR: In this paper, the structural and magnetic properties of amorphous Gd-Fe alloys were investigated at the nanoscale level. And the authors proposed a possible explanation based on a combination of the Dzyaloshinskii-Moriya interaction and exchange frustration, modeled by an antiferromagnetic second-neighbor exchange interaction between Gd atoms in the Gdrich region.
Abstract: In recent years there has been an intense interest in understanding the microscopic mechanism of thermally induced magnetization switching driven by a femtosecond laser pulse. Most of the effort has been dedicated to periodic crystalline structures while the amorphous counterparts have been less studied. By using a multiscale approach, i.e., first-principles density functional theory combined with atomistic spin dynamics, we report here on the very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale level. Both structural and dynamical properties of Gd-Fe alloys reported in this work are in good agreement with previous experiments. We calculated the dynamic behavior of homogeneous and inhomogeneous amorphous Gd-Fe alloys and their response under the influence of a femtosecond laser pulse. In the homogeneous sample, the Fe sublattice switches its magnetization before the Gd one. However, the temporal sequence of the switching of the two sublattices is reversed in the inhomogeneous sample. We propose a possible explanation based on a mechanism driven by a combination of the Dzyaloshinskii-Moriya interaction and exchange frustration, modeled by an antiferromagnetic second-neighbor exchange interaction between Gd atoms in the Gd-rich region. We also report on the influence of laser fluence and damping effects in the all-thermal switching.

Journal ArticleDOI
TL;DR: It is shown that an intense femtosecond light pulse with photon energies higher than that of the bandgap, triggers spin waves in EuTe, and the ability to control and monitor the dynamics of the exchange energy with an all-optical technique opens up new opportunities for the manipulation of magnetism at ultrafast time-scales.
Abstract: We demonstrate that the ultrafast fast dynamics of the d–f exchange interaction, between conduction band electrons and lattice spins in EuTe, can be accessed using an all-optical technique. Our results reveal, in full detail, the time evolution of the d–f exchange interaction induced by a femtosecond laser pulse. Specifically, by monitoring the time resolved dynamics of the reflectivity changes and Kerr rotation of a weak light pulse reflected from the surface of the sample, it is shown that an intense femtosecond light pulse with photon energies higher than that of the bandgap, triggers spin waves in EuTe. The laser-induced spin waves modulate the d–f exchange interaction, and cause the bandgap to oscillate with an amplitude reaching 1 meV, at frequencies up to tens of GHz. The ability to control and monitor the dynamics of the exchange energy with our all-optical technique opens up new opportunities for the manipulation of magnetism at ultrafast time-scales.

Journal ArticleDOI
TL;DR: Recent ab initio calculations confirm that the alkali-doped fullerides are a new type of unconventional superconductors, where the unusual synergy between the phonons and Coulomb interactions drives the high-Tc superconductivity.
Abstract: Alkali-doped fullerides (A3C60 with A=K, Rb, Cs) show a surprising phase diagram, in which a high transition-temperature (Tc) s-wave superconducting state emerges next to a Mott insulating phase as a function of the lattice spacing. This is in contrast with the common belief that Mott physics and phonon-driven s-wave superconductivity are incompatible, raising a fundamental question on the mechanism of the high-Tc superconductivity. This article reviews recent ab initio calculations, which have succeeded in reproducing comprehensively the experimental phase diagram with high accuracy and elucidated an unusual cooperation between the electron-phonon coupling and the electron-electron interactions leading to Mott localization to realize an unconventional s-wave superconductivity in the alkali-doped fullerides. A driving force behind the exotic physics is unusual intramolecular interactions, characterized by the coexistence of a strongly repulsive Coulomb interaction and a small effectively negative exchange interaction. This is realized by a subtle energy balance between the coupling with the Jahn-Teller phonons and Hund's coupling within the C60 molecule. The unusual form of the interaction leads to a formation of pairs of up- and down-spin electrons on the molecules, which enables the s-wave pairing. The emergent superconductivity crucially relies on the presence of the Jahn-Teller phonons, but surprisingly benefits from the strong correlations because the correlations suppress the kinetic energy of the electrons and help the formation of the electron pairs, in agreement with previous model calculations. This confirms that the alkali-doped fullerides are a new type of unconventional superconductors, where the unusual synergy between the phonons and Coulomb interactions drives the high-Tc superconductivity.

Journal ArticleDOI
TL;DR: In this article, the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two adatom magnetic impurities placed on zigzag silicene nanoribbons (ZSNR) in the presence of the out-of-plane electric field was studied.
Abstract: We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two adatom magnetic impurities placed on zigzag silicene nanoribbon (ZSNR) in the presence of the out-of-plane electric field. We calculate the interaction mediated by the charge carriers of the pristine ZSNR at half filling. We show that the rotation of the spin of the itinerant electrons due to the exchange interaction causes a twisted RKKY interaction between localized spins which consists of the Heisenberg, Dzyaloshinsky-Moriya, and Ising interactions and explore the tunability of the RKKY interaction terms in respect with the perpendicular electric field. We numerically find that, due to the zero-energy edge state of ZSNR, the RKKY coupling is significantly enhanced when impurities are located on the zigzag edges. We also examine the RKKY interaction in different electric fields and explore that, in the topological insulator phase, the RKKY interaction is much greater than that when the system is in the band insulator region.

Journal ArticleDOI
TL;DR: In this article, room temperature ferromagnetism and p-type conductivity of Na doped ZnO films have been observed with a crossover of positive to negative magnetoresistance with the variation of Na doping concentrations.
Abstract: Na doped ZnO films were fabricated via a hydrothermal process. The films have shown room temperature ferromagnetism and p-type conductivity. Crossover of positive to negative magnetoresistance has been observed with the variation of Na doping concentrations. The positive MR is due to p-p exchange interaction induced Zeeman splitting to suppress the hopping path of holes. The ferromagnetism is attributed to the formation of a Zn vacancy complex. The negative magnetoresistance is due to the minimization of spin-dependent scattering by the applied magnetic field.