Spin Energy Gap Research Articles

Magnetothermal properties of CoO2 monolayer from first-principles and Monte Carlo simulations

Cobalt oxides are known for their excellent heat transfer properties. The main component of cobalt oxides is the CoO2 monolayer, which exhibits high-temperature superconductivity caused by strong electron–phonon coupling (EPC). We here systematically investigate the structural stability, electronic structure, and magnetism of the CoO2 monolayer using first-principles and Monte Carlo simulations. On this basis, we further study the changes in the spin energy gap, magnetic axis direction, and other properties of the CoO2 monolayer with the changes in carrier concentration. By appropriately doping the CoO2 monolayer with holes, the magnetic axis direction of the CoO2 monolayer can be reversed, thereby enhancing its potential application in the field of spin electronic devices. Monte Carlo simulation is used to study the regulation of different factors on the magnetothermal properties of the CoO2 monolayer. Through the analysis of physical parameters such as Curie temperature (TC) and bandgap, we find that the appropriate carrier concentration and magnetic field can not only regulate the magnetothermal properties of materials but also further improve the efficiency of materials in low-temperature environments.

Structural, dielectric, magnetic, optical, and electrical properties of double‐perovskite Sr1.5Mn0.5Fe0.5Hf1.5O6 oxides

AbstractHalf‐metallic ferromagnets (HMFs) exhibit semiconductor behavior for one spin projection and metal characteristics for the other, which have promising applications in spintronics. It is a great challengeable for HMFs to have wide spin energy gap, large saturation magnetization (MS), and high Curie temperature (TC) simultaneously. Here, we report the Sr1.5Mn0.5Fe0.5Hf1.5O6 (SMFHO) double perovskite compounds synthesized by solid‐state reaction and display a magnetic TC up to 804 K due to the strong Mn3+(↑)Fe3+(↑) spin interactions, which is the reported record in perovskite‐type HMFs so far. The measured MS value at 300 K was 4.87 μB/f.u., and it further increased up to 6.09 μB/f.u. at 2 K. The SMFHO powders exhibited butterfly‐shaped magnetoresistance (MR)–H curve at 2 K, and the MR (2 K, 7 T) value was measured as −3.86% due to the intergranular magnetic tunneling effect. Variation of the resistivity of the SMFHO ceramics versus temperature exhibits a semiconductor behavior, and the electrical transport properties of the SMFHO ceramics are well studied by Mott's range jump model, thermally activated semiconductor conductance model and small polaron jumping model, respectively. The optical absorption spectrum of the SMFHO powders demonstrates an indirect optical bandgap of 3.56 eV. Our present work demonstrates the intriguing properties of the SMFHO oxides where high TC, wide energy gap, and large MS value are preserved at the same time, which open the way to their promising applications in advanced spintronic devices at or beyond room temperature.

Spin- and valley-polarized Goos–Hänchen-like shift in ferromagnetic mass graphene junction with circularly polarized light

We investigate the band structure and Goos–Hänchen-like shift in ferromagnetic mass graphene junction modulated by the circularly polarized light. It is found that both spin and valley-related energy gaps can be opened by employing the circularly polarized light and the exchange field in mass graphene. The valley-polarized Goos–Hänchen-like shift can be identified in the presence of circularly polarized light, and the spin-polarized Goos–Hänchen-like shift can be realized with introduction of exchange field in mass graphene. Furthermore, the spin and valley polarization-related Goos–Hänchen-like shift can be achieved by combination of circularly polarized light and exchange field in mass graphene. It is hopeful that our work will be more conducive for future applications in graphene polarization transport devices.

Realization of a Half Metal with a Record-High Curie Temperature in Perovskite Oxides.

Half metals, in which one spin channel is conducting while the other is insulating with an energy gap, are theoretically considered to comprise 100% spin-polarized conducting electrons, and thus have promising applications in high-efficiency magnetic sensors, computer memory, magnetic recording, and so on. However, for practical applications, a high Curie temperature combined with a wide spin energy gap and large magnetization is required. Realizing such a high-performance combination is a key challenge. Herein, a novel A- and B-site ordered quadruple perovskite oxide LaCu3 Fe2 Re2 O12 with the charge format of Cu2+ /Fe3+ /Re4.5+ is reported. The strong Cu2+ (↑)Fe3+ (↑)Re4.5+ (↓) spin interactions lead to a ferrimagnetic Curie temperature as high as 710 K, which is the reported record in perovskite-type half metals thus far. The saturated magnetic moment determined at 300 K is 7.0 μB f.u.-1 and further increases to 8.0 μB f.u.-1 at 2 K. First-principles calculations reveal a half-metallic nature with a spin-down conducting band while a spin-up insulating band with a large energy gap up to 2.27eV. The currently unprecedented realization of record Curie temperature coupling with the wide energy gap and large moment in LaCu3 Fe2 Re2 O12 opens a way for potential applications in advanced spintronic devices at/above room temperature.

Highly efficient near-infrared thermally activated delayed fluorescence material based on a spirobifluorene decorated donor

The development of red/near-infrared (NIR) thermally activated delayed fluorescence (TADF) emitters are relatively lagging due to the spin statistics and energy gap law. Herein, we designed and synthesized a new NIR TADF emitter, 3-(4-(9,9′-spirobi[fluorene]-3-yl(phenyl)amino)phenyl)acenaphtho[1,2-b]pyrazine-8,9-dicarboni-trile (SDPA-APDC), by incorporating a spiro-type electron-donating moiety N,N-diphenyl-9,9′-spirobi[fluorene]-2-amine (SDPA) to an electron-withdrawing unit acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile (APDC). The photophysical, electrochemical and thermal properties of SDPA-APDC have been systematically explored. Consequently, the emitter was found high photoluminescence quantum yield (PLQY), narrow bandgap, small singlet-triplet energy gap (ΔEST) and excellent thermal stability. Furthermore, SDPA-APDC was developed for electroluminescence devices. The doped devices of SDPA-APDC achieved a red emission peak at 696 nm with a maximum external quantum efficiency (EQE) of 10.75%. And the non-doped device exhibited a NIR emission peak at 782 nm with a maximum EQE of 2.55%

A high-spin ground-state donor-acceptor conjugated polymer.

Interest in high-spin organic materials is driven by opportunities to enable far-reaching fundamental science and develop technologies that integrate light element spin, magnetic, and quantum functionalities. Although extensively studied, the intrinsic instability of these materials complicates synthesis and precludes an understanding of how fundamental properties associated with the nature of the chemical bond and electron pairing in organic materials systems manifest in practical applications. Here, we demonstrate a conjugated polymer semiconductor, based on alternating cyclopentadithiophene and thiadiazoloquinoxaline units, that is a ground-state triplet in its neutral form. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-to-low spin energy gap of 9.30 × 10-3 kcal mol-1. The strongly correlated electronic structure, very narrow bandgap, intramolecular ferromagnetic coupling, high electrical conductivity, solution processability, and robust stability open access to a broad variety of technologically relevant applications once thought of as beyond the current scope of organic semiconductors.

Widening the Spin Energy Gap

Widening the Spin Energy Gap

Quantitative prediction and interpretation of spin energy gaps in polyradicals: the virtual magnetic balance.

Open-shell organic molecules possessing more than two unpaired electrons and sufficient stability even at room temperature are very unusual, but few were recently synthesized that promise a number of fascinating applications. Unfortunately, reliable structural information is not available and only lower limits can be estimated for energy splittings between the different spin states. On these grounds, we introduce here an effective 'virtual magnetic balance', a robust and user-friendly tool purposely tailored for polyradicals and devised to be used in parallel with experimental studies. The main objective of this tool is to provide reliable structures and quantitative splittings of spin states of large, complex molecules. We achieved this objective with reasonable computation times and in a theoretical framework that allows disentanglement of different stereo-electronic effects contributing to the overall experimental result. A recently synthesized tetraradical with remarkable chemical stability was used as a case study.

Generation of full polarization in ferromagnetic graphene with spin energy gap

We propose a workable scheme for the generation of full spin polarization in ferromagnetic graphene system with strain or Rashba spin-orbit interaction. A spin energy gap can be opened in ferromagnetic graphene system in the presence of strain or Rashba spin-orbit interaction, leading to the full polarization in the spin energy gap. In addition, under the combined modulation of strain and Rashba spin-orbit interaction, the ferromagnetic graphene system can generate significantly large spin-polarized current with a full polarization in the spin energy gap. It is anticipated to apply such a phenomenon to design the electron spin devices based on the graphene.

Enhanced spin polarization in graphene with spin energy gap induced by spin-orbit coupling and strain

We investigate the possibility of spin polarization in graphene. The result shows that a spin energy gap can be opened in the presence of both spin-orbit coupling and strain. We find that high spin polarization with large spin-polarized current is achieved in the spin energy gap. However, only one of the two modulations is present, no spin polarization can be generated. So the combination of the two modulations provides a way to design tunable spin polarization without need for a magnetic element or an external magnetic field.

New attractive-force concept for Cooper pairs and theoretical evaluation of critical temperature and critical-current density in high-temperature superconductors

We propose a new attractive interaction between pairs of holes that results in Cooper pairs and a linear temperature dependence of the spin-energy gap derived from Fermi statistics. This interaction is a Lorentz force between two moving holes with equal velocity. This force is analogous to the attractive electromagnetic force between parallel current-carrying leads; local currents exist at a CuO2 surface. Combining the spin-energy gap and the proposed attractive force, we derive a critical temperature equation that gives the dependence of critical temperature on doping. This equation contains electric charge, coherence, Debye temperature, hole concentration, and forbidden band gap. It does not contain numerical or fitting parameters. By comparing the values obtained by this equation with experimental results, we find that the proposed theory agrees with the results for doping dependence. Furthermore, we use the spin-energy gap to obtain results for the temperature dependence of critical current density.

Nonlinear magnetotransport phenomena in high-mobility two-dimensional electrons in InGaAs/InP and GaAs/AlGaAs

This paper reports on the observation and analysis of magnetotransport phenomena in the nonlinear differential resistance ${r}_{xx}=d{V}_{xx}/dI$ of high-mobility In${}_{x}$Ga${}_{1\ensuremath{-}x}$As/InP and GaAs/Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As Hall bar samples driven by direct current ${I}_{\mathrm{dc}}$. Specifically, it is observed that Shubnikov--de Haas (SdH) oscillations at large filling factors invert their phase at sufficiently large values of ${I}_{\mathrm{dc}}$. This phase inversion is explained as being due to an electron heating effect. In the quantum Hall effect regime the ${r}_{xx}$ oscillations transform into diamond-shaped patterns with different slopes corresponding to odd and even filling factors. The diamond-shaped features at odd filling factors can be used as a probe to determine spin energy gaps. A zero current anomaly (ZCA) which manifests itself as a narrow dip in the ${r}_{xx}({I}_{\mathrm{dc}})$ characteristics at zero current is also observed. The ZCA effect strongly depends upon temperature, vanishing above 1 K while the transport diamonds persist to higher temperatures. The transport diamonds and ZCA are fully reproduced in a higher mobility GaAs/AlGaAs Hall bar structure confirming that these phenomena reflect intrinsic properties of two-dimensional systems.

First-principles research on influence of C dopants on magnetic and electric properties of rocksalt MgS

The 2×2×1 rocksalt C-doped MgS supercells are optimized and their magnetic and electric properties, including the half-metallicity, the conductivity and the supercell magnetic moments, are calculated or analysed by the first-principles researches based on the density functional theory. Results show that the concentration of C-dopants may cause important influence on the magnetic and the electric properties of rocksalt MgS. C dopants are inclined to have a scattering distribution. MgC0.0625S0.9375, aMgC0.1250S0.8750 and MgC0.1875S0.8125 have evident half-metallicity. They have wide spin energy gaps, thus high Curie temperature possibly. Their supercell magnetic moments are near to integral numbers 2.0, 4.0 and 6.0 μB. The main reason for spin polarization and half-metallicity of C-doped MgS is that there are sp hybridized orbitals in ligand compound ML6 caused by covalent interaction between C-ions and Mg-ions.

Doping effect of nonmagnetic impurity on the spin-Peierls-like transition in the quasi-one-dimensional (quasi-1D) spin system of 1-(4′-nitrobenzyl)pyridinium bis(maleonitriledithiolato)nickelate

A nonmagnetic compound, [NO 2BzPy][Cu(mnt) 2] (mnt 2− = maleonitriledithiolate; NO 2BzPy + = 1-(4′-nitrobenzyl)pyridinium), is isostructural with [NO 2BzPy][Ni(mnt) 2], which is a quasi-1D spin system and exhibits a spin-Peierls-like transition with J = 192 K in the gapless state and spin energy gap = 738 K in the dimerization state, respectively. Further, five nonmagnetic impurity doped compounds [NO 2BzPy][Cu x Ni 1− x (mnt) 2] ( x = 0.04–0.74) were prepared, and their crystal structures as well as magnetic properties were investigated. The nonmagnetic doping causes the suppression of the spin transition with an average rate of 139(13) K/percentage of dopant concentration, and the transition collapse is estimated at around x > 0.5.

Structural and Magnetic Investigations for the Doping Effect of Nonmagnetic Impurity on the Spin-Peierls-like Transition in a Quasi-One-Dimensional Magnet: 1-(4‘-Nitrobenzyl)pyridinium Bis(maleonitriledithiolato)nickelate

A nonmagnetic compound, [NO(2)BzPy][Au(mnt)(2)] (NO(2)BzPy(+) = 1-(4'-nitrobenzyl)pyridinium; mnt(2-) = maleonitriledithiolate), was synthesized and characterized structurally, which is isostructural with [NO(2)BzPy][Ni(mnt)(2)] that is a quasi-one-dimensional magnet and possesses a spin-Peierls-like transition with J = 192 K in the gapless state and spin energy gap = 738 K in the dimerization state, respectively. Further, ten nonmagnetic impurity doped compounds with a formula [NO(2)BzPy][Au(x)Ni(1-x)(mnt)(2)] (x = 0.01-0.73) were prepared and investigated by crystal structural determinations and magnetic susceptibility measurements. The nonmagnetic doping causes the suppression of the spin transition with an average rate of 221(12) K/percentage of dopant concentration. From the plots of chi(m)-T, the transition collapse (the characteristic of the transition is the sudden drop of chi(m) upon cooling, and the disappearance of this characteristic is considered as the criterion for the transition collapse) is estimated at around x > 0.27. In heavier doped system x = 0.49, the spin gap vanishes and a gapless phase is achieved again.

The computational design of zinc-blende half-metals and their nanostructures

The influence of atomic disorder and heterointerfaces with III–V semiconductors on theelectronic and magnetic properties of zinc-blende (ZB) CrAs is studied by ab initiocalculations based on density-functional theory. Antisite Cr spins are coupledantiferromagnetically with the Cr spins at the ordinary sites, while the ferromagneticcoupling between the Cr spins at the ordinary sites is robust against defect formation. Thedegree of spin polarization is not reduced significantly by the impurity bands formed in theminority spin energy gap. In the ZB CrAs/GaAs junction, relatively high spin polarizationis retained even at the interface in contrast to usual half-metal/semiconductorheterojunctions. Complete spin polarization is also preserved throughout ZB CrAs/GaAsmultilayers and it is insensitive to the substitutional disorder between Cr and Ga sites.

Green’s function treatment of the localized antiferromagnetic system

An extended Green's function method is presented for a localized antiferromagnetic (AF) system with mixed AF and ferromagnetic couplings, and single-ion anisotropy. Expressions are given for the N\'eel temperature, the temperature-dependent magnetic order parameter (MOP), and the spin component along the preferred direction of the single-ion anisotropy in the ground state for $A$-type AF $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$. These predict the MOP found by inelastic neutron scattering. Our theory exhibits a nontrivial internal consistency with four measured parameters, including ${T}_{N}$. Our theory also predicts a temperature-dependent spin energy gap that is different from the Holstein-Primakoff theory.

Spin dynamics in the BEC phase of the S=1/2 quantum spin system TlCuCl 3

The copper salt S=1/2 TlCuCl 3 is a three-dimensional quantum spin system with a singlet ground-state of dimer origin. A spin energy gap Δ≈0.7 meV separates the ground-state from the triplet waves, which propagate along all directions in reciprocal space. Across the quantum critical point at H c= Δ/ gμ B Bose–Einstein condensation of the triplet quasi-particles has been observed. The inelastic neutron scattering spectra at H> H c proof the coexistence of a gapless Goldstone mode, which has a linear dispersion, with renormalized quadratic Zeeman modes.

Local spin susceptibility in KCuCl 3

S= 1 2 KCuCl 3 has a singlet ground state with a finite spin energy gap Δ=31 K to triplet excited states. The bulk magnetic susceptibility vanishes for T⪡ Δ according to the singlet nature of the ground state. Astonishingly, recent muon spin relaxation (μSR) measurements reveal fluctuating internal fields down to T=5 K , with the onset of a quasi-static regime below T=5 K . The presence of free electronic magnetic moments and their possible freezing at low T contradicts the bulk picture of KCuCl 3. By means of polarized neutron scattering and μSR measurements on single crystals of the same batch, the nature of the local spin susceptibility is studied. The claims of a substantial muon influence in KCuCl 3 are reinforced.

Spin dynamics in the high-field phase of quantum-critical S =1/2 TlCuCl 3

An external magnetic field suppresses the spin-energy gap in singlet ground state S=1/2 TlCuCl3. The system becomes quantum-critical at Hc≈5.7 T, where the energy of the lowest Zeeman-split triplet excitation crosses the nonmagnetic ground state. Antiferromagnetic ordering is reported above Hc, which underlines the three-dimensional nature of the observed quantum phase transition. The intrinsic parameters of S=1/2 TlCuCl3 allow us to access the critical region microscopically by neutron scattering. A substantial study of the spin dynamics in the high-field phase of TlCuCl3 at T=1.5 K up to H=12 T was performed for the first time. The results possibly indicate two dynamical regimes, which can be understood within characteristically renormalized triplet modes and a low-lying dynamics of potentially collective origin.