Magnetic Band Structure Research Articles

Unveiling the interplay of magnetic order and electronic band structure in the evolution of the anomalous Hall effect in single crystalline MnPtGa

Unveiling the interplay of magnetic order and electronic band structure in the evolution of the anomalous Hall effect in single crystalline MnPtGa

Magnetic proximity effect on the spin-valley coupling in two-dimensional Cr2Ge2Te6/2H-TMD van der Waals heterostructures

Two-dimensional (2D) transition metal dichalcogenides with 2H-phases, as a unique platform of valleytronics, display valley polarization and the well-known anomalous valley Hall effect when stacking with 2D magnetic substrates. In this study, we employ first-principles calculations to investigate the magnetic states, band structures, and magnetic proximity-dependent valley properties of 2D van der Waals heterostructures Cr2Ge2Te6/2H-MX2 (M = Mo, W, and X = S, Se, Te). Our findings reveal that the heterostructures possess stacking-dependent spontaneous valley polarization as well as pristine perpendicular magnetic anisotropy. Additionally, the Berry curvature and circular polarization demonstrate the presence of spin–momentum coupling characteristics, while the calculated non-zero Hall voltage indicates that the anomalous valley Hall effect can be achieved in valley-polarized systems. Furthermore, due to the strain effect and the electronic polarization at the interface, Cr2Ge2Te6/2H-MX2 heterostructures undergo the transition from semiconductors to semimetals upon substitution of early chalcogen elements. These calculations provide valuable insights for practical applications of valleytronics in 2D van der Waals heterostructure systems.

Tunable magnetic and electronic properties of armchair BeN4 nanoribbons.

Recently, layered BeN4 as a novel Dirac semimetal has been fabricated (M. Bykov, T. Fedotenko, S. Chariton et al. Phys. Rev. Lett., 2021, 126, 175501). Motivated by the experiment, we perform first-principles calculations to predict the stability, magnetic configurations, and electronic structures of unsaturated BeN4 nanoribbons with an armchair-terminated edge. The magnetic interactions and electronic properties of BeN4 nanoribbons are sensitively influenced by the edge morphology. The BeN4 nanoribbons with both edges occupied by Be atoms undergo a transition from a ferromagnetic (FM) metal to an antiferromagnetic (AFM) semiconductor with the increase of ribbon width. The configurations with edges situated by Be and N atoms are FM/ferrimagnetic (FIM) metals or nearly half-metals, and the spin polarizability is as high as 85% when the ribbon width is N = 5. The nanoribbons with both edge sites occupied by pentagonal N atoms are nonmagnetic (NM), while the nanoribbons terminated by N atoms in a hexagonal ring are FM metals. We also explore the magnetic properties and band structures of BeN4 nanoribbons with hydrogen passivation. Our results open up a versatile edge engineering avenue to design BeN4-based spintronic and nanoelectronic devices.

Interplay between magnetic order and electronic band structure in ultrathin GdGe2 metalloxene films.

Dimensionality can strongly influence the magnetic structure of solid systems. Here, we predict theoretically and confirm experimentally that the antiferromagnetic (AFM) ground state of bulk gadolinium germanide metalloxene, which has a quasi-layered defective GdGe2 structure, is preserved in the ultrathin film limit. Ab initio calculations demonstrate that ultrathin GdGe2 films present in-plane intra-layer ferromagnetic coupling and AFM inter-layer coupling in the ground state. Angle-resolved photoemission spectroscopy finds the AFM-induced band splitting expected for the 2 and 3 GdGe2 trilayer (TL) films, which disappear above the Néel temperature. The comparative analysis of isostructural ultrathin DyGe2 and GdSi2 films confirms the magnetic origin of the observed band splitting. These findings are in contrast with the recent report of ferromagnetism in ultrathin metalloxene films, which we ascribe to the presence of uncompensated magnetic moments.

Atomic-Scale Determination of Cation and Magnetic Order in the Triple Perovskite Sr3Fe2ReO9

Abstract Pseudo-cubic (pc) perovskite oxides (ABO3) that can have different magnetic cations with different types and degrees of order at B sites have attracted considerable interest as a result of their tunable magnetic properties. Nanoscale inhomogeneity in cation order on the B sites can lead to different magnetic ground states and electronic band structures in local sample regions. Here, we determine cation order on the atomic scale in a nanosized Sr3Fe2ReO9 phase that has a 1:2 B-site-ordered triple perovskite structure using aberration-corrected analytical transmission electron microscopy (TEM), revealing that the Fe and Re cations form tripled-layered repeats with –[Fe–Fe–Re]n– sequences along [111]pc and an ordering vector of 1/3[111]*. To the best of our knowledge, this 1:2 B-site-ordered triple perovskite Sr3Fe2ReO9 phase has not been reported before. Based on a relaxed theoretical model that is consistent with the experimental images, density functional theory calculations are performed to determine the magnetic ground states and exchange parameters of the newly discovered Sr3Fe2ReO9 phase, in which nearest-neighbour Fe and Re cations are coupled antiferromagnetically. This combination of aberration-corrected analytical TEM and ab initio calculations provides physical insight into cation order and magnetic coupling in perovskite oxides at the atomic level.

XA yapıdaki Ti2RuSn ters-Heusler alaşımının yapısal, elektronik, elastik ve termodinamik özelliklerinin teorik olarak incelenmesi

Ters-Heusler (XA) yapıdaki üçlü Ti2RuSn alaşımının bazı yapısal özellikleri, manyetikliği, elektronik yapısı ve ayrıca elastisitesi DFT olarak bilinen yoğunluk fonksiyonel teorisi içerisinde yer alan genelleştirilmiş grandyent kabulleri kullanılarak incelenmiştir. Hesaplanan optimize edilmiş örgü sabitinin, toplam manyetik momentin ve elektronik bant yapısının literatürde yer alan mevcut verilerle uyumlu olduğu görüldü. Spin polarize bant hesaplamasından, Ti2RuSn alaşımı spin-aşağı yöneliminde 0.405 eV'luk bir yasak enerji aralığına sahipken, spin-yukarı yöneliminde metalik özellik göstermekte olduğu ve bundan dolayı yarı metalik ferromanyetik bir doğaya sahip olduğu tahmin edildi. İkinci dereceden elastik sabitlerinin sonuçlarının, bu alaşım için Born mekanik kararlılık kriterlerini karşıladığı ve mekanik olarak kararlı olduğu anlaşıldı. Ayrıca, Pugh kriterlerine göre B/G oranı 3.23 olan bu alaşımın 1.75 sınırından büyük olması nedeniyle sünek bir doğaya sahip olduğu ve anizotropi faktörü (A) 2.88 olması nedeniyle anizotropik davranış gösterdiği tespit edildi. İncelenen bu alaşımın spin elektroniği uygulamaları için umut verici bir aday olduğu söylenebilir.

High-spin states of $$^{204}$$At: isomeric states and shears band structure

High-spin states of neutron-deficient trans-lead nucleus \(^{204}\hbox {At}\) were populated up to \(\sim 8\,\mathrm{MeV}\) excitation through the \(^{12}\hbox {C} + ^{197}\hbox {Au}\) fusion evaporation reaction. Decay of the associated levels through prompt and delayed \(\gamma \)-ray emissions were studied to evaluate the underlying nuclear structure. The level scheme, which was partly known, was extended further. An isomeric \(16^+\) level with observed mean lifetime \(\tau =52 \pm 5\, \mathrm{ns}\), was established from our measurements. Attempts were made to interpret the excited states based on multi quasiparticle and hole structures involving \(2f_{5/2}\), \(1h_{9/2}\), and \(1i_{13/2}\) shell model states, along with moderate core excitation. Magnetic dipole band structure over the spin parity range: \(16^+\)–\(23^+\) was confirmed and evaluated in more detail, including the missing cross-over E2 transitions. Band-crossing along the shears band was observed and compared with the evidence of similar phenomena in the neighbouring \(^{202}\hbox {Bi}\), \(^{205}\hbox {Rn}\) isotones and the \(^{203}\hbox {At}\) isotope. Based on comparison of the measured B(M1)/B(E2) values for transitions along the band with the semiclassical model based estimates, the shears band of \(^{204}\hbox {At}\) was established along with the level scheme.

Intrinsic Magnetic Topological Insulator State Induced by the Jahn-Teller Effect.

The Jahn-Teller effect is a geometrical distortion which lowers the system symmetry and lifts orbital degeneracy in molecules and solids. It affects a wide range of properties, including magnetic and band structures. In this work we propose a family of Cr-containing intrinsic magnetic topological insulator materials which are subjected to a pseudo-Jahn-Teller effect-CrBi2Se4, CrBi2Te2Se2, and CrBi2Te4. Using first-principles calculations we study their properties and investigate the impact of Jahn-Teller distortions on the electronic and magnetic properties. We show that these distortions can significantly affect magnetic anisotropy energy and band structure. Without the distortions accounted for, all three of the compounds exhibit a semimetallic band structure. The distortions open a band gap, which in the cases of CrBi2Te2Se2 and CrBi2Te4 is inverted. We also investigate the CrBi2Te2Se2 and CrBi2Te4 surface band structure and demonstrate that the surface states have a topological origin.

Thermal Hall and Nernst responses in ultrathin magnetic films of pyrochlore lattice

The ultrathin films of pyrochlore lattice along [111] direction provide a fertile ground for exploring topological states in oxide materials, where the electron correlations are strong and magnetically ordered states are favored. As a result of strong interactions, most of states are magnetic insulators with strong Dzyaloshinskii–Moriya and anisotropic spin interactions. We considered three types of heterostructures, a bilayer of triangular-kagome (TK), and two trilayers of triangular-kagome-triangular (TKT) and kagome-triangular-kagome (KTK) thin films. The ground states are magnetically ordered with the low-energy excitations, the magnons, characterized by nontrivial topology. While for TK model the lowest magnon band is trivial, the lowest bands in TKT and KTK model are topological. These features have direct consequences on the thermal Hall conductivity, where we observed that the latter quantity onsets at lower temperature in multilayer models, suggesting that the thermal Hall response can be controlled by engineering lattice-driven magnetic band structure. We also introduce the thermal sublattice Nernst effect in these layers and found the corresponding response is as large as the thermal Hall response.

Out-of-plane magnetic anisotropy engineered via band distortion in two-dimensional materials

We unveil the connection between magnetic anisotropy and band structures in $\mathrm{\ensuremath{\Gamma}}$-specialized ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}/{\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ heterostructure, through first-principles calculations. A significant band distortion occurs near the $\mathrm{\ensuremath{\Gamma}}$ point with a formed valley. As the band-distortion-caused valley moves relative to the quasivalence band maximum, the magnetic anisotropy switches between in-plane and out-of-plane magnetic anisotropy. Such a rule applies to either the strained or polarization-switched ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}/{\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ heterostructure. These findings demonstrate the feasibility of predicting the magnetic anisotropy by band structure modification in two-dimensional magnetic systems.

Hofstadter butterfly and the quantum Hall effect in twisted double bilayer graphene

We study the energy spectrum and quantum Hall effects of the twisted double bilayer graphene in uniform magnetic field. We investigate two different arrangements, AB-AB and AB-BA, which differ in the relative orientation but have very similar band structures in the absence of a magnetic field. For each system, we calculate the energy spectrum and quantized Hall conductivities at each spectral gap by using a continuum Hamiltonian that satisfies the magneto-translation condition. We show that the Hofstadter butterfly spectra of AB-AB and AB-BA stackings differ significantly, even though their zero magnetic field band structures closely resemble; the spectrum of AB-AB has valley degeneracy, which can be lifted by applying interlayer potential asymmetry, while the spectrum of AB-BA has no such degeneracy in any case. We explain the origin of the difference from the perspectives of lattice symmetry and band topology.

Unconventional valley-dependent optical selection rules and landau level mixing in bilayer graphene

Selection rules are of vital importance in determining the basic optical properties of atoms, molecules and semiconductors. They provide general insights into the symmetry of the system and the nature of relevant electronic states. A two-dimensional electron gas in a magnetic field is a model system where optical transitions between Landau levels (LLs) are described by simple selection rules associated with the LL index N. Here we examine the inter-LL optical transitions of high-quality bilayer graphene by photocurrent spectroscopy measurement. We observed valley-dependent optical transitions that violate the conventional selection rules Δ|N| = ± 1. Moreover, we can tune the relative oscillator strength by tuning the bilayer graphene bandgap. Our findings provide insights into the interplay between magnetic field, band structure and many-body interactions in tunable semiconductor systems, and the experimental technique can be generalized to study symmetry-broken states and low energy magneto-optical properties of other nano and quantum materials.

Electronic and magnetic properties of zigzag α-graphyne nanoribbons with edge fluorine modification

Using the first principles calculations, we investigate the stability of hydrogen, fluorine and oxygen atoms saturated the edge of zigzag α-graphyne nanoribbons (ZαGYNRs), and find edge fluorination is the most stable one. Then we probe the magnetic properties and electronic structures of three different edge fluorinated ZαGYNRs. The results show that the magnetic properties and band structures have a close relationship with the edge fluorinated manners. For the symmetric edge fluorination: (1) Monofluorinated ZαGYNRs shows spin-degenerated semiconductor property in the ground state (antiferromagnetic state). (2) For difluorinated ZαGYNRs, the magnetism and band structures are relying on the ribbon width. It shows nonmagnetic semiconductor property when the ribbon width is smaller than 6. Otherwise, it shows spin-degenerated semiconductor property in its ground state (antiferromagnetic state). While the asymmetric edge fluorination can break the spin-degeneracy and introduce 1μB magnetic moment for ZαGYNRs in its ground state (ferromagnetic state), making the system shows bipolar magnetic semiconductor property. All these results are beneficial for designing spin semiconductor devices.

Strain-tunable magnetic order and electronic structure in 2D CrAsS4

Using first-principles calculations based on density-functional theory, the effects of strain on the magnetic order and band structure of single-layer CrAsS4 have been investigated systematically. By applying a uniaxial tensile strain of 2.99% along the y-direction or a compressive strain of 1.76% along the x-direction, the single-layer CrAsS4 is determined to be an antiferromagnetic (AFM) semiconductor that undergoes a phase transition from AFM to ferromagnetic (FM) state. Further, the physical mechanism underlying the strain-dependent magnetic stability is discovered to be a competition between direct exchange and indirect superexchange interactions between the nearest neighbour Cr atoms. Moreover, the bandgap exhibits an abrupt change that accompanies the phase transition in the magnetic order. Our study provides an intuitive approach to design the strain-modulated spintronic devices.

Electronic and phonon structure of nickel hydroxide: first-principles calculation study

We carried out a complete study (magnetic, electronic, lattice dynamic, and point defects) of the β-nickel hydroxide (β-Ni(OH)2) from first-principles calculations based on density functional theory. It is found that both of the magnetic ground state and band structure of β-Ni(OH)2 are strongly dependent on the correlation effect of Ni d-electrons. Experimental founded antiferromagnetic ground state with spin coupling along c direction has been confirmed by DFT+U method, and the predicated band structure shows a direct band gap about 3.5 eV with the highest occupied valence and lowest occupied bands mainly composed by O p-electron and Ni d-electron. Negative longitude acoustic phonon frequency around K point has been found, which is originated from the weak OH bond. High frequency localized vibration of hydrogen atom makes it easy to break away, and so form a vacancy, in agreement with the prediction that H+ vacancy has the lowest formation energy.

Magnetism in the strained ordered phases of PtxFe1-x and PtxCo1-x (x=0.25, 0.5, and 0.75)

The response of the magnetic moment of the L10 and L12 ordered phases of PtxM1-x (M = Fe,Co) (x = 0.25, 0.5, and 0.75) to compressive and tensile strains have been investigated using density functional theory (DFT). The magnetic moment of the Pt0.75M0.25 and Pt0.50M0.50 phases varies linearly compared to the response of the Pt0.25M0.75 alloys which shows a transition in the rate of change of magnetic moment at approximately zero strain. For all phases the mechanism of magnetic moment change under strain is shown to be intra-orbital charge transfer within the Pt, Fe and Co d shells. The strained and equilibrium magnetic band structures of each phase contain spin-orbit effects which are presented and discussed.

Generalized Pseudopotentials for the Anisotropic Fractional Quantum Hall Effect.

We generalize the notion of Haldane pseudopotentials to anisotropic fractional quantum Hall (FQH) systems that are physically realized, e.g., in tilted magnetic field experiments or anisotropic band structures. This formalism allows us to expand any translation-invariant interaction over a complete basis, and directly reveals the intrinsic metric of incompressible FQH fluids. We show that purely anisotropic pseudopotentials give rise to new types of bound states for small particle clusters in the infinite plane, and can be used as a diagnostic of FQH nematic order. We also demonstrate that generalized pseudopotentials quantify the anisotropic contribution to the effective interaction potential, which can be particularly large in models of fractional Chern insulators.

Colloquium: Atomic quantum gases in periodically driven optical lattices

Time periodic forcing in the form of coherent radiation is a standard tool for the coherent manipulation of small quantum systems like single atoms. In the last years, periodic driving has more and more also been considered as a means for the coherent control of many-body systems. In particular, experiments with ultracold quantum gases in optical lattices subjected to periodic driving in the lower kilohertz regime have attracted a lot of attention. Milestones include the observation of dynamic localization, the dynamic control of the quantum phase transition between a bosonic superfluid and a Mott insulator, as well as the dynamic creation of strong artificial magnetic fields and topological band structures. This article reviews these recent experiments and their theoretical description. Moreover, fundamental properties of periodically driven many-body systems are discussed within the framework of Floquet theory, including heating, relaxation dynamics, anomalous topological edge states, and the response to slow parameter variations.

Microscopic mechanism of the giant magnetocaloric effect in MnCoGe alloys probed by x-ray magnetic circular dichroism

One important aspect of the magneto-structural transition in MnCoGe-based materials is the reduction in saturation magnetization from the orthorhombic to the hexagonal phase. Here, by combining an element specific magnetization probe such as x-ray magnetic circular dichroism and band structure calculations, we show that the magnetic moment instability between orthorhombic and hexagonal structures originates from a reduction in the Mn sub-lattice magnetization.

Thin films of a three-dimensional topological insulator in a strong magnetic field: Microscopic study

The response of thin films of Bi$_2$Se$_3$ to a strong perpendicular magnetic field is investigated by performing magnetic bandstructure calculations for a realistic multi-band tight-binding model. Several crucial features of Landau quantization in a realistic three-dimensional topological insulator are revealed. The $n=0$ Landau level is absent in ultra-thin films, in agreement with experiment. In films with a crossover thickness of five quintuple layers, there is a signature of the $n=0$ level, whose overall trend as a function of magnetic field matches the established low-energy effective-model result. Importantly, we find a field-dependent splitting and a strong spin-polarization of the $n=0$ level which can be measured experimentally at reasonable field strengths. Our calculations show mixing between the surface and bulk Landau levels which causes the character of levels to evolve with magnetic field.