Magnetic Band Structure Research Articles

Magnetic property and possible half-metal behavior in Co-doped graphene

The magnetic property and band structures of Co-monolayer doped graphene were examined on the basis of density-functional theory. The magnetic moment of the system is closely related to the interfacial spacing. Magnetic-nonmagnetic transition would be produced by decreasing the layer distance. Although the magnetic moment can also be reduced by increasing the lattice constant, the ground states are magnetic states under tension. Besides, the increase of lattice constant greatly enlarges the direct and indirect gaps of spin-down bands near the Fermi level. With a little increase of the Fermi level or the electron density, half-metal behavior would be expectable in the Co-doped graphene under tension.

Theoretical investigation of the magnetic ordering and the superconducting state in rare earth iron pnictides

The superconductivity and magnetic phenomena of the rare earth iron pnictides R FePO (R = La , Sm ) are analyzed using ab initio density functional theory in the local density approximation (LDA) with the on-site Hubbard U eff parameter (LDA + U). The results of the magnetic properties, band structures and Fermi surfaces of the recently synthesized tetragonal layered iron pnictides superconductor SmFePO are presented in comparison with the superconductor LaFePO . The anti-ferromagnetic state [ Sm : AFM, Fe AFM_stripe] is found to be the most stable than the other possible phases, which is in agreement with experiment. The effect of the Hubbard correction is investigated and is found to be a necessary requirement for the accurate description of both the electronic structure and the Fermi surfaces.

Improved luminescence from tensile-strained compound semiconductor quantum wells with dielectric-rod photonic crystals

We investigated the use of dielectric-rod photonic crystals on a tensile-strained dilute nitride gain medium. After applying the patterned photonic crystal rods to GaInNAs quantum wells with a heterostructure having optical confinement, we observed enhanced extracted luminescence efficiency from the surface depending on the correlated variation in the transverse magnetic photonic band structure.

Magnetotransport Subband Spectroscopy in InAs Nanowires

We report on magnetotransport measurements in InAs nanowires under a large magnetic field (up to 55T), providing a spectroscopy of the one-dimensional electronic band structure. Large modulations of the conductance mediated by a control of the Fermi energy reveal the Landau fragmentation, carrying the fingerprints of the confined InAs material. Our numerical simulations of the magnetic band structure consistently support the experimental results and reveal key parameters of the electronic confinement.

Nature of the Bad Metallic Behavior ofFe1.06TeInferred from Its Evolution in the Magnetic State

We investigate with angle-resolved photoelectron spectroscopy the changes of the Fermi surface and the main bands from the paramagnetic state to the antiferromagnetic (AFM) state occurring below 72 K in Fe1.06Te. The evolution is completely different from that observed in Fe pnictides, as nesting is absent. The AFM state is a rather good metal, in agreement with our magnetic band structure calculation. On the other hand, the paramagnetic state is very anomalous with a large pseudogap of ~65 meV on the electron pocket that closes in the AFM state. We discuss this behavior in connection with spin fluctuations existing above the magnetic transition and the correlations predicted in the spin-freezing regime of the incoherent metallic state.

Superconductivity in the Mn5Si3-type Zr5Sb3system

We report the discovery of superconductivity at 2.3 K in Zr5Sb3, the first superconducting member in the large compound family of the Mn5Si3-structure type. Transport, magnetic, and calorimetric measurements and band structure calculations show it to be a phonon-mediated BCS superconductor, with a relatively large density of states at the Fermi level associated with the d-electrons of Zr and substantially larger electron-phonon coupling compared to the Sn counterpart compound Zr5Sn3. More superconductors with even higher transition temperatures are expected to be found in this family of compounds.

Invariants of the single-impurity Anderson model and implications for conductance functionals

An exact relation between the conductance maximum G0 at zero temperature and a ratio of lead densities is derived within the framework of the single-impurity Anderson model: , where and , denote the excess density in the left/right lead at distance x due to the presence of the impurity at the origin, x = 0. The relation constitutes a parameter-free expression of the conductance of the model in terms of the ground-state density that generalizes an earlier result to the generic case of asymmetric lead couplings. It turns out that the specific density ratio, , is independent of the distance to the impurity x, the (magnetic) band structure and filling fraction of the contacting wires, the strength of the onsite interaction, the gate voltage and the temperature. Disorder-induced backscattering in the contacting wires has an impact on that we discuss. Our result suggests that it should be possible, in principle, to determine experimentally the peak conductance of the Anderson impurity by performing a combination of measurements of ground-state densities.

Double trigonal warping and the anomalous quantum Hall step in bilayer graphene with Rashba spin–orbit coupling

We demonstrate that the trigonal warping observed in bilayer graphene is doubled in the presence of Rashba spin–orbit (RSO) coupling, i.e. the Dirac points along the three-fold symmetry axis are doubled. There are now seven Dirac points. Furthermore, the RSO interaction breaks the electron–hole symmetry of the magnetic band structure. The most intriguing feature is that the step of the quantum Hall plateau at zero energy is four times that at finite energy. The number of Dirac points and the zero energy Hall step are only determined by the existence of RSO coupling, but are independent of the strength of the coupling. The robustness of these phenomena suggests equivalence between the RSO coupling and the topological effect in bilayer coupling.

Synthesis, Crystal Structures and Properties of the Zintl Phases Sr2ZnP2, Sr2ZnAs2, A2ZnSb2 and A2ZnBi2 (A = Sr and Eu)

AbstractThe new intermetallic compounds Sr2ZnP2, Sr2ZnAs2, A2ZnSb2 and A2ZnBi2 (A = Sr, Eu) have been synthesized from the corresponding elements through high‐temperature reactions using the flux‐growth method. Their structures have been established by single‐crystal and powder X‐ray diffraction. In all cases, the X‐ray diffraction patterns can be successfully indexed based on hexagonal cells in the space group P63/mmc (no. 194) with lattice parameters in the range a = 4.31–4.73 Å and c = 7.9–8.55 Å. The average structure can be described in the ZrBeSi type (Pearson symbol hP6; 3 unique positions) with defects on the zinc site – structure refinements indicate that every second zinc position is vacant, i.e., their formula unit is AZn1–xPn with x = 0.5(A = Sr, Eu; Pn = P, As, Sb, Bi). No stoichiometry breadth was observed, which could imply that a super‐structure with a long‐range order of the zinc vacancies is plausible and evidence for such was sought using electron diffraction. The results from these experiments, as well as magnetic susceptibility measurements and band structure calculations using the LMTO code are also discussed.

First-principles study of magnetic properties in Mo-doped graphene

The geometric structure, electronic structure and magnetic properties of substitutionally Mo-doped graphene are studied based on first-principles calculations. Mo introduces a magnetic moment of 2 μB in graphene. The magnetic properties and band structure can be well understood using a hybridization model. Magnetic coupling between two Mo impurities is also discussed. Depending on the relative position of the two Mo impurities, the ground state of the system can be ferromagnetic, antiferromagnetic or paramagnetic. A Ruderman–Kittel–Kasuya–Yosida (RKKY)-like behavior is observed when the distance between Mo atoms is relatively large. However, when the distance between Mo atoms is rather small, the RKKY model is not suitable to describe the magnetic ordering due to their non-neglectable direct interactions.

K 2Mg 5− xSn 3 and K 3Mg 18Tt11 ( Tt=Sn, pb) with two types of Mg–Sn/Pb frameworks

K 2Mg 5− x Sn 3 ( x=0.28) and K 3Mg 18 Tt 11 ( Tt=Sn, Pb) have been synthesized by reacting the mixture of the corresponding pure elements at high temperature, and structurally characterized by single-crystal X-ray diffraction studies. K 2Mg 5− x Sn 3 ( x=0.28) is isostructural with Ni 7− x Sb Q 2 ( Q=Se, Te) series and features 2D corrugated [Mg 5− x Sn 3] layers that are separated by K + cations. The structure of K 3Mg 18 Tt 11 ( Tt=Sn, Pb) is closely related to the Ho 2Rh 12As 7 structural type and features 3D [Mg 18 Tt 11] framework composed of 1D [Mg 18 Tt 11] columns that are interconnected via Mg– Tt bonds, forming 1D hexagonal tunnels occupied by the K+ cations. Electronic structure calculations indicate that Mg atoms can function as either electron donor or as a participator in the network along with Tt atoms. Magnetic property measurements and band structure calculations indicate that these compounds are metallic.

On the R 5d band polarization in rare-earth–transition metal compounds

Magnetic measurements and band structure calculations were performed onRT2 andRT5 compounds, where R is a heavy rare-earth andT = Fe, Co, Ni, Al, as well as on pseudobinary compoundsGdCo2 − xAx (A = Ni, Cu,Si), YFe2 − xVx and YCo4 − xNixB. The calculated moments per formula unit described well the experimentally determinedmagnetizations. By considering the 4f–5d–3d exchange interactions, we evaluate thecontributions of local 4f–5d and short range 5d–3d interactions to R 5d and Y 4d bandpolarizations. The 4f–5d induced polarizations are proportional to the De Gennes factorand are the same for a given R and a similar type structure. The R 5d and Y 4d bandpolarizations induced by R 5d–T3d or Y 4d–T3d hybridizations are proportional to thenumber of neighbouring T atoms, to a given R, and their magnetic moments. Previousresults on the matter are also discussed.

Static equation of state of bcc iron

Body-centered-cubic (bcc) iron is one of the most investigated solid-state systems. Using four different density-functional methods, we show that there is a magnetic transition close to the ground-state volume of bcc Fe, which originates from the particular magnetic band structure. The common equation of state functions, used to determine the basic ground-state physical quantities from the calculated total energies, cannot capture the physics of this magnetic transition leading to serious underestimation of the Fe bulk modulus. Ignorance of the magnetic transition found here is reflected by large scatter of the published theoretical bulk moduli of ferromagnetic bcc Fe. Due to the low performance of the exchange-correlation functionals, most of the erroneous results are accidentally in good agreement with the experimental values. The present finding is of fundamental importance, especially taking into account that bcc Fe is frequently used as a test system in assessing the performance of exchange-correlation approximations or total-energy methods.

Tuning Ferro- and Metamagnetic Transitions in Rare-Earth Cobalt Phosphides La1−xPrxCo2P2

A family of rare-earth cobalt phosphides La1−xPrxCo2P2 (0 ≤ x ≤ 1) that belong to the ThCr2Si2 structure type has been prepared and characterized by structural and magnetic methods and electronic band structure calculations. All studied quaternary phases exhibit multiple magnetic transitions, leading to an observation of ferro- and metamagnetism and magnetic pole reversal. The ferromagnetic transition temperature of LaCo2P2 (132 K) increases dramatically upon substitution of Pr for La and reaches 268 K for La0.25Pr0.75Co2P2. This increase is accompanied by elongation of intralayer Co−Co distances. Variable temperature X-ray diffraction data revealed that temperature dependences of unit cell parameters, and, correspondingly, both intra- and interlayer Co−Co separations show anomalous changes at temperatures close to the ferromagnetic transition. The electronic structure calculations reveal a strong peak in the nonmagnetic density of states (DOS). This instability is removed in the spin-polarized DOS due to...

Molecular Conductors Based on the Mixed-Valence Polyoxometalates [SMo12O40]n− (n = 3 and 4) and the Organic Donors Bis(ethylenedithio)tetrathiafulvalene and Bis(ethylenedithio)tetraselenafulvalene

The synthesis, crystal structure, and physical characterization of two new radical salts formed by the organic donors bis(ethylenedithio)tetrathiafulvalene (ET) and bis(ethylenediseleno)tetrathiafulvalene (BETS) and the Keggin polyoxometalate (POM) [SMo(12)O(40)](n-) are reported. The salts isolated are ET(8)[SMo(12)O(40)] x 10 H(2)O (1) (crystal data: (1) monoclinic, space group I2/m with a = 13.9300(10) A, b = 43.467(3) A, c = 13.9929(13) A, beta = 107.979(6) degrees, V = 8058.9(11) A(3), Z = 2) and BETS(8)[SMo(12)O(40)] x 10 H(2)O (2) (crystal data: monoclinic, space group I2/m with a = 14.0878(2) A, b = 44.1010(6) A, c = 14.0930(2) A, beta = 106.739(3) degrees, V = 8384.8 A(3), Z = 2). Both compounds are isostructural and consist of alternating layers of the organic donors (with an alpha or theta(42+40) packing mode) and POM anions. The structural data, as well as the magnetic susceptibility, ESR measurements, and band structure calculations, indicate that the Keggin POMs have been reduced by one electron in 1 and by two electrons in 2, leading to the POMs [SMo(12)O(40)](3-) and [SMo(12)O(40)](4-) in 1 and 2, respectively. At ambient pressure 1 is a classical semiconductor with a room temperature conductivity of about 1 S cm(-1) and an activation energy of about 130 meV. Salt 2 also exhibits an activated behavior although it does not follow a classical semiconducting regime. The conductivity of 2 under applied pressure shows an enhanced conductivity, in agreement with the analysis of the electronic structure, although no metallic behavior is detected below about 10 kbar.

Topological Surface States in Three-Dimensional Magnetic Insulators

An electron moving in a magnetically ordered background feels an effective magnetic field that can be both stronger and more rapidly varying than typical externally applied fields. One consequence is that insulating magnetic materials in three dimensions can have topologically nontrivial properties of the effective band structure. For the simplest case of two bands, these "Hopf insulators" are characterized by a topological invariant as in quantum Hall states and Z2 topological insulators, but instead of a Chern number or parity, the underlying invariant is the Hopf invariant that classifies maps from the three-sphere to the two-sphere. This Letter gives an efficient algorithm to compute whether a given magnetic band structure has nontrivial Hopf invariant, a double-exchange-like tight-binding model that realizes the nontrivial case, and a numerical study of the surface states of this model.

Pband in a rotating optical lattice

We investigate the effects of rotation on the excited bands of a tight-binding lattice, focusing particularly on the first excited $(p)$ band. Both the on-site energies and the hopping between lattice sites are modified by the effective magnetic field created by rotation, causing a nontrivial splitting and magnetic fine structure of the $p$ band. We show that Peierls substitution can be modified to describe $p$ band under rotation, and use this method to derive an effective Hamiltonian. We compare the spectrum of the effective Hamiltonian with a first-principles calculation of the magnetic band structure and find excellent agreement, confirming the validity of our approach. We also discuss the on-site interaction terms for bosons and argue that many-particle phenomena in a rotating $p$ band can be investigated starting from this effective Hamiltonian.

Preface: phys. stat. sol. (c) 4/9

AbstractII–VI semiconductors were already systematically investigated in the 1930s, however, research in this area faded in the 50s in favor of type IV and III–V semiconductors. II–VI research then was resumed in the 70s. The establishment of extensive II–VI molecular beam epitaxy facilities in Würzburg in the early 90s created ideal conditions for a Collaborative Research Centre on these materials. In 1994, the Würzburg Sonderforschungsbereich (SFB 410) on II–VI physics and chemistry was established by the Deutsche Forschungsgemeinschaft. A variety of powerful characterization methods available in Würzburg were successfully employed, involving (low temperature) optical, magneto‐optical, and transport methods, surface analysis and X‐ray techniques, and, especially in recent years, advanced lithography. The experiments were accompanied by detailed theoretical work, and a close cooperation between all groups was successfully established.The establishment of SFB 410 allowed for long‐term planning and resulted in continuity. During the 12 years of its existence, a large number of interesting and important results were obtained. In the present special issue a selection of results obtained during the last few years is presented. Some of the contributions in this issue have partly review character, but all of them contain new results. The titles of the 25 contributions to this issue give a good impression of the addressed topics and the employed methods.While in the early years of the SFB, much of the research supported the application of II–VI semiconductors as lasers in the blue/green spectral range. In recent years the emphasis shifted to compounds containing manganese ions (diluted magnetic semiconductors). The availability of these materials has allowed the SFB to play a leading role in the development of the research area of spintronics (the control of electron spins by an electric field) with the demonstration of II–VI facilitating spin injection and the creation of magnetic resonant tunneling diodes that function as spin filters. Closely related to spintronics is the study of the influence of spin–orbit coupling on quantum transport. The outstanding II–VI material here is HgTe which boosts mobilities as well as spin–orbit splittings that are much larger than those available so far in III–Vs. The development of lithographic techniques tailored for this material has enabled the fabrication of nanostructures exhibiting novel phenomena such as the Aharonov‐Casher phase and the spin Hall effect. Highly interesting (magneto‐)optical properties have been found in quantum dots and micro resonators.Many of the results were only achieved because of a considerable improvement and thorough, also theoretical, understanding of epitaxial layer and self‐organized quantum dot growth, as well as a detailed control of surfaces and interfaces. Careful surface studies employing a variety of electronic and geometric surface techniques have added considerable knowledge on the two‐dimensional (interface) properties of many II–VI combinations. Ad hoc developed theories on magnetic transport and band structure calculations have largely improved the physical understanding of the experimental results and of electronic, (magneto‐)optical, and (magneto‐)transport properties of some II–VI materials. Finally, a large effort was made to synthesize, model, and, in particular, characterize II–VI nanoparticles, with (or without) organic (or inorganic) shells. A significant effort was made to analyze in detail especially those nanoparticles that are neither molecules nor solids having diameters below three nanometers.Altogether, this collaborative research effort (“Sonderforschungsbereich”) was very successful in many respects, for example in the number and quality of the research results which accordingly lead to numerous publications and presentations, in various international co‐operations, in the education of many diploma and doctoral students, in the development of several young scientists who got their habilitation and became professors, and in the further development of and improved collaboration between the involved three faculties of Würzburg University. Most of the results would not have been achieved without the extremely well developed co‐operation between all groups of SFB 410.The three speakers of SFB 410 would like to thank their colleagues for this extraordinary co‐operation, the DFG for 12 years of financial support, the reviewers for their critical monitoring, and the administrative people for their effective involvement.

Electronic structure calculations in a uniform magnetic field using periodic supercells

We have recently presented a method that allows one to use periodic supercells in ab initio electronic structure calculations in the presence of a finite magnetic field [Phys. Rev. Lett. 92 (2004) 186402]. This method retains the simplicity and efficiency of plane-wave basis sets and Fourier transforms. The original formulation was developed for cubic cell and for the k=(0,0,0) point of the supercell Brillouin zone, and here we extend the formalism to arbitrarily tilted supercells and to the case of non-zero k-points. Implementation details are discussed, together with numerical benchmarks. Finally, first principles calculations of magnetic band structures are presented.

Magnetic quantum oscillations of electrons on a two-dimensional lattice: Numerical simulations and the magnetic breakdown approach

We report on analytical and numerical studies of the magnetic quantum oscillations---the de Haas--van Alphen (dHvA) effect---in a model of spinless electrons on a two-dimensional (2D) square lattice in a magnetic field $B$ known as the Azbel-Hofstadter (AH) problem. The numerical simulations of the dHvA oscillations are based on the magnetic band structure, which is obtained by numerical diagonalization of the one-electron model Hamiltonian on a square lattice in the magnetic field. At zero magnetic field and for a filling of 0.55 electrons per unit cell, the shape of the Fermi surface (FS) is close to a chessboard in which white and black cells are related to holes and electrons, respectively. The numerical simulations of the dHvA oscillations for this FS clearly display the magnetic breakdown (MB) features in the Fourier spectrum: the combinational and forbidden frequencies, which cannot be explained within the standard Lifshitz-Kosevich theory. Because of the idealized conditions of our numerical simulation (zero temperature, ideal crystal lattice) the only reason for deviation of the dHvA oscillations from the sawtooth magnetization of a 2D electron gas is the fractal magnetic band structure of the AH problem. We have also developed a semiclassical analytical approach to the dHvA oscillations taking account of the MB at the corners of the hole and electron orbits of the FS, the probability of which $0\ensuremath{\le}W(B)=\mathrm{exp}(\ensuremath{-}{B}_{\mathrm{MB}}∕B)\ensuremath{\le}1$ depends on $B$ and the MB field ${B}_{\mathrm{MB}}$. We have shown that the dHvA oscillations in this case are completely determined by the quantization of the closed hole and electron orbits and by special MB factors for these orbits ${I}^{e(h)}(p)$ ($p=1,2,3,\dots{}$, is the harmonic index). The factors ${I}^{e(h)}(p)$ not only strongly modulate the amplitudes of harmonics in the magnetization but act also through the changes in the chemical potential (CP) oscillations. The latter are very important: they control the shape of the magnetization (which can be close either to a direct or an inverted sawtoothlike profile) and add forbidden peaks in the Fourier spectrum. In the case of a fixed CP the forbidden peaks vanish and the magnetization profile is close to the direct sawtooth. As a function of the MB probability $W(B)$, the MB factors ${I}^{e(h)}(p)$ are polynomials of degree $p$ with the following fixed values at the ends: 1 at $W(B)=0$ and 0 at $W(B)=1$. Our analytic expression for the magnetization as a function of magnetic field $B$ agrees pretty well with our numerical simulations using only one fitting parameter---the MB field ${B}_{\mathrm{MB}}$. Small deviations are due to the fractal fine structure of the magnetic bands.