Doping Bi$_2$Se$_3$ by magnetic ions represents an interesting problem since it may break the time reversal symmetry needed to maintain the topological insulator character. Mn dopants in Bi$_2$Se$_3$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the Bi$_2$Se$_3$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T_{\rm{C}}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T_{\rm{C}}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.
We calculate the photoemission spectra of suspended and epitaxial doped graphene using an ab initio cumulant expansion of the Green's function based on the GW self-energy. Our results are compared to experiment and to standard GW calculations. For doped graphene on a silicon carbide substrate, we find, in contrast to earlier calculations, that the spectral function from GW only does not reproduce experimental satellite properties. However, ab initio GW plus cumulant theory combined with an accurate description of the substrate screening results in good agreement with experiment, but gives no p…
The electronic and structural properties of (i) boron doped graphene sheets, and (ii) the chemisorption processes of hydrogen adatoms on the boron doped graphene sheets have been examined by {\it ab initio} total energy calculations.
The chirality associated with broken time reversal symmetry in magnetically doped topological insulators has important implications to the quantum transport phenomena. Here we report the anomalous Hall effect studies in Mn- and Cr-doped Bi$_2$Te$_3$ topological insulators with varied thickness and doping content. By tracing the chirality of the Hall loops, we find that the Mn-type anomalous Hall effect with clockwise chirality is strengthened by the reduction of film thickness, which is opposite to that of the Cr-type anomalous Hall effect with counterclockwise chirality. We provide a phenomen…
We study the magnetic properties of topological insulators doped with isoelectronic magnetic impurities. We obtain that at zero temperature the impurities order ferromagnetically, but when raising the temperature the topological insulator undergoes a first order phase transition to a spin density wave phase before the system reaches the paramagnetic phase. The origin of this phase is the non-trivial dependence of the topological insulator spin susceptibility on the momentum. We analyze the coupling of the non-uniform magnetic phase with the Dirac electronic system that occurs at the surfaces o…
A theory of self-excitation in the metal-insulator-metal structure doped with metal nanowires is developed for the case where the power is provided by an external source of radio waves. Both the transient stage of self-excitation and the steady-state regime of self-oscillation are analyzed in a fully analytical form. The numerical estimates demonstrate that this effect can be used for diverse practical purposes, in particular, for radio frequency power harvesting. These findings extend the approach developed in nano-optics to the field of electrical engineering.
A topological insulator is a new state of matter, possessing gapless spin-locking surface states across the bulk band gap which has created new opportunities from novel electronics to energy conversion. However, the large concentration of bulk residual carriers has been a major challenge for revealing the property of the topological surface state via electron transport measurement. Here we report surface state dominated transport in Sb-doped Bi2Se3 nanoribbons with very low bulk electron concentrations. In the nanoribbons with sub-10nm thickness protected by a ZnO layer, we demonstrate complet…
We present a method to microscopically derive a small-size k$\cdot$p Hamiltonian in a Hilbert space spanned by physically chosen ab initio spinor wave functions. Without imposing any complementary symmetry constraints, our formalism equally treats three- and two-dimensional systems and simultaneously yields the Hamiltonian parameters and the true $\mathbb{Z}_2$ topological invariant. We consider bulk crystals and thin films of Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and Sb$_{2}$Te$_{3}$. It turns out that the effective continuous k$\cdot$p models with open boundary conditions often incorrectly pre…
To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration $n_c$ for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of p…
Following the finite-temperature ab initio calculation framework based on the relativistic disordered local moments, we computationally demonstrate the possibility of doping-enhanced coercivity at high-temperatures, taking YCo$_5$ as a working material in order to extract the $3d$-electron part of the electronic structure of the rare-earth permanent magnets. Alkaline-earth dopants are shown to be the candidates to realize the proposed phenomenon.
The quasiparticle interference (QPI) technique is a powerful tool that allows to uncover the structure and properties of electronic structure of a material combined with scattering properties of defects at surfaces. Recently this technique has been pivotal in proving the unique properties of the surface state of topological insulators which manifests itself in the absence of backscattering. In this work we derive a Green function based formalism for the ab initio computation of Fourier-transformed QPI images. We show the efficiency of our new implementation at the examples of QPI that forms ar…
We report the instrumentation and experimental results of a cryogenic scanning microwave impedance microscope. The microwave probe and the scanning stage are located inside the variable temperature insert of a helium cryostat. Microwave signals in the distance modulation mode are used for monitoring the tip-sample distance and adjusting the phase of the two output channels. The ability to spatially resolve the metal-insulator transition in a doped silicon sample is demonstrated. The data agree with a semi-quantitative finite-element simulation. Effects of the thermal energy and electric fields…
We present the first ab-initio band-theory-based description of spin-compensated polarons (known as Zhang-Rice singlets) in a hole-doped cuprate, specifically one-dimensional Ca_{2+x} Y_{2-x} Cu_5 O_10. Zhang-Rice singlets are many-particle configurations relevant to the exotic behavior of hole-doped cuprates, stemming from spontaneous charge localization. They appear in our case-study material above a threshold doping, successively turning the insulating undoped antiferromagnet into a gap insulator, a singlet-rich metallic paramagnet, and finally, a singlet-saturated insulating diamagnet.
Based on first-principles calculation, it has been predicted that the magnetic anisotropy energy (MAE) in Co-doped ZnO (Co:ZnO) depends on electron-filling. Results show that the charge neutral Co:ZnO presents a "easy plane" magnetic state. While modifying the total number of electrons, the easy axis rotates from in-plane to out-of-plane. The alternation of the MAE is considered to be the change of the ground state of Co ion, resulting from the relocating of electrons on Co d-orbitals with electron-filling.
Using ab-initio calculations, we study the electronic structure of gallium-doped germanium, which was found recently to be a superconductor, with a critical temperature of 0.5 Kelvins, and a particularly low density of Cooper pairs. The calculations of the electronic properties reveal that no sign of an impurity band is observed, and that the Fermi level lies in the valence band of Germanium. Moreover, the calculation of the phonon frequencies shows that a new mode associated to the Ga atom is appearing, around 175$ cm-1.
Because of their unique structure, it has been proposed that carbon nanotube ropes may well provide an ideal container for the storage of molecular hydrogen. Indeed, there has been some experimental evidence of enhanced hydrogen uptake in doped Li and other alkali metal systems [Chen et al, Science 285, 91 (1999)]. We have therefore addressed this issue of hydrogen storage in Li-doped graphite and carbon nanotube systems theoretically with ab initio simulations. Our results find no evidence for such enhanced storage, based on the induced structural changes. In addition, we have also investigat…
The local magnetic moment of Ti:ZnO is calculated from first principles by using the corrected-band-gap scheme (CBGS). The results shows that the system is magnetic with the magnetization of 0.699 $μ_B$ per dopant. The origin of the local magnetic moment is considered to be the impurity band partially occupied by the donor electrons in the conduction band. Further, the impacts of applying Hubbard U to Ti-d orbital on the magnetic moment have been investigated.
SmCo5 permanent magnets exhibit enormous uniaxial magnetocrystalline anisotropy energy and have a high Curie temperature. However, a low energy product presents a significant drawback in the performance of SmCo5 permanent magnets. In order to increase the energy product in SmCo5, we propose substituting fixed amount of cobalt with iron in a new magnet, SmFe3CoNi, where inclusion of nickel metal makes this magnet thermodynamically stable. We further discuss some basic theoretical magnetic properties of the SmCo5 compound.
The way doping affects the spin wave stiffness and the exchange stiffness of permalloy (Py) is investigated via ab-initio calculations, using the Korringa-Kohn-Rostoker (KKR) Green function formalism. By considering various types of dopants of different nature (V, Gd, and Pt), we are able to draw general conclusions. To describe the trends of the stiffness with doping is it sufficient to account for the exchange coupling between nearest neighbors. The polarizability of the impurities is not an important factor for the spin wave stiffness. Rather, the decisive factor is the hybridization betwee…
We present an ab initio study of dopant-dopant interactions in beryllium-doped InGaAs. We consider defect formation energies of various interstitial and substitutional defects and their combinations. We find that all substitutional-substitutional interactions can be neglected. On the other hand, interactions involving an interstitial defect are significant. Specially, interstitial Be is stabilized by about 0.9/1.0 eV in the presence of one/two BeGa substitutionals. Ga interstitial is also substantially stabilized by Be interstitials. Two Be interstitials can form a metastable Be-Be-Ga complex …
We present a novel technique for calculating the dielectric response of metal/insulator heterostructures. This scheme allows, for the first time, the fully first-principles calculation of the microscopic properties of thin-film capacitors at finite bias potential. The method can be readily applied to pure insulators, where it provides an interesting alternative to conventional finite-field techniques based on the Berry-phase formalism. We demonstrate the effectiveness of our method by performing comprehensive numerical tests on a model Ag/MgO/Ag heterostructure.
Transport properties of permalloy doped with V, Co, Pt, and Au are explored via ab-initio calculations. The Kubo-Bastin formula is evaluated within the fully relativistic Korringa-Kohn-Rostoker Green function formalism. Finite temperature effects are treated by means of the alloy analogy model. It is shown that the fact that the host is disordered and not crystalline has a profound effect on how the conductivities characterizing the anomalous Hall effect and the spin Hall effect depend on the dopant concentration. Several relationships between quantities characterizing charge and spin transpor…
We use magneto-conductivity to study the magnetic proximity effect on surface states of doped topological insulators. Our bilayers consist of a layer of Fe$_7$Se$_8$, which is a metallic ferrimagnet and a layer of Bi$_{0.8}$Sb$_{1.2}$Te$_{3}$ which is a highly hole-doped topological insulator. Using transport measurements and a modified Hikami-Larkin-Nagaoka model, we show that the ferromagnet shortens significantly the effective coherence length of the surface states, suggesting that a gap is opened at the Dirac point. We show that the magnetically induced gap persists on surface states which…
We present a microscopic picture of silver dynamics in GeSe$_{3}$:Ag glass obtained from {\it ab initio} simulation. The dynamics of Ag is explored at two temperatures, 300K and 700K. In the relaxed network, Ag occupies bond centers between suitably separated host sites. At 700K, Ag motion proceeds via a trapping-release dynamics, between "super traps" or cages consisting of multiple bond-center sites in a small volume. Our work offers a first principles identification of trapping centers invoked in current theories, with a description of their properties. We compute the charge state of the Ag…
Thermoelectric properties of the system La$_2$NiO$_{4+δ}$ have been studied ab initio. Large Seebeck coefficient values are predicted for the parent compound, and to some extent remain in the hole-doped metallic phase, accompanied of an increase in the conductivity. This system, due to its layered structure would be a suitable candidate for an improvement of its thermoelectric figure of merit by nanostructurization in thin films, that has already been shown to increase the electrical conductivity ($σ$). Our calculations show that in the region around La$_2$NiO$_{4.05}$ the system has a large t…
The electronic and magnetic properties of iron-doped topological insulator Bi1.84-xFe0.16CaxSe3 single crystals were studied. By co-doping Fe and Ca atoms, ferromagnetic bulk states with different carrier density (from n-type to p-type) were obtained. Effective magnetic moments for each Fe atom was estimated as small as about 0.07$μ$B. Magnetic and non-magnetic phases separation was observed in all samples. Our results suggest that the bulk ferromagnetism in Fe-doped Bi2Se3 is not intrinsic and regardless of carrier density.
The differences in energy between electronic bands due to valley splitting are of paramount importance in interpreting transport spectroscopy experiments on state-of-the-art quantum devices defined by scanning tunneling microscope lithography. We develop a plane-wave density functional theory description of these systems which is size-limited due to computational tractability. We then develop a less resource-intensive alternative via localized basis functions, retaining the physics of the plane-wave description, and extend this model beyond the capability of plane-wave methods to determine the…
Magnetic interaction with the gapless surface states in topological insulator (TI) has been predicted to give rise to a few exotic quantum phenomena. However, the effective magnetic doping of TI is still challenging in experiment. Using first-principles calculations, the magnetic doping properties (V, Cr, Mn and Fe) in three strong TIs (Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$) are investigated. We find that for all three TIs the cation-site substitutional doping is most energetically favorable with anion-rich environment as the optimal growth condition. Further our results show…
Heterostructures between topological insulators (TI) and magnetic insulators represent a pathway to realize the quantum anomalous Hall effect (QAHE). Using density functional theory based systematic screening and investigation of thermodynamic, magnetic and topological properties of heterostructures, we demonstrate that forming a type-I heterostructure between a wide gap antiferromagnetic insulator Cr$_2$O$_3$ and a TI-film, such as Sb$_2$Te$_3$, can lead to pinning of the Fermi-level at the center of the gap, even when magnetically doped. Cr-doping in the heterostructure increases the gap to …
We present a study of the structural and electronic properties of highly doped topological insulator Bi2Se3 single crystals synthesized by the Bridgman method. Lattice structural characterizations by X-ray diffraction, scanning tunneling microscopy, and Raman spectroscopy confirmed the high quality of the as-grown single crystals. The topological surface states in the electronic band structure were directly re- vealed by angle-resolved photoemission spectroscopy. Transport measurements showed that the conduction was dominated by the bulk carriers and confirmed a previously observed bulk quantu…
We argue that many properties of the half-doped manganites may be understood in terms of a new two-(eg electron)-fluid description, which is energetically favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a competition between canting of the core spins of Mn promoting mobile carriers and polaronic trapping of carriers by JT defects, in the presence of CE, orbital and charge order. We show that this explains several features of the doping and magnetic field induced insulator-metal transitions, as the particle-hole asymmetry and the smallness of the transition fields.
In this paper we present an approach aimed at performing many-body calculations of Born-effective charges of crystalline insulators, by including the electron-correlation effects. The scheme is implemented entirely in the real space, using Wannier-functions as single-particle orbitals. Correlation effects are computed by including virtual excitations from the Hartree-Fock mean field, and the excitations are organized as per a Bethe-Goldstone-like many-body hierarchy. The results of our calculations suggest that the approach presented here is promising.
Possible high-Tc superconductivity at 45 K in the Ge-doped cluster Mott insulator GaNb4Se8
Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences（中国科学院物理研究所与北京凝聚态物理国家实验室）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）
We study magneto-transport phenomena in two rare-earth doped topological insulators, SmxFexSb2-2xTe3 and SmxBi2-xTe2Se single crystals. The magneto-transport behaviours in both compounds exhibit a systematic crossover between weak anti-localization (positive magnetoresistance) and weak localization (negative magnetoresistance) with changes in temperatures and magnetic fields. The weak localization is caused by rare-earth-doping induced magnetization, and the weak anti-localization originates from topologically protected surface states. The transition between weak localization and weak anti-loc…
We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi2-xCrxSe3, via the proximity effect using a high-TC ferrimagnetic insulator Y3Fe5O12. The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 K to 50 …
Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped Pb$_{1-x}$Sn$_x$Te compounds that manifest huge bulk resistivities together with strong evidence of topological surface states, based on thickness-dependent…
Quantum spin liquid states, in which spins are quantum-mechanically delocalized in direction, have been so far studied for charge-localized Mott insulators arising from strong repulsive interaction. Recently, however, it was found that the doped Mott insulator with a triangular lattice, $κ$-(ET)$_4$Hg$_{2.89}$Br$_8$, exhibits both spin-liquid-like magnetism and metallic electrical conduction. Thus, it is now possible to experimentally explore how the spin liquidity affects the electrical conduction, an issue that has received a great deal of theoretical attention. Here, with a newly developed …
In this paper we explore the different mechanisms that affect the thermopower of a band insulating perovskite (in this case, SrTiO$_3$) when subject to strain (both compressive or tensile). We analyze the high temperature, entropy dominated limit and the lower temperature, energy-transport regime. We observe that the effect of strain in the high-temperature Seebeck coefficient is small at the concentration levels of interest for thermoelectric applications. However, the effective mass changes substantially with strain, which produces an opposite effect to that of the degeneracy-breakups produc…
In this paper, we present ab initio computer models of Cu-doped amorphous Ta2O5 , a promising candidate for Conducting Bridge Random Access Memory (CBRAM) memory devices, and study the structural, electronic, charge transport and vibrational properties based on plane-wave density functional methods. We offer an atomistic picture of the process of phase segregation/separation between Cu and Ta2O5 subnetworks. Electronic calculations show that the models are conducting with extended Kohn-Sham orbitals around the Fermi level. In addition to that, we also characterize the electronic transport usin…
Local density calculations of the electronic structure of FeSi, FeSi_{1-x}Al_x and Fe_{1-x}Ir_xSi systems in the B20 structure are presented. Pure FeSi has a semi-conducting gap of 6 mRy at 0 K. Effects of temperature (T) in terms of electronic and vibrational excitations are included. Various measurable properties, such as magnetic susceptibility chi(T), electronic specific heat C(T), thermoelectric power S(T), relative variations in resistivity rho(T), and peak positions in the density-of-states (DOS) are calculated. The feedback from vibrational disorder onto the electronic structure is fou…
Ab-initio density functional theory (DFT) calculations of the relative stability of anatase and rutile polymorphs of TiO2 were carried using all-electron atomic orbitals methods with local density approximation (LDA). The rutile phase exhibited a moderate margin of stability of ~ 3 meV relative to the anatase phase in pristine material. From computational analysis of the formation energies of Si, Al, Fe and F dopants of various charge states across different Fermi level energies in anatase and in rutile, it was found that the cationic dopants are most stable in Ti substitutional lattice positi…
We report a numerical analysis of Anderson localization in a model of a doped semiconductor. The model incorporates the disorder arising from the random spatial distribution of the donor impurities and takes account of the electron-electron interactions between the carriers using density functional theory in the local density approximation. Preliminary results suggest that the model exhibits a metal-insulator transition.
In this work we perform a detailed first-principles analysis of the electronic and optical properties of NiBr$_2$ within the state-of-the-art $GW$+BSE scheme to determine whether this system displays negative excitonic energies, which would identify it as an (half) excitonic insulator. Particular attention is payed to the convergence of the $GW$ band structure and to the consistency between approximations employed in the ground-state calculations and approximations employed in the linear response calculations. We show that these two issues play a crucial role in identifying the excitonic natur…
We propose that a specific spatial configuration of lattice sites that energetically favor {\it 3+} or {\it 4+} Mn ions in moderately doped manganites constitutes approximately a spatially random two-energy-level system. Such an effect results in a mechanism of metal-insulator transitions that appears to be different from both the Anderson transition and the Mott-Hubbard transition. Correspondingly, a disordered Kondo lattice model is put forward, whose dynamical mean-field solution agrees reasonably with experiments.
Heterointerfaces in complex oxide systems open new arenas in which to test models of strongly correlated material, explore the role of dimensionality in metal-insulator-transitions (MITs) and small polaron formation. Close to the quantum critical point Mott MITs depend on band filling controlled by random disordered substitutional doping. Delta-doped Mott insulators are potentially free of random disorder and introduce a new arena in which to explore the effect of electron correlations and dimensionality. Epitaxial films of the prototypical Mott insulator GdTiO3 are delta-doped by substituting…
We present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide band gap spinel ZnGa$_2$O$_4$. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined.
Metal-ion doping can effectively regulate the metal-insulator transition temperature in $\mathrm{VO}_2$. Experiments found that the pentavalent and hexavalent ion doping dramatically reduces the transition temperature while the trivalent ion doping increases the transition temperature and induces intermediate phases. Based on the phase-field model of the metal-insulator transition in $\mathrm{VO}_2$ we developed previously, we formulate a Landau potential of the metal-ion-doped $\mathrm{VO}_2$ taking account of the effects of doping on the electron correlation and lattice structure. The effect…
Polar metals are rare because free carriers in metals screen electrostatic potential and eliminate internal dipoles. Degenerate doped ferroelectrics may create an approximate polar metallic phase. We use first-principles calculations to investigate $n$-doped LiNbO$_3$-type oxides (LiNbO$_3$ as the prototype) and compare to widely studied perovskite oxides (BaTiO$_3$ as the prototype). In the rigid-band approximation, substantial polar displacements in $n$-doped LiNbO$_3$ persist even at 0.3 $e$/f.u. ($\simeq$ 10$^{21}$ cm$^{-3}$), while polar displacements in $n$-doped BaTiO$_3$ quickly get su…
The structural, magnetotransport, and angle-resolved photoemission spectroscopy (ARPES) of Ag-doped Bi2Se3 single crystals are presented. Temperature dependent resistivity exhibits metallic behavior with a slope change above 200 K for Ag-doped Bi2Se3. The magnetoresistance shows positive quadratic dependence at low fields satisfying Kohler's rule. Hall resistivity measurement shows that electrons are dominant charge carriers. Furthermore, these results agree well with the ARPES spectra observed at T = 20 K, where the Fermi level lies inside the bulk conduction band. The Dirac point of the topo…
Thermoelectric properties of the system La$_2$NiO$_{4+δ}$ have been recently discussed [Phys. Rev. B 86, 165114 (2012)] via ab initio calculations. An optimum hole-doping value was obtained with reasonable thermopower and thermoelectric figure of merit being calculated. Here, a large increase in the thermoelectric performance through lattice strain and the corresponding atomic relaxations is predicted. This increase would be experimentally attainable via growth in thin films of the material on top of different substrates. A small tensile strain would produce large thermoelectric figures of mer…
We have investigated possible spin and charge ordered states in 3d transition-metal oxides with small or negative charge-transfer energy, which can be regarded as self-doped Mott insulators, using Hartree-Fock calculations on d-p-type lattice models. It was found that an antiferromagnetic state with charge ordering in oxygen 2p orbitals is favored for relatively large charge-transfer energy and may be relevant for PrNiO$_3$ and NdNiO$_3$. On the other hand, an antiferromagnetic state with charge ordering in transition-metal 3$d$ orbitals tends to be stable for highly negative charge-transfer e…
We study how the Mott metal-insulator transition (MIT) is affected when we have to deal with electrons with different angular momentum quantum numbers. For that purpose we apply ab-initio quantum-chemical methods to lithium rings in order to investigate the analogue of a MIT. By changing the interatomic distance we analyse the character of the many-body wavefunction and discuss the importance of the $s-p$ orbital quasi-degeneracy within the metallic regime. The charge gap (ionization potential minus electron affinity) shows a minimum and the static electric dipole polarizability has a pronounc…
In this paper we present an approach aimed at calculating the optical dielectric constant of crystalline insulators both at the Hartree-Fock, and correlated levels. Our scheme employs a real-space methodology, employing Wannier functions as the basic building blocks. The scheme has been applied to compute the optical dielectric constants of LiF, LiCl, MgO, and Li2O. In all the cases, results of correlated calculations, based upon systematic many-body corrections beyond Hartree-Fock, exhibit excellent agreement with the experiments. To the best of our knowledge, the calculations presented here …
We estimate the contribution of the long-wavelength el-TO-ph interaction and discuss the effect it has on electron pairing in doped polar systems like SrTiO3 and PbTe. The analytical and numerical results presented in the study indicate that the el-TO-ph interaction tends to contribute little to the total strength of electron-phonon coupling in these and related materials. To explain this fact we consider possible reasons why the effect of the polar long-wavelength transverse optical phonons on the coupling constant "lambda" is far less than one might suppose.
We show that metal-insulator transition in Pr0.5Ca0.5Mn1-xCrxO3 (x = 0.015-0.025) is unstable against thermal cycling. Insulator-metal transition shifts down and low temperature resistivity increases each time when the sample is cycled between a starting temperature TS and a final temperature TF. The effect is dramatic lower is x. Insulator-metal transition in x = 0.015 can be completely destroyed by thermal cycling in absence of magnetic field as well as under H = 2 T. Magnetic measurements suggest that ferromagnetic phase fraction decreases with thermal cycling. We suggest that increase in s…
Magnetic insulators are often antiferromagnetic (AFM) and layered AFM compounds usually show low ordering temperature. On the other hand, layered ferromagnetic (FM) insulators with high-TC are very rare although they could be quite useful for spintronic applications. Here, using crystal field level analysis in combination with density functional theory calculations as well as Monte Carlo simulations, we predict that the layered insulator Sr2NiRuO4 would have a strong FM coupling with TC as high as 240 K. The tetragonal crystal field in the Ni-O-Ru square plane stabilizes the S=1/2 Ni+ and S=3/…
A heterojunction Mott field effect transistor (FET) is proposed that consists of an epitaxial channel material that exhibits an electron-correlation-induced Mott metal-to-insulator transition. The Mott material is remotely (modulation) doped with a degenerately doped conventional band insulator. An applied voltage modulates the electron transfer from the doped band insulator to the Mott material and produces transistor action by inducing an insulator-to-metal transition. Materials parameters from rare-earth nickelates and SrTiO3 are used to assess the potential of the "modulation-doped Mott FE…
