|Home Open Positions
|| Research Topics
||Contact Me NEWS|
|70+ publications and a book chapter
2200+ citations; H-index is 28; i10-index is 50 (GoogleScholar)
|Combining Optical Transparency with Electrical Conductivity: Challenges and Prospects (pages 1–29)|
Julia E. Medvedeva
in Transparent Electronics: From Synthesis to Applications Editors: Antonio Facchetti and Tobin J. Marks
|Probing the Unique Role of Gallium in Amorphous Oxide
Semiconductors through Structure–Property Relationships
S.L. Moffitt, Q. Zhu, Q. Ma, A.F. Falduto, D.B. Buchholz, R.P.H. Chang, T.O. Mason, J.E. Medvedeva, T.J. Marks, and M.J. Bedzyk
Advanced Electronic Materials, 1700189 (2017)
This study explores the unique role of Ga in amorphous (a-) In-Ga-O oxide semiconductors through combined theory and experiment. It reveals substitutional effects that have not previously been attributed to Ga, and that are investigated by examining how Ga influences structure–property relationships in a series of pulsed laser deposited a-In-Ga-O thin films. Element-specific structural studies (X-ray absorption and anomalous scattering) show good agreement with the results of ab initio molecular dynamics simulations. This structural knowledge is used to understand the results of air-annealing and Hall effect electrical measurements. The crystallization temperature of a-IO is shown to increase by as much as 325 °C on substituting Ga for In. This increased thermal stability is understood on the basis of the large changes in local structure that Ga undergoes, as compared to In, during crystallization. Hall measurements reveal an initial sharp drop in both carrier concentration and mobility with increasing Ga incorporation, which moderates at >20 at% Ga content. This decline in both the carrier concentration and mobility with increasing Ga is attributed to dilution of the charge-carrying In-O matrix and to increased structural disorder. The latter effect saturates at high at% Ga.
|Recent advances in understanding the structure and properties of amorphous oxide semiconductors (Review)
J.E. Medvedeva, D.B. Buchholz, R.P.H. Chang
Advanced Electronic Materials, 3, 1700082 (2017)
Amorphous oxide semiconductors (AOSs) – ternary or quaternary oxides of post-transition metals such as In-Sn-O, Zn-Sn-O, or In-Ga-Zn-O – have been known for a decade and have attracted a lot of attention due to several technological advantages including low-temperature large-area deposition, mechanical flexibility, smooth surfaces, as well as high carrier mobility which is an order of magnitude larger than that of amorphous silicon (a-Si:H). Compared to their crystalline counterparts, the structure of AOSs is extremely sensitive to deposition conditions, stoichiometry, and composition, giving rise to a wide range of tunable optical and electrical properties. The large parameter space and the resulting complex deposition-structure-property relationships in AOSs, make the currently available theoretical and experimental research data rather scattered and the design of new materials difficult. In this work, the key properties of several In-based AOSs are studied as a function of cooling rates, oxygen stoichiometry, cation composition, or lattice strain. Based on a thorough comparison of the results of ab-initio modeling, comprehensive structural analysis, accurate property calculations, and systematic experimental measurements, a four-dimensional parameter space for AOSs is derived, serving as a solid foundation for property optimization in known AOSs and for design of next-generation transparent amorphous semiconductors.
PDF Figure 4 update
NiSe2 Film: Bifunctional Electrocatalyst for Full Water Splitting at
Remarkably Low Overpotential with High Energy Efficiency
A.T. Swesi, J. Masud, W.P.R. Liyanage, S. Umapathi, E. Bohannan, J. Medvedeva, M. Nath
Scientific Reports, 7, 2401 (2017)
Herein we have shown that electrodeposited NiSe2 can be used as a bifunctional electrocatalyst under alkaline conditions to split water at very low potential by catalyzing both oxygen evolution and hydrogen evolution reactions at anode and cathode, respectively, achieving a very high electrolysis energy efficiency exceeding 80% at considerably high current densities (100 mA cm−2). The OER catalytic activity as well as electrolysis energy efficiency surpasses any previously reported OER electrocatalyst in alkaline medium and energy efficiency of an electrolyzer using state-of-the-art Pt and RuO2 as the HER and OER catalyst, respectively. Through detailed electrochemical and structural characterization, we have shown that the enhanced catalytic activity is attributed to directional growth of the electrodeposited film that exposes a Ni-rich lattice plane as the terminating plane, as well as increased covalency of the selenide lattice which decreases the Ni(II) to Ni(III) oxidation potential. Thereby, the high efficiency along with extended stability makes NiSe2 as the most efficient water electrolyzer known to-date.
|New insights into the structure, chemistry, and properties of Cu4SnS4
Amitava Choudhury, Sudip Mohapatra, Hooman Yaghoobnejad Asl, Seng Huat Lee, Yew San Hor, Julia Medvedeva, Devon L McClane, Gregory E Hilmas, Michael A McGuire, Andrew F May, Hsin Wang, Shreeram Dash, Aaron Welton, Punit Boolchand, Kasey P Devlin, Jennifer Aitken, Regine Herbst-Irmer, Váčlav Petříček
Journal of Solid State Chemistry, 253, 192–201 (2017)
The ambient temperature structure of Cu4SnS4 has been revisited and the recently reported low temperature structure has been confirmed from single-crystal X-ray diffraction data. A structural phase transition from a large monoclinic unit cell at low temperature to a smaller orthorhombic unit cell at high temperature has been observed. The room temperature phase exhibited disorder in the two copper sites, which is a different finding from earlier reports. The low temperature monoclinic form crystallizes in P21/c space group, which is isostructural with Cu4GeS4. The phase transition has also been studied with variable temperature powder X-ray diffraction and 119Sn Mössbauer spectroscopy. The Seebeck coefficients and electrical resistivity of polycrystalline Cu4SnS4 are reported from 16 to 400 K on hot pressed pellets. Thermal conductivity measurements at high temperatures, 350 – 750 K exhibited very low thermal conductivities in the range 0.28 – 0.35 W K–1 m–1. In all the transport measurements the phase transition has been observed at around 232 K. Resistivity decreases, while Seebeck coefficient increases after the phase transition during warming up from low to high temperatures. This change in resistivity has been correlated with the results of first-principles electronic band structure calculations using highly-accurate screened-exchange local density approximation. It was found that both the low hole effective mass of 0.63 me for the Γ→Y crystallographic direction and small band gap, 0.49 eV, are likely to contribute to the observed higher conductivity of the orthorhombic phase. Cu4SnS4 is also electrochemically active and shows reversible reaction with lithium between 1.7 and 3.5 volts.
|Thermal Conductivity Comparison of Indium Gallium Zinc Oxide Thin
Films: Dependence on Temperature, Crystallinity, and Porosity
B. Cui, L. Zeng, D. Keane, M.J. Bedzyk, D.B. Buchholz, R.P.H. Chang, X. Yu, J. Smith, T.J. Marks, Y. Xia, A.F. Facchetti, J.E. Medvedeva, and M. Grayson
J. Phys. Chem. C 120, 7467−7475 (2016)
The cross-plane thermal conductivity of InGaZnO (IGZO) thin films was measured using the 3ω technique from 18 to 300 K. The studied morphologies
include amorphous (a-IGZO), semicrystalline (semi-c-IGZO), and c-axis-aligned single-crystal-like IGZO (c-IGZO) grown by pulsed laser deposition (PLD) as well as a-IGZO deposited by sputtering and by solution combustion processing. The atomic structures of the amorphous and crystalline films were simulated with ab initio molecular dynamics. The film quality and texturing information was assessed by X-ray diffraction and grazing incidence wide-angle X-ray scattering. X-ray reflectivity was also conducted to quantify film densities and porosities. All the high-density films exhibit an empirical powerlaw temperature dependence of the thermal conductivity κ ∼ T0.6 in the specified temperature range. Among the PLD dense films, semi-c-IGZO exhibits the highest thermal conductivity, remarkably exceeding both films with more order (c-IGZO) and with less order (a-IGZO) by a factor of 4. The less dense combustion-synthesized films, on the other hand, exhibited lower thermal conductivity, quantitatively consistent with a porous film using either an effective medium or percolation model. All samples are consistent with the porosity-adapted Cahill−Pohl (p-CP) model of minimum thermal conductivity.
|Selective Crystal Growth and Structural, Optical, and Electronic Studies of Mn3Ta2O8
K. Rickert, E.A. Pozzi, R. Khanal, M. Onoue, G. Trimarchi, J.E. Medvedeva, M.C. Hersam, R.P. Van Duyne, K.R. Poeppelmeier,
Inorganic Chemistry, 54, 6513-6519 (2015)
Mn3Ta2O8, a stable targeted material with an unusual and complex cation topology in the complicated Mn−Ta−O phase space, has been grown as a ≈3-cm-long single crystal via the optical floating-zone technique.
Single-crystal absorbance studies determine the band gap as 1.89 eV, which
agrees with the value obtained from density functional theory electronic band structure calculations. The valence band consists of the hybridized Mn d−O p
states, whereas the bottom of the conduction band is formed by the Ta d states. Furthermore, out of the three crystallographically distinct Mn atoms that are four-, seven-, or eight-coordinate, only the former two contribute their states
near the top of the valence band and hence govern the electronic transitions
across the band gap. PDF
|Optical Conductivity of Mayenite: From Insulator to Metal
R. Lobo, N. Bontemps, M.I. Bertoni, T.O. Mason, K. Poeppelmeier, A.J. Freeman, M.S. Park, and J.E. Medvedeva,
Journal of Physical Chemistry C, 119, 8849–8856 (2015)
Mayenite-based oxides [12CaO·7Al2O3] starting from [Ca24Al28O64]4+ + 2O2− (insulator) and subsequently annealed so as to obtain [Ca24Al28O64]4+ + 4e− (metal) were studied by reflectance and transmission on seven samples with dc conductivities spanning the range 10−10 to 1500 Ω−1 cm−1. As the samples become increasingly metallic, we observe an enhancement of spectral weight in the visible range. Simultaneously, the reflectance and the resulting optical conductivity develop a stronger component in the infrared, characteristic of mobile electrons. This electronic response appears to be strongly coupled to the phonons, as shown by their Fano profiles present in the metallic samples. Our results indicate that free carriers promote the formation of polarons and that these two excitations contribute to the dc conducting properties of mayenites. This overall electronic picture is consistent with first principles calculations of the density of states in mayenites. The observed strong electron−phonon coupling may be of interest in view of the superconductivity properties found in the metallic samples.
size effects on the electronic and structural properties of solution-processed In-X-O thin films
J. Smith, L. Zeng, R. Khanal, K. Stallings, A.F. Facchetti, J.E. Medvedeva, M.J. Bedzyk, and T.J. Marks
Advanced Electronic Materials, 1(7) 1500146 (2015)
The nature of charge transport and local structure are investigated in amorphous indium oxide-based thin films fabricated by spin-coating and modeled by ab-initio molecular dynamics liquid-quench simulations. The In-X-O series where X = Sc, Y, or La is investigated to understand the effects of varying both the X cation ionic radius (0.89 to 1.17 Å) and the film processing temperature (250 to 300˚C).
structural and transport properties of amorphous transparent conducting
R. Khanal, D.B. Buchholz, R.P.H. Chang, and J.E. Medvedeva,
Physical Review B, 91, 205203 (2015)
Based on the obtained interconnectivity and spatial distribution of the InO6 and XOx polyhedra in amorphous In-X-O, composition-dependent structural models of amorphous In-X-O oxides with X=Sn, Zn, Ga, or Ge, are derived based on ab-initio molecular dynamics liquid-quench simulations. The results help explain Hall mobility measurements in In−X−O thin films grown by pulsed-laser deposition and highlight the importance of long-range structural correlations in the formation of amorphous oxides and their transport properties.
It is found that indium retains its average coordination of 5.0 upon 20% X fractional substitution for In, whereasX cations satisfy their natural coordination with oxygen atoms. This finding suggests that the carrier generation is primarily governed by In atoms, in accord with the observed carrier concentration in amorphous In-O and In-X-O. At the same time, the presence of X affects the number of six-coordinated In atoms as well as the oxygen sharing between the InO6 polyhedra.
structural correlations in amorphous ternary In-based oxides
J.E. Medvedeva and R. Khanal,
Vacuum, Special issue on Oxide Coatings, 114, 142-149 (2015)
Systematic investigations of ternary In-based amorphous oxides, In-X-O with X = Sn, Zn, Ga, Cd, Ge, Sc, Y, or La, are performed using ab-initio molecular dynamics liquid-quench simulations. The results reveal that the local MeO structure remains nearly intact upon crystalline to amorphous transition and exhibit weak dependence on the composition. In marked contrast, the structural characteristics of the metal-metal shell, namely, the M-M distances and M-O-M angles that determine how MO polyhedra are connected into a network, are affected by the presence of X. Complex interplay between several factors such as the cation ionic size, metaleoxygen bond strength, as well as the natural preference for edge, corner, or face-sharing between the MO polyhedra, leads to a correlated behavior in the long-range structure. These findings highlight the mechanisms of the amorphous structure formation as well as the specifics of the carrier transport in these oxides.
Structure and Properties of Amorphous Indium Oxide
D. Buchholz, Q. Ma, D. Alducin, A. Ponce, M. Jose-Yacaman, R. Khanal, J.E. Medvedeva, R.P.H. Chang,
Chemistry of Materials, 26, 5401-5411 (2014)
The amorphous-tocrystalline transition and the structure of amorphous In2O3 were investigated by grazing angle X-ray diffraction (GIXRD), Hall transport measurement, high resolution transmission electron microscopy (HRTEM), electron diffraction, extended X-ray absorption fine structure (EXAFS), and ab initio molecular dynamics (MD) liquid-quench simulation. On the basis of excellent agreement between the EXAFS and MD results, a model of the amorphous oxide structure as a network of InOx polyhedra was constructed. Mechanisms for the transport properties observed in the crystalline, amorphous-to-crystalline, and amorphous deposition regions are presented, highlighting a unique structure−property relationship.
PDF Slide1 Slide2
initio simulation of alloying effect on stacking fault energy in fcc Fe
K.R. Limmer, J.E. Medvedeva, D.C. Van Aken, and N.I. Medvedeva
Computational Material Science, 99, 253-255 (2015)
The effect of 3d and 4d transition metal (TM) additions on the intrinsic stacking fault energy (SFE) in fcc Fe is studied to elucidate the role of alloying in the deformation mechanisms in austenitic steels. The results of ab initio calculations reveal that only Mn reduces the SFE, stabilizing the local hcp structure, whereas all other d-additions are expected to decrease the hcp-fcc transformation temperature and restrain the e-martensite formation. We predict a parabolic dependence of SFE on the atomic number of d-element across the series, with the largest increase in SFE obtained for the early and late elements in the d-series that follow the difference in the valence electrons between the TM and Fe atoms. To understand the SFE behavior in fcc Fe alloys, the driving forces for the fcc to hcp phase transformations of transition metal X and solid solution Fe–X were considered with an ab initio approach. It is found that the solution model explains the SFE trends for all TM additions except the late TMs with fully occupied d-shells (Cu and Ag). PDF
Generation in Multicomponent Wide-Bandgap Oxides: InGaZnO4
A. Murat, A. Adler, T.O. Mason, and J.E. Medvedeva
Journal of the American Chemical Society, 135, 5685-5692 (2013)
In this work, thorough theoretical and experimental investigations are combined in
order to explain the carrier generation and transport in crystalline InGaZnO4. Using first-principles density functional approach, we calculate the formation energies and transition levels of possible acceptor and donor point defects as well as the implied defect complexes in InGaZnO4 and determine the equilibrium defect and electron densities as a function of growth temperature and oxygen partial pressure. An excellent agreement of the theoretical results with our Brouwer analysis of the bulk electrical measurements for InGaZnO4 establishes the Ga antisite defect, GaZn, as the major electron donor in InGaZnO4. Moreover, we show that the oxygen vacancies, long believed to be the carrier source in this oxide, are scarce. The proposed carrier generation mechanism also explains the observed intriguing behavior of the conductivity in In-rich vs Ga-rich InGaZnO4.
oxygen vacancy formation in multicomponent wide-band-gap oxides
A. Murat, and J. E. Medvedeva
Physical Review B, 86, 085123 (2012)
The formation and distribution of oxygen vacancy defects in layered multicomponent InAMO4 oxides with A3+ = Al or Ga and M2+ = Ca or Zn and in the corresponding binary oxide constituents is investigated using first-principles density functional calculations. Based on the results obtained, we derive general rules on the role of chemical composition, local coordination, and atomic relaxation in the defect formation and propose an alternative light-metal oxide as a promising constituent of multicomponent functional materials with tunable properties.
properties of layered multicomponent wide-bandgap oxides: a
A. Murat, and J. E. Medvedeva
Physical Review B, 85, 155101 (2012)
The structural, electronic, and optical properties of 12 multicomponent oxides with layered structure RAMO4, where R3+ =In or Sc,A3+ =Al or Ga, andM2+ =Ca,Cd, Mg, or Zn, are investigated using first-principles density functional approach. The compositional complexity of RAMO4 leads to a wide range of band-gap values varying from 2.45 eV for InGaCdO4 to 6.29 eV for ScAlMgO4 as obtained from our self-consistent screened-exchange local density approximation calculations. Strikingly, despite the different band gaps in the oxide constituents, namely, 2–4 eV in CdO, In2O3, or ZnO, 5–6 eV for Ga2O3 or Sc2O3, and 7–9 eV in CaO, MgO, or Al2O3, the bottom of the conduction band in the multicomponent oxides is formed from the s states of all cations and their neighboring oxygen p states. We show that the hybrid nature of the conduction band in multicomponent oxides
originates from the unusual fivefold atomic coordination of A3+ and M2+ cations, which enables the interaction between the spatially spread s orbitals of adjacent cations via shared oxygen atoms. The effect of the local atomic coordination on the band gap, the electron effective mass, the orbital composition of the conduction band, and the expected (an)isotropic character of the electron transport in layered RAMO4 is thoroughly discussed. PDF
the properties of complex transparent conducting oxides: Role of
crystal symmetry, chemical composition, and carrier generation
J.E. Medvedeva, and C.L. Hettiarachchi
Physical Review B, 81, 125116 (2010)
The electronic properties of single- and multi-cation transparent conducting oxides (TCOs) are investigated using first-principles density functional approach. A detailed comparison of the electronic band structure of stoichiometric and oxygen deficient In2O3, alpha- and beta-Ga2O3, rock salt and wurtzite ZnO, and layered InGaZnO4 reveals the role of the following factors which govern the transport and optical properties of these TCO materials: (i) the crystal symmetry of the oxides, including both the oxygen coordination and the long-range structural anisotropy; (ii) the electronic configuration of the cation(s), specifically, the type of orbital(s) -- s, p, or d -- which form the conduction band; and (iii) the strength of the hybridization between the cation's states and the p-states of the neighboring oxygen atoms.
|Averaging of the electron effective mass in multicomponent transparent conducting oxides
Europhysics Letters, 78, 57004-6 (2007)
layered materials composed of various oxides of cations with s2 electronic
configuration, XY2O4, X =In or Sc, Y =Ga, Zn, Al, Cd and/or Mg, exhibit isotropic electron effective mass which can be obtained via averaging over those of the corresponding single-cation oxide constituents. This effect is due to a hybrid nature of the conduction band formed from the s-states of all cations and the oxygen p-states. Moreover, the observed insensitivity of the electron effective mass to the oxygen coordination and to the distortions in the cation-oxygen chains suggests that a similar behavior can be expected in the technologically important amorphous state. These findings significantly broaden the range of materials as efficient transparent conductor hosts. PDF
the properties of transparent oxide conductors. Dopant ion size and
electronic structure effects on CdO-based transparent conducting
oxides. Ga- and In-doped CdO thin films grown by MOCVD
S. Jin, Y.Yang, J.E. Medvedeva, L. Wang, S. Li, N. Cortes, J.R. Ireland, A.W. Metz, J. Ni, M.C. Hersam, A.J. Freeman, T.J. Marks
Chemistry of Materials, 20, 220-230 (2008)
A combined experimental and theoretical/band structure investigation is reported of Ga-doped CdO (CGO) and In-doped CdO (CIO) thin films grown on both amorphous glass and single-crystal MgO(100) substrates at 410 °C by metal–organic chemical vapor deposition (MOCVD). Film phase structure, microstructure, and electrical and optical properties are systematically investigated as a function of doping stoichiometry and growth conditions. Together, the experimental and theoretical results reveal that dopant ionic radius and electronic configuration have a significant influence on the CdO based TCO structural, electronic, and optical properties: (1) lattice parameters contract as a function of dopant ionic radius in the order Y (1.09 Å) < In (0.94 Å) < Sc (0.89 Å), Ga (0.76 Å), with the smallest radius ion among the four dopants only shrinking the lattice marginally and exhibiting low doping efficiency; (2) carrier mobilities and doping efficiencies decrease in the order In > Y > Sc > Ga; (3) the Sc and Y dopant d states have substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics. PDF
|Unconventional approaches to combine optical transparency with electrical conductivity
Applied Physics A, 89, 43-47 (2007)
The combination of electrical conductivity and optical transparency in the same material - known to be a prerogative of only a few oxides of post-transition metals, such as In, Sn, Zn and Cd - manifests itself in a distinctive band structure of the transparent conductor host. While the oxides
of other elements with s2 electronic configuration, for example, Mg, Ca, Sc and Al, also exhibit the desired optical and electronic features, they have not been considered as candidates for achieving good electrical conductivity because of the challenges of efficient carrier generation in these wide-bandgap materials.
Here we demonstrate that alternative approaches to the problem not only allow for attaining the transport and optical properties which compete with those in currently utilized transparent conducting oxides (TCO), but also significantly broaden the range of materials with a potential of being developed into novel functional transparent conductors.
|Magnetically Mediated Transparent Conductors: In2O3 doped with Molybdenum
Physical Review Letters, 97, 086401 (2006)
First-principles band structure investigations of the electronic, optical, and magnetic properties of Mo-doped In2O3 reveal the vital role of magnetic interactions in determining both the electrical conductivity and the Burstein-Moss shift which governs optical absorption. We demonstrate the advantages of the transition metal doping which results in smaller effective mass, larger fundamental band gap, and better overall optical transmission in the visible as compared to commercial Sn-doped In2O3. Similar behavior is expected upon doping with other transition metals opening up an avenue for the family of efficient transparent conductors mediated by magnetic interactions.
|Role of Embedded Clustering in Dilute Magnetic Semiconductors: Cr doped GaN
X.Y. Cui, J.E. Medvedeva, B. Delley, A.J. Freeman, N. Newman, C. Stampfl
Physical Review Letters, 95, 256404 (2005)
Results of extensive density-functional studies provide direct evidence that Cr atoms in Cr:GaN have a strong tendency to form embedded clusters, occupying Ga sites. Significantly, for larger than 2-Cr-atom clusters, states containing antiferromagnetic coupling with net spin in the range 0:06–1:47 B=Cr are
favored. We propose a picture where various configurations coexist and the statistical distribution and associated magnetism will depend sensitively on the growth details. Such a view may elucidate many puzzling observations related to the structural and magnetic properties of III-N and other dilute semiconductors.
as the Archetypical Transparent Conducting Oxide. Systematics of Dopant
Ionic Radius and Electronic Structure Effects on Charge Transport and
Y. Yang, S. Jin, J.E. Medvedeva, J.R. Ireland, A.W. Metz, J. Ni, M.C. Hersam, A.J. Freeman, T.J. Marks
Journal of the American Chemical Society, 127, 8796 (2005)
A series of yttrium-doped CdO (CYO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 410 °C by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties have been investigated. electronic band structure calculations are carried out to systematically compare the structural, electronic, and optical properties of the In-, Sc-, and Y-doped CdO systems. Both experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: (1) lattice parameters contract as a function of dopant ionic radii in the order Y (1.09 Å) < In (0.94 Å) < Sc (0.89 Å); (2) the carrier mobilities and doping efficiencies decrease in the order In > Y > Sc; (3) the dopant d state has substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics. PDF
|Tunable conductivity and conduction mechanism in an ultraviolet light activated electronic conductor
M.I. Bertoni, T.O. Mason, J.E. Medvedeva, A.J. Freeman, K.R. Poeppelmeier, B. Delley
Journal of Applied Physics, 97, 103713 (2005)
A tunable conductivity has been achieved by controllable substitution of an ultraviolet light activated electronic conductor. The transparent conducting oxide system H-doped Ca12−xMgxAl14O33 (x=0, 0.1, 0.3, 0.5, 0.8, 1.0) presents a conductivity that is strongly dependent on the substitution level and temperature. Four-point dc-conductivity decreases with x from 0.26 S/cm (x=0) to 0.106 S/cm (x=1) at room temperature. At each composition the conductivity increases (reversibly with temperature) until a decomposition temperature is reached; above this value, the conductivity drops dramatically due to hydrogen recombination and loss. The observed conductivity behavior is consistent with the predictions of our first principles density functional calculations for the Mg-substituted system with x=0, 1, and 2. The Seebeck coefficient is essentially composition and temperature independent, the later suggesting the existence of an activated mobility associated with small polaron conduction. The optical gap measured remains constant near 2.6 eV while transparency increases with the substitution level, concomitant with a decrease in carrier
|Half-metallicity and efficient spin injection in AlN/GaN:Cr (0001) heterostructure
J.E. Medvedeva, A.J. Freeman, X.Y. Cui, C. Stampfl, N. Newman
Physical Review Letters, 94, 146602 (2005)
First-principles investigations of the structural, electronic, and magnetic properties of Cr-doped AlN/GaN (0001) heterostructures reveal the possibility of efficient spin injection from a ferromagnetic GaN:Cr electrode through an AlN tunnel barrier. We demonstrate that Cr atoms segregate into the GaN
region and that these interfaces retain their half-metallic behavior leading to a complete, i.e., 100%, spin polarization of the conduction electrons. This property makes the wide band-gap nitrides doped with Cr to be excellent candidates for high-efficiency magnetoelectronic devices. PDF
|Combining high conductivity with complete optical transparency: A band-structure approach
J.E. Medvedeva, A.J. Freeman
Europhysics Letters, 69, 583 (2005)
A comparison of the structural, optical and electronic properties of the recently
discovered transparent conducting oxide (TCO), nanoporous Ca12Al14O33, with those of the conventional TCOs (such as Sc-doped CdO) indicates that this material belongs conceptually to a new class of transparent conductors. For this class of materials, we formulate criteria for the successful combination of high electrical conductivity with complete transparency in the visible range and emphasize the significant correlation between their structural characteristics and electronic and optical properties. Our analysis suggests that this set of requirements can be met for a group of novel materials called electrides which may have desired features such as connected structural cavities, large bandgaps and near-metallic electronic conductivity.
|Hopping versus bulk conductivity in transparent oxides: 12CaO.7Al2O3
J.E. Medvedeva, A.J. Freeman
Applied Physical Letters, 85, 955 (2004)
First-principles calculations of the mayenite-based oxide, [Ca12Al14O32] 2+(2e−), reveal the mechanism responsible for its high conductivity. A detailed comparison of the electronic and optical properties of this material with those of the recently discovered transparent conducting oxide, H-doped UV-activated Ca12Al14O33, allowed us to conclude that the enhanced conductivity in
[Ca12Al14O32]2+(2e−) is achieved by elimination of the Coulomb blockade of the charge carriers. This results in a transition from variable range-hopping behavior with a Coulomb gap in H-doped UV-irradiated Ca12Al14O33, to bulk conductivity in [Ca12Al14O32]2+(2e−). Further, the high degree of delocalization of the conduction electrons obtained in [Ca12Al14O32]2+(2e−) indicates that it cannot be classified as an electride, as originally suggested.
|Electronic structure and light-induced conductivity in a transparent refractory oxide
J.E. Medvedeva, A.J. Freeman, M.I. Bertoni, T.O. Mason
Physical Review Letters, 93, 016408 (2004)
Combined first-principles and experimental investigations reveal the underlying mechanism responsible for a drastic change of the conductivity (by 10 orders of magnitude) following hydrogen annealing and UV irradiation in a transparent oxide, 12CaO.Al2O3, found by Hayashi et al. [Nature (London)
419, 462 (2002).] The charge transport associated with photoexcitation of an electron from H- occurs by electron hopping.We identify the atoms participating in the hops, determine the exact paths for the carrier migration, estimate the temperature behavior of the hopping transport, and predict a way to enhance the conductivity by specific doping.
PDF Slide1 Slide2 Slide3 Slide4
|Orbital ordering in paramagnetic LaMnO3 and KCuF3
J.E. Medvedeva, M.K. Korotin, V.I. Anisimov, A.J. Freeman
Physical Review B, 65, 172413 (2002)
Ab initio studies of the stability of orbital ordering, its coupling to magnetic structure and its possible origins (electron-phonon and/or electron-electron interactions) are reported for two perovskite systems, LaMnO3 and KCuF3. We present an average spin state calculational scheme that allowed us to treat a paramagnetic state and to succesfully describe the experimental magnetic or orbital phase diagram of both LaMnO3 and KCuF3 in crystal structures when the Jahn-Teller distortions are neglected. Hence, we conclude that the orbital ordering in both compounds is purely electronic in origin.
|Role of Coulomb correlation on magnetic and transport properties of doped manganites: La0.5Sr0.5MnO3 and LaSr2Mn2O7
J.E. Medvedeva, V.I. Anisimov, O.N. Mryasov, A.J. Freeman
Journal of Physics: Condens. Matter, 14, 4533 (2002)
Results of local spin-density approximation (LSDA) andLSDA+U calculations
of the electronic structure and magnetic configurations of the 50% hole-doped
pseudocubic perovskite La0.5Sr0.5MnO3 and double-layered LaSr2Mn2O7 are presented. We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons has a very different influence on the (i) band structures, (ii) magnetic ground states, (iii) interlayer exchange interactions, and (iv) anisotropy of the electrical transport in these two manganites. A p–d hybridization analysis is employed to look for possible explanations for the LSDA failing to predict the observed magnetic and transport properties of the double-layered compound—in contrast to the case for the doped perovskite manganite. PDF
|Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and Tc in MgB2
N.I. Medvedeva, A.L. Ivanovskii, J.E. Medvedeva, A.J.Freeman, D.L. Novikov
Physical Review B, 65, 052501 (2001)
Results of full-potential linear muffin-tin orbital generalized gradient approximation calculations of the band structure and boron electric field gradients (EFG’s) for the new medium-Tc superconductor MgB2 and related
diborides MB2, M5Be, Al, Sc, Ti, V, Cr, Mo, and Ta are reported. The boron EFG variations are found to be related to specific features of their band structure and particularly to the M-B hybridization. The strong charge
anisotropy at the B site in MgB2 is completely defined by the valence electrons—a property which sets MgB2 apart from other diborides. The boron EFG in MgB2 is weakly dependent on applied pressure: the B p-electron
anisotropy increases with pressure, but it is partly compensated by the increase of core charge asymmetry. The concentration of holes in bonding s bands is found to decrease slightly from 0.067 to 0.062 holes/B under a
pressure of 10 GPa. Despite a small decrease of N(EF), the Hopfield parameter increases with pressure and we believe that the main reason for the reduction under pressure of the superconducting transition temperature Tc
is the strong pressure dependence of phonon frequencies, which is sufficient to compensate for the electronic effects. PDF
|Electronic structure of superconducting MgB2 and related binary and ternary borides
N.I. Medvedeva, A.L. Ivanovskii, J.E. Medvedeva, A.J.Freeman
Physical Review B, 64, 020502(R) (2001)
First-principles full potential linear muffin-tin orbital–generalized gradient approximation electronic structure calculations of the new medium-TC superconductor (MTSC) MgB2 and related diborides indicate that superconductivity in these compounds is related to the existence of px,y-band holes at the G point. Based on these calculations, we explain the absence of medium-TC superconductivity for BeB2, AlB2 , ScB2, and YB2. The simulation of a number of MgB2-based ternary systems using a supercell approach demonstrates that (i) the electron doping of MgB2 (i.e., MgB22yXy with X=Be, C, N, O) and the creation of defects in the boron sublattice ~nonstoichiometric MgB22y) are not favorable for superconductivity, and (ii) a possible way of
searching for similar or higher MTSC should be via hole doping of MgB2 (CaB2) or isoelectronic substitution of Mg (i.e., Mg12xMxB2 with M=Be, Ca, Li, Na, Cu, Zn) or creating layered superstructures of the MgB2 /CaB2 type.
|Coulomb correlation and magnetic ordering in double-layered LaSr2Mn2O7
J.E. Medvedeva, V.A. Anisimov, M.K. Korotin, O.N. Mryasov, A.J. Freeman
Journal of Magnetism and Magnetic Materials, 237, 47 (2001)
A detailed study of the electronic structure and magnetic configurations of the 50% hole-doped double-layered manganite LaSr2Mn2O7 is presented. We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons
(i) significantly modifies the electronic structure, magnetic ordering (from FM to AFM), and interlayer exchange interactions, and (ii) promotes strong anisotropy in electrical transport, reducing the effective hopping parameter along the c-axis for electrically active eg electrons. This finding is consistent with observations of anisotropic transport property which sets this manganite apart from conventional 3D systems. A half-metallic band structure is predicted
with both the LSDA and LSDA+U methods. The experimentally observed A-type AFM ordering in LaSr2Mn2O7 is found to be energetically more favorable with U>7 eV. A simple interpretation of interlayer exchange coupling is given within double and super-exchange mechanisms based on the dependencies on U of the effective exchange parameters and eg state sub-band widths.
|The effect of Coulomb interaction and magnetic ordering on the electronic structure of two hexagonal YMnO3 phases
J.E. Medvedeva, O.N. Mryasov, M.K. Korotin, V.A. Anisimov, A.J. Freeman
J. Phys.: Condens. Matter, 12, 4947 (2000)
The electronic structure of YMnO3 in its high- and low-temperature hexagonal phases has been investigated within the local spin-density approximation (LSDA) and by the LSDA + U method which takes into account the local Coulomb interaction between d electrons of transitionmetal ions. In contrast to the case for orthorhombic manganites, the d4-configuration degeneracy is
already lifted in the high-temperature symmetric hexagonal phase, indicating that Mn3+ is not a Jahn–Teller ion; hence, we argue that the lowering of the symmetry is not connected with Jahn–Teller instability in hexagonal YMnO3. Each of these two hexagonal phases is found to be semiconducting, with a band gap of about 1.5 eV. It is shown that magnetism and correlation
effects are important in band-gap formation for both crystal structures. Using the Green function method, we estimated the Neel temperature from the calculated effective exchange interaction parameters, and found it to be in good agreement with experiment. PDF