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Alex Zunger

Publications and source records attributed to Alex Zunger.

At least 109 records · Page 6Linked to original sources
arXiv2004

Trends in ferromagnetism, hole localization, and acceptor level depth for Mn substitution in

We examine the intrinsic mechanism of ferromagnetism in dilute magnetic semiconductors by analyzing the trends in the electronic structure as the host is changed from GaN to GaP, GaAs and GaSb, keeping the transition metal impurity fixed. In contrast with earlier interpretations which depended on the host semiconductor, we found that a single mechanism is sufficient to explain the ferromagnetic stabilization energy for the entire series.

cond-mat.other↗
arXiv2004

Unusual direction dependence of exchange energies in GaAs:Mn - Is the RKKY description relevant

Ferromagnetism in Mn-doped GaAs, the prototypical dilute magnetic semiconductor, has so far been attributed to hole mediated RKKY-type interactions. First-principles calculations reveal a strong direction dependence of the ferromagnetic (FM) stabilization energy of two magnetic ions, a dependence that cannot be explained within RKKY. In the limit of host-like hole (engineered here by an GGA+U approach with large U) where the RKKY model is applicable, we find that the exchange energies are strongly reduced, suggesting that this limit cannot explain the observed ferromagnetism. The dominant contribution stabilizing the FM state is found to be maximal for <110>-oriented pairs and minimal for <100> oriented pairs, providing an alternate explanation for magnetism in such materials in terms of energy lowering due to p-d hopping interactions, and offering a new design degree of freedom to enhance FM.

cond-mat.other↗
arXiv2004

Can a half-metallic zincblende-type structure be stabilized via epitaxy?

The need for spin-injectors having the same zincblende-type crystal structure as conventional semiconductor substrates has created significant interests in theoretical predictions of possible metastable ``half-metallic'' zincblende ferromagnets, which are normally more stable in other structure-types, e.g., NiAs. Such predictions were based in the past on differences $Δ_{\rm bulk}$ in the total-energies of the respective {\em bulk} crystal forms (zincblende and NiAs). We show here that the appropriate criterion is comparing difference $Δ_{\rm epi}(a_s)$ in {\em epitaxial} total-energies. This reveals that even if $Δ_{\rm bulk}$ is small, still for MnAs, CrSb, CrAs, CrTe, $Δ_{\rm epi}(a_s) > 0$ for all substrate lattice constant $a_s$, so the zincblende phase is not stabilized. For CrS we find $Δ_{\rm epi}(a_s) <0$, but the system is antiferromagnetic, thus not half-metallic. Finally, zincblende CrSe is predicted to be epitaxially stable for $a_s > 6.2$ Å and half-metallic.

cond-mat.mtrl-sci↗
arXiv2004

Theory of excitonic spectra and entanglement engineering in dot molecules

We present results of correlated pseudopotential calculations of an exciton in a pair of vertically stacked InGaAs/GaAs dots. Competing effects of strain, geometry, and band mixing lead to many unexpected features missing in contemporary models. The first four excitonic states are all optically active at small interdot separation, due to the broken symmetry of the single-particle states. We quantify the degree of entanglement of the exciton wavefunctions and show its sensitivity to interdot separation. We suggest ways to spectroscopically identify and maximize the entanglement of exciton states.

cond-mat.mtrl-sci↗
arXiv2004

Metal-nonmetal transition and excitonic ground state in InAs/InSb quantum dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict a metal-nonmetal transition and an excitonic ground state in the InAs/InSb quantum dot (QD) system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the single-particle gap $E_g$ becomes very small. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wavefunctions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of fully discrete 0D confined energy levels. These correlation effects overcome $E_g$, leading to the formation of a bi-excitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by $\sim$ 15 meV) than the state without excitons. We discuss the excitonic phase transition on QD arrays in the low dot density limit.

cond-mat.mtrl-sci↗
arXiv2003

Ferromagnetism in Mn doped GaAs due to substitutional-interstitial complexes

While most calculations on the properties of the ferromagnetic semiconductor GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn$_{Ga}$), we investigate here whether alternate lattice sites are favored and what the magnetic consequences of this might be. Under As-rich (Ga-poor) conditions prevalent at growth, we find that the formation energies are lower for Mn$_{Ga}$ over interstitial Mn (Mn$_i$).As the Fermi energy is shifted towards the valence band maximum via external $p$-doping, the formation energy of Mn$_i$ is reduced relative to Mn$_{Ga}$. Furthermore, under epitaxial growth conditions, the solubility of both substitutional and interstitial Mn are strongly enhanced over what is possible under bulk growth conditions. The high concentration of Mn attained under epitaxial growth of p-type material opens the possibility of Mn atoms forming small clusters. We consider various types of clusters, including the Coulomb-stabilized clusters involving two Mn$_{Ga}$ and one Mn$_i$. While isolated Mn$_i$ are hole killers (donors), and therefore destroy ferromagnetism,complexes such as Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$) are found to be more stable than complexes involving Mn$_{Ga}$-Mn$_{Ga}$-Mn$_{Ga}$. The former complexes exhibit partial or total quenching of holes, yet Mn$_i$ in these complexes provide a channel for a ferromagnetic arrangement of the spins on the two Mn$_{Ga}$ within the complex. This suggests that ferromagnetism in Mn doped GaAs arises both from holes due to isolated Mn$_{Ga}$ as well as from strongly Coulomb stabilized Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$ clusters.

cond-mat↗
arXiv2003

First-principles investigation of the assumptions underlying Model-Hamiltonian approaches to ferromagnetism of 3d impurities in III-V semiconductors

We use first-principle calculations for transition metal impurities V, Cr, Mn, Fe, Co and Ni in GaAs as well as Cr and Mn in GaN, GaP and GaSb to identify the basic features of the electronic structure of these systems. The microscopic details of the hole state such as the symmetry and the orbital character, as well as the nature of the coupling between the hole and the transition metal impurity are determined. This could help in the construction of model Hamiltonians to obtain a description of various properties beyond what current first-principle methods are capable of.

cond-mat↗
arXiv2001

Origin of room-temperature ferromagnetism in Mn doped semiconducting CdGeP2

CdGeP2 chalcopyrites doped with Mn have been recently found to exhibit room temperature ferromagnetism. Isovalent substitution of the Cd site is expected, however, to create antiferromagnetism, in analogy with the well-known CdTe:Mn (d^5) case. However, chalcopyrite semiconductors exhibit low-energy intrinsic defects. We show theoretically how ferromagnetism results from the interaction of Mn with hole-producing intrinsic defects.

cond-mat.str-el↗
arXiv2000

BAs and boride III-V alloys

Boron arsenide, the typically-ignored member of the III-V arsenide series BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an X_1c-like indirect band gap, and its bond charge is distributed almost equally on the two atoms in the unit cell, exhibiting nearly perfect covalency. The reasons for these are tracked down to the anomalously low atomic p orbital energy in the boron and to the unusually strong s-s repulsion in BAs relative to most other III-V compounds. We find unexpected valence band offsets of BAs with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is significantly higher than that of AlAs, despite the much smaller bond length of BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects result from the unusually strong mixing of the cation and anion states at the VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and composition-independent band gap bowing. This means that while addition of small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing enthalpies which are smaller than in GaN-GaAs alloys. The unique features of boride III-V alloys offer new opportunities in band gap engineering.

cond-mat.mtrl-sci↗
arXiv2000

Theoretical interpretation of the experimental electronic structure of lens shaped, self-assembled InAs/GaAs quantum dots

We adopt an atomistic pseudopotential description of the electronic structure of self-assembled, lens shaped InAs quantum dots within the ``linear combination of bulk bands'' method. We present a detailed comparison with experiment, including quantites such as the single particle electron and hole energy level spacings, the excitonic band gap, the electron-electron, hole-hole and electron hole Coulomb energies and the optical polarization anisotropy. We find a generally good agreement, which is improved even further for a dot composition where some Ga has diffused into the dots.

cond-mat.mtrl-sci↗
arXiv1999

Addition spectra of quantum dots: The role of dielectric mismatch

Using atomistic pseudopotential wave functions we calculate the electron and hole charging energies of InAs quantum dots. We find that the charging energies depend strongly on the dielectric constant epsilon_out of the surrounding material, and that when the latter is smaller than the dielectric constant of the dot the electron-electron and hole-hole interactions are dominated by surface-polarization effects. We predict the addition energies and the quasi-particle gap as a function of size and epsilon_out. We find excellent agreement with recent single-dot tunneling spectroscopy data for epsilon_out=6.

cond-mat.mtrl-sci↗
arXiv1999

A pseudopotential study of electron-hole excitations in colloidal, free-standing InAs quantum dots

Excitonic spectra are calculated for free-standing, surface passivated InAs quantum dots using atomic pseudopotentials for the single-particle states and screened Coulomb interactions for the two-body terms. We present an analysis of the single particle states involved in each excitation in terms of their angular momenta and Bloch-wave parentage. We find that (i) in agreement with other pseudopotential studies of CdSe and InP quantum dots, but in contrast to k.p calculations, dot states wavefunction exhibit strong odd-even angular momentum envelope function mixing (e.g. $s$ with $p$) and large valence-conduction coupling. (ii) While the pseudopotential approach produced very good agreement with experiment for free-standing, colloidal CdSe and InP dots, and for self-assembled (GaAs-embedded) InAs dots, here the predicted spectrum does {\em not} agree well with the measured (ensemble average over dot sizes) spectra. (1) Our calculated excitonic gap is larger than the PL measure one, and (2) while the spacing between the lowest excitons is reproduced, the spacings between higher excitons is not fit well. Discrepancy (1) could result from surface states emission. As for (2), agreement is improved when account is taken of the finite size distribution in the experimental data. (iii) We find that the single particle gap scales as $R^{-1.01}$ (not $R^{-2}$), that the screened (unscreened) electron-hole Coulomb interaction scales as $R^{-1.79}$ ($R^{-0.7}$), and that the eccitonic gap sclaes as $R^{-0.9}$. These scaling laws are different from those expected from simple models.

cond-mat.mtrl-sci↗
arXiv1998

Comment on "Quantum Confinement and Optical Gaps in Si Nanocrystals"

We show that the method used by Ogut, Chelikowsky and Louie (Phys. Rev. Lett. 79, 1770 (1997)) to calculate the optical gap of Si nanocrystals omits an electron-hole polarization energy. When this contribution is taken into account, the corrected optical gap is in excellent agreement with semi-empirical pseudopotential calculations.

cond-mat.mtrl-sci↗
arXiv1998

Prediction of a Strain Induced Conduction Band Minimum in Embedded Quantum Dots

Free standing InP quantum dots have previously been theoretically and experimentally shown to have a direct band gap across a large range of experimentally accessible sizes. We demonstrate that when these dots are embedded coherently within a GaP barrier material, the effects of quantum confinement in conjunction with coherent strain suggest there will be a critical diameter of dot (60A), above which the dot is direct, type I, and below which it is indirect, type II. However, the strain in the system acts to produce another conduction state with an even lower energy, in which electrons are localized in small pockets at the interface between the InP dot and the GaP barrier. Since this conduction state is GaP X_1c-derived and the highest occupied valence state is InP, Gamma-derived, the fundamental transition is predicted to be indirect in both real and reciprocal space (``type II'') for all dot sizes. This effect is peculiar to the strained dot, and is absent in the free-standing dot.

cond-mat.mtrl-sci↗
arXiv1998

Cation- and vacancy-ordering in Li_xCoO_2

Using a combination of first-principles total energies, a cluster expansion technique, and Monte Carlo simulations, we have studied the Li/Co ordering in LiCoO_2 and Li-vacancy/Co ordering in CoO_2. We find: (i) A ground state search of the space of substitutional cation configurations yields the (layered) CuPt structure as the lowest-energy state in the octahedral system LiCoO_2 (and CoO_2), in agreement with the experimentally observed phase. (ii) Finite temperature calculations predict that the solid-state order- disorder transitions for LiCoO_2 and CoO_2 occur at temperatures (~5100 K and ~4400 K, respectively) much higher than melting, thus making these transitions experimentally inaccessible. (iii) The energy of the reaction E(LiCoO_2) - E(CoO_2) - E(Li) gives the average battery voltage V of a Li_xCoO_2/Li cell. Searching the space of configurations for large average voltages, we find that CuPt (a monolayer <111> superlattice) has a high voltage (V=3.78 V), but that this could be increased by cation randomization (V=3.99 V), partial disordering (V=3.86 V), or by forming a 2-layer Li_2Co_2O_4 superlattice along <111> (V=4.90 V).

cond-mat.mtrl-sci↗
arXiv1998

Evaluating and improving the cluster variation method entropy functional for Ising alloys

The success of the "Cluster Variation Method" (CVM) in reproducing quite accurately the free energies of Monte Carlo (MC) calculations on Ising models is explained in terms of identifying a cancellation of errors: We show that the CVM produces correlation functions that are too close to zero, which leads to an overestimation of the exact energy, E, and at the same time, to an underestimation of -TS, so the free energy F=E-TS is more accurate than either of its parts. This insight explains a problem with "hybrid methods" using MC correlation functions in the CVM entropy expression: They give exact energies E and do not give significantly improved -TS relative to CVM, so they do not benefit from the above noted cancellation of errors. Additionally, "hybrid methods" suffer from the difficulty of adequately accounting for both ordered and disordered phases in a consistent way. A different technique, the "Entropic Monte Carlo" (EMC), is shown here to provide a means for critically evaluating the CVM entropy. Inspired by EMC results, we find a universal and simple correction to the CVM entropy which produces individual components of the free energy with MC accuracy, but is computationally much less expensive than either MC thermodynamic integration or EMC.

cond-mat.mtrl-sci↗
arXiv1997

Effects of anharmonic strain on phase stability of epitaxial films and superlattices: applications to noble metals

Epitaxial strain energies of epitaxial films and bulk superlattices are studied via first-principles total energy calculations using the local-density approximation. Anharmonic effects due to large lattice mismatch, beyond the reach of the harmonic elasticity theory, are found to be very important in Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that <001> is the elastically soft direction for biaxial expansion of Cu and Ni, but it is <201> for large biaxial compression of Cu, Ag, and Au. The stability of superlattices is discussed in terms of the coherency strain and interfacial energies. We find that in phase-separating systems such as Cu-Ag the superlattice formation energies decrease with superlattice period, and the interfacial energy is positive. Superlattices are formed easiest on (001) and hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the formation energy of superlattices increases with period, and interfacial energies are negative. These superlattices are formed easiest on (001) or (110) and hardest on (111) substrates. For Ni-Au we find a hybrid behavior: superlattices along <111> and <001> like in phase-separating systems, while for <110> they behave like in ordering systems. Finally, recent experimental results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys, immiscible in the bulk form, are explained in terms of destabilization of the phase separated state due to lattice mismatch between the substrate and constituents.

cond-mat.mtrl-sci↗