Article records
https://feeds.library.caltech.edu/people/Tahir-Kheli-J/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 14:22:51 +0000Spinons and holons for the one-dimensional three-band Hubbard models of high-temperature superconductors
https://resolver.caltech.edu/CaltechAUTHORS:20141208-150229740
Authors: {'items': [{'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 1993
DOI: 10.1073/pnas.90.21.9959
PMCID: PMC47692
The one-dimensional three-band Hubbard Hamiltonian is shown to be equivalent to an effective Hamiltonian that has independent spinon and holon quasiparticle excitations plus a weak coupling of the two. The spinon description includes both copper sites and oxygen hole sites leading to a one-dimensional antiferromagnet incommensurate with the copper lattice. The holons are spinless noninteracting fermions in a simple cosine band. Because the oxygen sites are in the Hamiltonian, the quasiparticles are much simpler than in the exact solution of the t-J model for 2t = ± J. If a similar description is correct for two dimensions, then the holons will attract in a p-wave potential.https://authors.library.caltech.edu/records/zwwf3-cdn04Antiferromagnetic band structure of La_2CuO_4: Becke-3–Lee-Yang-Parr calculations
https://resolver.caltech.edu/CaltechAUTHORS:20190626-121728673
Authors: {'items': [{'id': 'Perry-J-K', 'name': {'family': 'Perry', 'given': 'Jason K.'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2001
DOI: 10.1103/physrevb.63.144510
Using the Becke-3–Lee-Yang-Parr (B3LYP) functional, we have performed band-structure calculations on the high-temperature superconductor parent compound, La_2CuO_4. Under the restricted spin formalism (ρ_↑ = ρ_↓), B3LYP band structure agrees well with the standard local-density approximation (LDA) band structure. It is metallic with a single Cu_(x^2 − y^2)/Op_σ band crossing the Fermi level. Under the unrestricted spin formalism (ρ_↑ ≠ ρ_↓), the B3LYP band structure has a spin-polarized antiferromagnetic solution with a band gap of 2.0 eV, agreeing well with experiment. This state is 0.52 eV (per formula unit) lower than that calculated under the restricted spin formalism. The apparent high energy of the spin-restricted state is attributed to an overestimate of on-site Coulomb repulsion, which is corrected in the unrestricted spin calculations. The stabilization of the total energy with spin polarization arises primarily from the stabilization of the x^2 − y^2 band, such that the character of the eigenstates at the top of the valence band in the antiferromagnetic state becomes a strong mixture of Cu_(x^2 − y^2)/Op_σ and Cu_(z^2)/O′p_z. Since the Hohenberg-Kohn theorem requires the spin-restricted and spin-unrestricted calculations to give identical ground-state energies and total spatial densities for the exact functionals, this large disparity in energy reflects the inadequacy of current functionals for describing the cuprates. This calls into question the use of band structures based on current restricted spin-density functionals (including LDA) as a basis for single-band theories of superconductivity in these materials.https://authors.library.caltech.edu/records/04435-6r546Ab initio evidence for the formation of impurity d_(3z^2-r^2) holes in doped La_(2-x)Sr_xCuO_4
https://resolver.caltech.edu/CaltechAUTHORS:PERprb02
Authors: {'items': [{'id': 'Perry-J-K', 'name': {'family': 'Perry', 'given': 'Jason K.'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2002
DOI: 10.1103/PhysRevB.65.144501
Using the spin unrestricted Becke-3-Lee-Yang-Parr density functional, we computed the electronic structure of explicitly doped La2-xSrxCuO4 (x=0.125, 0.25, and 0.5). At each doping level, an impurity hole band is formed within the undoped insulating gap. This band is well localized to CuO6 octahedra adjacent to the Sr impurities. The nature of the impurity hole is A1g in symmetry, formed primarily from the z2 orbital on the Cu and pz orbitals on the apical O's. There is a strong triplet coupling of this hole with the intrinsic B1g Cu x2-y2/O1 pσ hole on the same site. Optimization of the c coordinate of the apical O's in the doped CuO6 octahedron leads to an asymmetric anti-Jahn-Teller distortion of the O_2 atoms toward the central Cu. In particular, the O_2 atom between the Cu and Sr is displaced 0.26 Å while the O_2 atom between the Cu and La is displaced 0.10 Å. Contrary to expectations, investigation of a 0.1 Å enhanced Jahn-Teller distortion of this octahedron does not force formation of an x^2 - y^2 hole, but instead leads to migration of the z^2 hole to the four other CuO_6 octahedra surrounding the Sr impurity. This latter observation offers a simple explanation for the bifurcation of the Sr-O_2 distance revealed in x-ray absorption fine structure data.https://authors.library.caltech.edu/records/ftsss-e8j53Numerical resistivity calculations for disordered three-dimensional metal models using tight-binding Hamiltonians
https://resolver.caltech.edu/CaltechAUTHORS:GILprb04
Authors: {'items': [{'id': 'Gilman-Y', 'name': {'family': 'Gilman', 'given': 'Yulia'}}, {'id': 'Allen-P-B', 'name': {'family': 'Allen', 'given': 'Philip B.'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2004
DOI: 10.1103/PhysRevB.70.224201
We calculate the zero-temperature resistivity of model three-dimensional disordered metals described by tight-binding Hamiltonians. Two different mechanisms of disorder are considered: diagonal disorder (random on-site potentials) and off-diagonal disorder (random hopping integrals). The nonequilibrium Green function formalism provides a Landauer-type formula for the conductance of arbitrary mesoscopic systems. We use this formula to calculate the resistance of finite-size disordered samples of different lengths. The resistance averaged over disorder configurations is linear in sample length and resistivity is found from the coefficient of proportionality. Two structures are considered: (1) a simple cubic lattice with one s-orbital per site, and (2) a simple cubic lattice with two d-orbitals. For small values of the disorder strength, our results agree with those obtained from the Boltzmann equation. Large off-diagonal disorder causes the resistivity to saturate, whereas increasing diagonal disorder causes the resistivity to increase faster than the Boltzmann result. The crossover toward localization starts when the Boltzmann mean free path l relative to the lattice constant a has a value between 0.5 and 2.0 and is strongly model dependent.https://authors.library.caltech.edu/records/pe6v8-3wb93First-principles approach to the charge-transport characteristics of monolayer molecular-electronics devices: Application to hexanedithiolate devices
https://resolver.caltech.edu/CaltechAUTHORS:KIMprb06
Authors: {'items': [{'id': 'Kim-Y-H', 'name': {'family': 'Kim', 'given': 'Yong-Hoon'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Schultz-P-A', 'name': {'family': 'Schultz', 'given': 'Peter A.'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2006
DOI: 10.1103/PhysRevB.73.235419
We report on the development of an accurate first-principles computational scheme for the charge transport characteristics of molecular monolayer junctions and its application to hexanedithiolate (C6DT) devices. Starting from the Gaussian basis set density-functional calculations of a junction model in the slab geometry and corresponding two bulk electrodes, we obtain the transmission function using the matrix Green's function method and analyze the nature of transmission channels via atomic projected density of states. Within the developed formalism, by treating isolated molecules with the supercell approach, we can investigate the current-voltage characteristics of single and parallel molecular wires in a consistent manner. For the case of single C6DT molecules stretched between Au(111) electrodes, we obtain reasonable quantitative agreement of computed conductance with a recent scanning tunneling microscope experiment result. Comparing the charge transport properties of C6DT single molecules and their monolayer counterparts in the stretched and tilted geometries, we find that the effect of intermolecular coupling and molecule tilting on the charge transport characteristics is negligible in these devices. We contrast this behavior to that of the pi-conjugated biphenyldithiolate devices we have previously considered and discuss the relative importance of molecular cores and molecule-electrode contacts for the charge transport in those devices.https://authors.library.caltech.edu/records/737y8-q4706Chiral plaquette polaron theory of cuprate superconductivity
https://resolver.caltech.edu/CaltechAUTHORS:TAHprb07
Authors: {'items': [{'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2007
DOI: 10.1103/PhysRevB.76.014514
Ab initio density functional calculations on explicitly doped La2−xSrxCuO4 find that doping creates localized holes in out-of-plane orbitals. A model for cuprate superconductivity is developed based on the assumption that doping leads to the formation of holes on a four-site Cu plaquette composed of the out-of-plane A1 orbitals apical O pz, planar Cu d3z2−r2, and planar O psigma. This is in contrast to the assumption of hole doping into planar Cu dx^2−y^2 and O psigma orbitals as in the t-J model. Allowing these holes to interact with the d^9 spin background leads to chiral polarons with either a clockwise or anticlockwise charge current. When the polaron plaquettes percolate through the crystal at x[approximate]0.05 for La2−xSrxCuO4, a Cu dx^2−y^2 and planar O psigma band is formed. The computed percolation doping of x[approximate]0.05 equals the observed transition to the "metallic" and superconducting phase for La2−xSrxCuO4. Spin exchange Coulomb repulsion with chiral polarons leads to d-wave superconducting pairing. The equivalent of the Debye energy in phonon superconductivity is the maximum energy separation between a chiral polaron and its time-reversed partner. This energy separation is on the order of the antiferromagnetic spin coupling energy, Jdd~0.1 eV, suggesting a higher critical temperature. An additive skew-scattering contribution to the Hall effect is induced by chiral polarons and leads to a temperature dependent Hall effect that fits the measured values for La2−xSrxCuO4. The integrated imaginary susceptibility, observed by neutron spin scattering, satisfies omega/T scaling due to chirality and spin-flip scattering of polarons along with a uniform distribution of polaron energy splittings. The derived functional form is compatible with experiments. The static spin structure factor for chiral spin coupling of the polarons to the undoped antiferromagnetic Cu d^9 spins is computed for classical spins on large two-dimensional lattices and is found to be incommensurate with a separation distance from (pi/a,pi/a) given by deltaQ[approximate](2pi/a)x, where x is the doping. When the perturbed x^2−y^2 band energy in mean field is included, incommensurability along the Cu-O bond direction is favored. A resistivity ~T^(µ+1) arises when the polaron energy separation density is of the form ~Deltaµ due to Coulomb scattering of the x^2−y^2 band with polarons. A uniform density leads to linear resistivity. The coupling of the x^2−y^2 band to the undoped Cu d^9 spins leads to the angle-resolved photoemission pseudogap and its qualitative doping and temperature dependence. The chiral plaquette polaron leads to an explanation of the evolution of the bilayer splitting in Bi-2212.https://authors.library.caltech.edu/records/1b5m9-c6655Dynamic admittance of carbon nanotube-based molecular electronic devices and their equivalent electric circuit
https://resolver.caltech.edu/CaltechAUTHORS:YAMnano08
Authors: {'items': [{'id': 'Yam-ChiYung', 'name': {'family': 'Yam', 'given': 'ChiYung'}}, {'id': 'Mo-Yan', 'name': {'family': 'Mo', 'given': 'Yan'}}, {'id': 'Wang-Fan', 'name': {'family': 'Wang', 'given': 'Fan'}}, {'id': 'Li-Xiaobao', 'name': {'family': 'Li', 'given': 'Xiaobao'}}, {'id': 'Chen-GuanHua', 'name': {'family': 'Chen', 'given': 'GuanHua'}}, {'id': 'Zheng-Xiao', 'name': {'family': 'Zheng', 'given': 'Xiao'}}, {'id': 'Matsuda-Yuki', 'name': {'family': 'Matsuda', 'given': 'Yuki'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2008
DOI: 10.1088/0957-4484/19/49/495203
We use first-principles quantum mechanics to simulate the transient electrical response through carbon nanotube-based conductors under time-dependent bias voltages. The dynamic admittance and time-dependent charge distribution are reported and analyzed. We find that the electrical response of these two-terminal molecular devices can be mapped onto an equivalent classical electric circuit and that the switching time of these end-on carbon nanotube devices is only a few femtoseconds. This result is confirmed by studying the electric response of a simple two-site model device and is thus generalized to other two-terminal molecular electronic devices.https://authors.library.caltech.edu/records/52fdd-5g217The Chiral Plaquette Polaron Paradigm (CPPP) for high temperature cuprate superconductors
https://resolver.caltech.edu/CaltechAUTHORS:20090916-092344893
Authors: {'items': [{'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2009
DOI: 10.1016/j.cplett.2009.02.025
A scientific revolution occurred in 1986–1994 in which the Tc for the best superconductors exploded from 23 K (Nb_3Ge) to 138 K (Hg_(0.2)Tl_(0.8)Ba_2Ca_2Cu_3O_(8.33)). Despite enormous effort over the last 21 years, the superconducting mechanism remains unknown. All previous attempts assumed that the doped holes were in the CuO_2 plane. We showed recently with improved quantum mechanics (QM) calculations that the hole is out of the CuO_2 plane and delocalized over four Cu atoms in a square Plaquette that forms the basis of our Chiral Plaquette Polaron Paradigm (CPPP). Here, we show how very simple geometric arguments provide a qualitative understanding of the broad range of cuprate phenomenology. This simple geometric analysis may be useful in guiding the development of materials for improved superconductors. The CPPP suggests that judicious control of the dopant distribution could possibly lead to a room-temperature T_c.https://authors.library.caltech.edu/records/87ddd-vcc85Universal Properties of Cuprate Superconductors: T_c Phase Diagram, Room-Temperature Thermopower, Neutron Spin Resonance, and STM Incommensurability Explained in Terms of Chiral Plaquette Pairing
https://resolver.caltech.edu/CaltechAUTHORS:20100528-082145664
Authors: {'items': [{'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2010
DOI: 10.1021/jz100265k
We report that four properties of cuprates and their evolution with
doping are consequences of simply counting four-site plaquettes arising from
doping, (1) the universal T_c phase diagram (superconductivity between ~0.05 and
~0.27 doping per CuO_2 plane and optimal T_c at ~0.16), (2) the universal doping
dependence of the room-temperature thermopower, (3) the superconducting
neutron spin resonance peak (the "41 meV peak"), and (4) the dispersionless
scanning tunneling conductance incommensurability. Properties (1), (3), and (4)
are explained with no adjustable parameters, and (2) is explained with exactly one.
The successful quantitative interpretation of four very distinct aspects of cuprate
phenomenology by a simple counting rule provides strong evidence for four-site
plaquette percolation in these materials. This suggests that inhomogeneity, percolation,
and plaquettes play an essential role in cuprates. This geometric analysis
may provide a useful guide to search for new compositions and structures with
improved superconducting properties.https://authors.library.caltech.edu/records/sqtmy-zf390Definitive Band Gaps for Single-Wall Carbon Nanotubes
https://resolver.caltech.edu/CaltechAUTHORS:20101028-080911813
Authors: {'items': [{'id': 'Matsuda-Y', 'name': {'family': 'Matsuda', 'given': 'Yuki'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A. III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2010
DOI: 10.1021/jz100889u
We report ab initio quantum mechanical calculations of band structures of single-walled carbon nanotubes (SWNTs) using the B3LYP flavor of density functional theory. In particular, we find excellent agreement with the small band gaps in "metallic" zigzag SWNTs observed by Lieber et al. [0.079 vs 0.080 eV for (9,0), 0.041 vs 0.042 eV for (12,0), and 0.036 eV vs 0.029 eV for (15,0)]. This contrasts with the results from LDA and PBE, which lead to band gaps 70−100% too small, and with those from the GW correction to LDA, which leads to a gap two times too large. Interestingly we find that the (5,0) system, expected to be a large gap semiconductor, is metallic. These results show that B3LYP leads to very accurate band gaps for CNTs, suggesting its use in designing CNT devices. We find that the effective mass of the CNT (significant in designing CNT devices) scales inversely proportional to the square of the diameter.https://authors.library.caltech.edu/records/00mrf-yk314Accurate Band Gaps for Semiconductors from Density Functional Theory
https://resolver.caltech.edu/CaltechAUTHORS:20110310-100110624
Authors: {'items': [{'id': 'Xiao-Hai', 'name': {'family': 'Xiao', 'given': 'Hai'}, 'orcid': '0000-0001-9399-1584'}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2011
DOI: 10.1021/jz101565j
An essential issue in developing semiconductor devices for photovoltaics and thermoelectrics is to design materials with appropriate band gaps plus the proper positioning of dopant levels relative to the bands. Local density (LDA)
and generalized gradient approximation (GGA) density functionals generally underestimate band gaps for semiconductors and sometimes incorrectly predict
a metal. Hybrid functionals that include some exact Hartree-Fock exchange are known to be better. We show here for CuInSe_2, the parent compound of the promising CIGS Cu(In_xGa_(1-x))Se_2 solar devices, that LDA and GGA obtain gaps of 0.0-0.01 eV (experiment is 1.04 eV), while the historically first global hybrid functional, B3PW91, is surprisingly better than B3LYP with band gaps of 1.07 and
0.95 eV, respectively. Furthermore, we show that for 27 related binary and ternary semiconductors, B3PW91 predicts gaps with a mean average deviation (MAD) of only 0.09 eV, which is substantially better than all modern hybrid functionals.https://authors.library.caltech.edu/records/hv4m5-3gc33The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory
https://resolver.caltech.edu/CaltechAUTHORS:20110512-100508849
Authors: {'items': [{'id': 'Cheng-Mu-Jeng', 'name': {'family': 'Cheng', 'given': 'Mu-Jeng'}, 'orcid': '0000-0002-8121-0485'}, {'id': 'Nielsen-R-J', 'name': {'family': 'Nielsen', 'given': 'Robert J.'}, 'orcid': '0000-0002-7962-0186'}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2011
DOI: 10.1039/C0CP02777D
We have studied the magnetic structure of the high
symmetry vanadyl pyrophosphate ((VO)_(2)P_(2)O)7, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J_(OPO) = −156.8 K and J_O = −68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J_layer = 19.2 K and J_chain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J_(OPO) originates from a super-exchange interaction through the bridging –O–P–O– unit. In contrast, J_O results from a direct overlap of 3d_(x^2 − y^2) orbitals on two vanadium atoms in the same V_(2)O_8 motif, making it very sensitive to structural fluctuations. Based on the variations in V–O bond length as a function of strain along a, we found that the V–O bonds of V–(OPO)_(2)–V are covalent and rigid, whereas the bonds of V–(O)_(2)–V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J_O, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J_O to be a constant over the range of 2–300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O_2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride.https://authors.library.caltech.edu/records/6b2tk-66654Surface and Electronic Properties of Hydrogen Terminated
Si [001] Nanowires
https://resolver.caltech.edu/CaltechAUTHORS:20110713-081531464
Authors: {'items': [{'id': 'Matsuda-Y', 'name': {'family': 'Matsuda', 'given': 'Yuki'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2011
DOI: 10.1021/jp106048u
The calculated band gaps reported previously for silicon nanowires (SiNW) have disagreed with the experimental values both in magnitude and in the behavior with radius. We resolve this discrepancy here. We report ab initio quantum mechanical calculations of hydrogen terminated Si [001] nanowires (H–SiNWs) as a function of diameter (d) and hydrogen coverage using the B3LYP density functional. For smaller diameters (d ≤ 1.9 nm) we find that the most stable surface is fully saturated with hydrogen leading to direct band gaps. For larger diameters, the surface dangling bonds are not saturated, leading to surface LUMO and HOMO states that lower the gap and lead to an indirect band gap. This transition from direct to indirect gap resolves the previous disagreement in the scaling of band gap with diameter. We conclude that the electronic properties of Si NW depend sensitively on controlling the diameter and surface hydrogen coverage.https://authors.library.caltech.edu/records/w6b4f-rec38Origin of the Pseudogap in High-Temperature Cuprate Superconductors
https://resolver.caltech.edu/CaltechAUTHORS:20111024-160452466
Authors: {'items': [{'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2011
DOI: 10.1021/jz200916t
Cuprate high-temperature superconductors exhibit a pseudogap in the normal state that decreases monotonically with increasing hole doping and closes at x ≈ 0.19 holes per planar CuO_2 while the superconducting doping range is 0.05 < x < 0.27 with optimal T_c at x ≈ 0.16. Using ab initio quantum calculations at the level that leads to accurate band gaps, we found that four-Cu-site plaquettes are created in the vicinity of dopants. At x ≈ 0.05, the plaquettes percolate, so that the Cu d_(x^2y^2)/O pσ orbitals inside the plaquettes now form a band of states along the percolating swath. This leads to metallic conductivity and, below T_c, to superconductivity. Plaquettes disconnected from the percolating swath are found to have degenerate states at the Fermi level that split and lead to the pseudogap. The pseudogap can be calculated by simply counting the spatial distribution of isolated plaquettes, leading to an excellent fit to experiment. This provides strong evidence in favor of inhomogeneous plaquettes in cuprates.https://authors.library.caltech.edu/records/mtjvj-e8w23Resistance of High-Temperature Cuprate Superconductors
https://resolver.caltech.edu/CaltechAUTHORS:20130520-104824705
Authors: {'items': [{'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}]}
Year: 2013
DOI: 10.1088/1367-2630/15/7/073020
Cuprate superconductors have many different atoms per unit cell. A large fraction of cells (5–25%) must be modified ('doped') before the material superconducts. Thus it is not surprising that there is little consensus on the superconducting mechanism, despite almost 200 000 papers (Mann 2011 Nature 475 280). Most astonishing is that for the simplest electrical property, the resistance, 'despite sustained theoretical efforts over the past two decades, its origin and its relation to the superconducting mechanism remain a profound, unsolved mystery' (Hussey et al 2011 Phil. Trans. R. Soc. A 369 1626). Currently, model parameters used to fit normal state properties are experiment specific and vary arbitrarily from one doping to the other. Here, we provide a quantitative explanation for the temperature and doping dependence of the resistivity in one self-consistent model by showing that cuprates are intrinsically inhomogeneous with a percolating metallic region and insulating regions. Using simple counting of dopant-induced plaquettes, we show that the superconducting pairing and resistivity are due to phonons.https://authors.library.caltech.edu/records/8zba6-5yf73Accurate Ab Initio Quantum Mechanics Simulations of Bi_2Se_3 and Bi_2Te_3 Topological Insulator Surfaces
https://resolver.caltech.edu/CaltechAUTHORS:20150914-105538415
Authors: {'items': [{'id': 'Crowley-J-M', 'name': {'family': 'Crowley', 'given': 'Jason M.'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2015
DOI: 10.1021/acs.jpclett.5b01586
It has been established experimentally that Bi_2Te_3 and Bi_2Se_3 are topological insulators, with zero band gap surface states exhibiting linear dispersion at the Fermi energy. Standard density functional theory (DFT) methods such as PBE lead to large errors in the band gaps for such strongly correlated systems, while more accurate GW methods are too expensive computationally to apply to the thin films studied experimentally. We show here that the hybrid B3PW91 density functional yields GW-quality results for these systems at a computational cost comparable to PBE. The efficiency of our approach stems from the use of Gaussian basis functions instead of plane waves or augmented plane waves. This remarkable success without empirical corrections of any kind opens the door to computational studies of real chemistry involving the topological surface state, and our approach is expected to be applicable to other semiconductors with strong spin-orbit coupling.https://authors.library.caltech.edu/records/w2ges-qc438Resolution of the Band Gap Prediction Problem for Materials Design
https://resolver.caltech.edu/CaltechAUTHORS:20160321-091914241
Authors: {'items': [{'id': 'Crowley-J-M', 'name': {'family': 'Crowley', 'given': 'Jason M.'}}, {'id': 'Tahir-Kheli-J', 'name': {'family': 'Tahir-Kheli', 'given': 'Jamil'}}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2016
DOI: 10.1021/acs.jpclett.5b02870
An important property with any new material is the band gap. Standard density functional theory methods grossly underestimate band gaps. This is known as the band gap problem. Here, we show that the hybrid B3PW91 density functional returns band gaps with a mean absolute deviation (MAD) from experiment of 0.22 eV over 64 insulators with gaps spanning a factor of 500 from 0.014 to 7 eV. The MAD is 0.28 eV over 70 compounds with gaps up to 14.2 eV, with a mean error of −0.03 eV. To benchmark the quality of the hybrid method, we compared the hybrid method to the rigorous GW many-body perturbation theory method. Surprisingly, the MAD for B3PW91 is about 1.5 times smaller than the MAD for GW. Furthermore, B3PW91 is 3–4 orders of magnitude faster computationally. Hence, B3PW91 is a practical tool for predicting band gaps of materials before they are synthesized and represents a solution to the band gap prediction problem.https://authors.library.caltech.edu/records/cm9jr-17a34