arXiv subjects

Jamil Tahir-Kheli

Publications and source records attributed to Jamil Tahir-Kheli.

16 recordsLinked to original sources

arXiv2024

Evidence for Atomic-Scale Inhomogeneity in Superconducting Cuprate NMR

In 1990, the Millis, Monien, and Pines (MMP) model and its improvement, the Zha, Barzykin, and Pines (ZBP) model in 1996, emerged as a realistic explanation of the cuprate NMR. These two models assume a single electronic component, translational symmetry, and that the electrons simultaneously have aspects of localized antiferromagnetic (AF) spins and delocalized Cu $d_{x^2-y^2}$ band states. NMR experiments were routinely fit to these models in the 1990s and early 2000s until they finally failed as NMR experiments developed further. It appears that cuprate theorists have given up on explaining the NMR and the NMR data is forgotten. Here, we assume a two-component model of electrons where the electrons reside in two regions, one metallic with delocalized band states, and the other antiferromagnetic with localized spins. This model breaks translational symmetry. We show that the normal state spin relaxation for the planar Cu, O, and Y atoms in $\mathrm{YBa_2Cu_3O_{7-δ}}$ and their Knight shifts are explained by this two-component model. The temperature dependence of the Cu spin relaxation rate anisotropy in the superconducting state is also explained qualitatively.

cond-mat.supr-con↗

arXiv2023

Potential Major Improvement in Superconductors for High-Field Magnets

Fusion reactors are limited by the magnetic field available to confine their plasma. The commercial fusion industry uses the larger magnetic field and higher operating temperature of the cuprate superconductor $\mathbf{YBa_{2}Cu_{3}O_{7-δ}}$ (YBCO) in order to confine their plasma into a dense volume. A superconductor is a macroscopic quantum state that is protected from the metallic (resistive) state by an energy gap. Unfortunately, YBCO has an anisotropic gap, known as D-wave because it has the shape of a $\mathbf{d_{x^2-y^2}}$ chemical orbital. This D-wave gap means that poly-crystalline wire cannot be made because a few degree misalignment between grains in the wire leads to a drastic loss in its supercurrent carrying ability, and thereby its magnetic field limit. The superconductor industry has responded by growing nearly-single-crystal superconducting YBCO films on carefully prepared substrate tapes kilometers in length. Heroic development programs have made such tapes commercially available, but they are very expensive and delicate. MRI magnet superconductors, such as $\mathbf{NbTi}$ and $\mathbf{Nb_{3}Sn}$, are formed into poly-crystalline wires because they have an isotropic gap in the shape of an s chemical orbital (called S-wave) that makes them insensitive to grain misalignment. However, these materials are limited to lower magnetic fields and liquid-He temperatures. Here, we modified YBCO by doping the Y site with Ca and Ce atoms to form $\mathbf{(Y_{1-x-y}Ca_{x}Ce_{y})Ba_{2}Cu_{3}O_{7-δ}}$, and show evidence that it changes to an S-wave gap. Its superconducting transition temperature, $\mathbf{T_c}$, of $\mathbf{\sim 70K}$, while lower than that of D-wave YBCO at $\mathbf{\sim 90K}$, is easily maintained using common, economic cryogenic equipment.

cond-mat.supr-con↗

arXiv2017

Latent Room-Temperature T$_c$ in Cuprate Superconductors

The ancient phrase, "All roads lead to Rome" applies to Chemistry and Physics. Both are highly evolved sciences, with their own history, traditions, language, and approaches to problems. Despite all these differences, these two roads generally lead to the same place. For high temperature cuprate superconductors however, the Chemistry and Physics roads do not meet or even come close to each other. In this paper, we analyze the physics and chemistry approaches to the doped electronic structure of cuprates and find the chemistry doped hole (out-of-the-CuO$\mathrm{_2}$-planes) leads to explanations of a vast array of normal state cuprate phenomenology using simple counting arguments. The chemistry picture suggests that phonons are responsible for superconductivity in cuprates. We identify the important phonon modes, and show that the observed T$\mathrm{_c} \sim 100$ K, the T$\mathrm{_c}$-dome as a function of hole doping, the change in T$\mathrm{_c}$ as a function of the number of CuO$\mathrm{_2}$ layers per unit cell, the lack of an isotope effect at optimal T$\mathrm{_c}$ doping, and the D-wave symmetry of the superconducting Cooper pair wavefunction are all explained by the chemistry picture. Finally, we show that "crowding" the dopants in cuprates leads to a pair wavefunction with S-wave symmetry and T$\mathrm{_c}\approx280-390$ K. Hence, we believe there is enormous "latent" T$\mathrm{_c}$ remaining in the cuprate class of superconductors.

cond-mat.supr-con↗

arXiv2013

Resistance of High-Temperature Cuprate Superconductors

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. 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." 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, Hall effect, and magnetoresistance 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.

cond-mat.supr-con↗

arXiv2011

Origin of the Pseudogap in High-Temperature Cuprate Superconductors

Cuprate high-temperature superconductors exhibit a pseudogap in the normal state that decreases monotonically with increasing hole doping and closes at x \approx 0.19 holes per planar CuO2 while the superconducting doping range is 0.05 < x < 0.27 with optimal Tc at x \approx 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 \approx 0.05 the plaquettes percolate, so that the Cu dx2y2/O pσ orbitals inside the plaquettes now form a band of states along the percolating swath. This leads to metallic conductivity and below Tc 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.

cond-mat.supr-con↗

arXiv2010

Universal Properties of Cuprate Superconductors: Tc Phase Diagram, Room-Temperature Thermopower, Neutron Spin Resonance, and STM Incommensurability Explained in Terms of Chiral Plaquette Pairing

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 Tc phase diagram (superconductivity between \approx0.05 and \approx0.27 doping per CuO2 plane, and optimal Tc at \approx0.16), (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.

cond-mat.supr-con↗

arXiv2008

Equivalent electric circuit of a carbon nanotube based molecular conductor

We apply our first-principles method to simulate the transient electrical response through carbon nanotube based conductors under time-dependent bias voltages, and report the dynamic conductance for a specific system. We find that the electrical response of the carbon nanotube device can be mapped onto an equivalent classical electric circuit. This is confirmed by studying the electric response of a simple model system and its equivalent circuit.

physics.chem-ph↗

arXiv2007

Chiral Plaquette Polaron Theory of Cuprate Superconductivity

Ab-initio density functional calculations on explicitly doped La(2-x)Sr(x)CuO4 find doping creates localized holes in out-of-plane orbitals. A model for 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 dz2, and planar O psigma. This is in contrast to the assumption of hole doping into planar Cu dx2-y2 and O psigma orbitals as in the t-J model. Interaction of holes with the d9 spin background leads to chiral polarons with either a clockwise or anti-clockwise charge current. When the polaron plaquettes percolate through the crystal at x~0.05 for LaSrCuO, a Cu dx2-y2 and planar O psigma band is formed. Spin exchange Coulomb repulsion with chiral polarons leads to D-wave superconductivity. The equivalent of the Debye energy in phonon superconductivity is the maximum energy separation between a chiral polaron and its time-reversed partner. 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 LaSrCuO. The integrated imaginary susceptibility 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 is computed and is incommensurate with a separation distance from (pi,pi) given by ~(2pi)x. Coulomb scattering of the x2-y2 band with polarons leads to linear resistivity. Coupling of the x2-y2 band to the undoped Cu d9 spins leads to the ARPES pseudogap and its doping and temperature dependence.

cond-mat.supr-con↗

arXiv2004

Numerical study of resistivity of model disordered three-dimensional metals

We calculate the zero-temperature resistivity of model 3-dimensional disordered metals described by tight-binding Hamiltonians. Two different mechanisms of disorder are considered: diagonal and off-diagonal. The non-equilibrium 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, (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 relative to the lattice constant has a value between 0.5 and 2.0 and is strongly model dependent.

cond-mat.dis-nn↗

arXiv2001

Ab Initio Evidence for the Formation of Impurity d(3z^2-r^2) Holes in Doped La_{2-x}Sr_xCuO_4

Using the spin unrestricted Becke-3-Lee-Yang-Parr density functional, we computed the electronic structure of explicitly doped La_{2-x}Sr_xCuO_4 (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 CuO_6 octahedra adjacent to the Sr impurities. The nature of the impurity hole is A_{1g} in symmetry, formed primarily from the z^2 orbital on the Cu and p_z orbitals on the apical O's. There is a strong triplet coupling of this hole with the intrinsic B_{1g} Cu x^2-y^2/O1 p_{sigma} hole on the same site. Optimization of the c coordinate of the apical O's in the doped CuO_6 octahedron lead to an asymmetric anti-Jahn-Teller distortion of the O2 atoms toward the central Cu. In particular, the O2 atom between the Cu and Sr is displaced 0.26 A while the O2 atom between the Cu and La is displaced 0.10 A. Contrary to expectations, investigation of a 0.1 A 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-O2 distance revealed in x-ray absorption fine structure data.

cond-mat.supr-con↗

arXiv2001

The Antiferromagnetic Band Structure of La2CuO4 Revisited

Using the Becke-3-LYP functional, we have performed band structure calculations on the high temperature superconductor parent compound, La2CuO4. Under the restricted spin formalism (rho(alpha) equal to rho(beta)), the R-B3LYP band structure agrees well with the standard LDA band structure. It is metallic with a single Cu x2-y2/O p(sigma) band crossing the Fermi level. Under the unrestricted spin formalism (rho(alpha) not equal to rho(beta)), the UB3LYP band structure has a spin polarized antiferromagnetic solution with a band gap of 2.0 eV, agreeing well with experiment. This state is 1.0 eV (per formula unit) lower than that calculated from the R-B3LYP. 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 x2-y2 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 x2-y2/O p(sigma) and Cu z2/O' p(z). Since the Hohenberg-Kohn theorem requires the spin restricted and spin unrestricted calculations give exactly the same ground state energy and total density 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.

cond-mat.supr-con↗

arXiv1999

The ARPES Pseudogap and Anomalous Background of Underdoped BiSCO 2212 as Evidence for a Fermi Level Band Crossing

We show that the unusual observations of a pseudogap in the normal state of underdoped Bi_2Sr_2CaCu_2O_8+delta (BiSCO) using angle resolved photoemission spectroscopy (ARPES) is consistent with a new band structure for the cuprate superconductors in which the x2-y2 and z2 bands are seen to cross at the Fermi level. Limitations in the experimental method prevent the narrow 3D z2 band from being fully resolved, leading instead to a broad background with "stepfunction" character. As a consequence, the Fermi surface is mis-assigned and a pseudogap of approximately d-wave symmetry develops.

cond-mat.supr-con↗

arXiv1999

The NMR of High Temperature Superconductors without Anti-Ferromagnetic Spin Fluctuations

A microscopic theory for the NMR anomalies of the planar Cu and O sites in superconducting La_1.85Sr_0.15CuO_4 is presented that quantitatively explains the observations without the need to invoke anit-ferromagnetic spin fluctuations on the planar Cu sites and its significant discrepancy with the observed incommensurate neutron spin fluctuations. The theory is derived from the recently published ab-initio band structure calculations that correct LDA computations tendency to overestimate the self-coulomb repulsion for the half-filled Cu d_x2-y2 orbital for these ionic systems. The new band structure leads to two bands at the Fermi level with holes in the Cu d_z2 and apical O p_z orbitals in addition to the standard Cu d_x2-y2 and planar O p_sigma orbitals. This band structure is part of a new theory for the cuprates that explains a broad range of experiments and is based upon the formation of Cooper pairs comprised of a k up spin electron from one band and a -k down spin electron from another band (Interband Pairing Model).

cond-mat.supr-con↗

arXiv1999

Electron Correlation and the c-axis Dispersion of Cu d_z^2: a New Band Structure for High Temperature Superconductors

Previously we showed the major effect of electron correlation in the cuprate superconductors is to lower the energy of the Cu d_x^2-y^2/O p_sigma (x^2-y^2) band with respect to the Cu d_z^2/O' p_z (z^2) band. In our 2D Hubbard model for La_1.85Sr_0.15CuO_4 (LaSCO), the z^2 band is narrow and crosses the standard x^2-y^2 band just below the Fermi level. In this work, we introduce c-axis dispersion to the model and find the z^2 band to have considerable anisotropic 3D character. An additional hole-like surface opens up in the z^2 band at (0,0,2pi/c) which expands with doping. At sufficient doping levels, a symmetry allowed x^2-y^2/z^2 band crossing along the (0,0)-(pi,pi) direction of the Brillouin zone appears at the Fermi level. At this point, Cooper pairs between the two bands (e.g. (k uparrow x^2-y^2/k downarrow z^2)) can form, providing the basis for the Interband Pairing Theory of superconductivity in these materials.

cond-mat.supr-con↗

arXiv1997

Electronic Structure of La1.85Sr0.15CuO4: Characterization of a Fermi Level Band Crossing

We present the results of a new Hubbard model for optimally doped LaSrCuO. This model uses parameters derived from BLYP calculations on the cluster CuO6. It explicitly includes the Cu dx2-y2 and dz2 orbitals, the O psigma orbitals, and the apical O pz orbitals. When correlation is properly included in the Hubbard model, we find that there is a crossing of two bands in the vicinity of the Fermi level for the optimally doped superconductor. This crossing rigorously occurs along the (0,0)-(pi,pi) direction of the 2D Brillouin zone. The crossing arises due to the overlap of a broad "B1g" band dominated by Cu dx2-y2 character and a narrower "A1g" band dominated by Cu dz2 character. We conclude that optimal doping of LaSrCuO and related materials is achieved when the Fermi levels coincides with this crossing. At this point, formation of Cooper pairs between the two bands (i.e. inter-band or IBP) leads to superconductivity. Furthermore, using geometric considerations, we extend our conclusions to YBaCuO and offer a simple explanation for the seemingly complex behavior of Tc as a function of doping in this material. This behavior can be understood on the basis of multiple band crossings.

cond-mat.supr-con↗

arXiv1997

Inter-Band Pairing Theory of Superconductivity

A model for high temperature superconductors based on the idea of Cooper pairs comprised of electrons from different bands is studied. We propose that the two bands relevant for the cuprates are comprised of Cu dx2-y2, dz2, planar O psigma, and apical O pz orbitals. Along the diagonal, kx=ky in the Brillouin zone, the two band Fermi surfaces may cross. We associate the optimal doping for the highest Tc with this point because only in the vicinity of this touching point are inter-band Cooper pairs energetically possible. Due to the lack of time reversal invariance of an inter-band Cooper pair with itself, the standard interpretation of Josephson tunneling is altered such that the detailed nature of the single particle tunneling matrix elements contributes to the supercurrent. The dx2-y2 gap observations from Josephson tunneling are shown to arise from our model with pairing due to phonons. A Hubbard model is written down for the two bands at the Fermi energy with realistic parameters for LaSrCuO. The anomalous normal state features in the nmr are calculated and qualitatively explained as due to the character of the two bands in the vicinity of the crossing point. The Hall effect is calculated using standard Bloch-Boltzmann transport theory. The observed strong temperature dependence of the Hall coefficient is reproduced and is due to the strong reshaping of the current carrying band Fermi surface due to band repulsion with the other band for dopings very close to the Fermi surface touching point. Reasonable quantitative agreement is also obtained for the nmr and Hall effect. A linear resistivity at optimal doping is expected due to the proximity of the second band in k space which can strongly relax the current and the "smallness" of the current carrying Fermi surface.

cond-mat.supr-con↗