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Manfred Bucher

Publications and source records attributed to Manfred Bucher.

30 records · Page 2Linked to original sources
arXiv2017

Fermi arc, pseudogap and strange-metal phase in hole-dopd lanthanum cuprates

Hole doping of La_{2-x}Ae_xCuO_4 (Ae=Sr,Ba) and La_{2-y-x}Ln_ySr_xCuO_4 (Ln = Nd, Eu; y = 0.4, 0.2) introduces unidirectional charge density waves (CDWs) of incommensurability delta_c(x) in domains of the CuO_2 planes. A periodic structure, each CDW gives rise to a Bragg-reflection mirror of extension delta_c(x) that attaches to a nodal point Q on the planar diagonal in reciprocal space. This confines itinerant holes to a Fermi arc about Q, leaving a pseudogap along the remainder of the underlying Fermi surface. The length of the Fermi arc and the magnitude of the pseudogap both are determined by δ_c(x). The pseudogap closes when the Fermi arc reaches the antinodal symmetry points M. This is the case at a doping level x^*_0 = 0.182 for La_{2-x}Ae_xCuO_4 at T=0 (quantum critical point, QCP) and otherwise at a doping-dependent pseudogap temperature T^*(x) that marks the boundary between the compounds' pseudogap phase and strange-metal phase. The different value of the observed QCP in La_{2-y-x}Ln_ySr_xCuO_4, x^*_0 = 0.235, is attributed to extra magnetic order from Ln^{3+} ions with a finite magnetic moment instead of La^{3+} with none. The possibility of quantum oscillations in La_{2-y-x}Ln_ySr_xCuO_4 in the high-end doping interval of their pseudogap phase, 0.182 < x < 0.235, is raised. The strange-metal phase is interpreted as a consequence of conflicting Bragg reflection conditions for the crystals' itinerant charge carriers when boundaries of the BZ and the CDW mirrors coincide, frustrating umklapp processes of carrier-carrier scattering.

physics.gen-ph↗
arXiv2017

Universality of density waves in p-doped La2CuO4 and n-doped Nd2CuO4+y

The contribution of $O^{2-}$ ions to antiferromagnetism in $La_{2-x}Ae_xCuO_4$ ($Ae = Sr, Ba)$ is highly sensitive to doped holes. In contrast, the contribution of $Cu^{2+}$ ions to antiferromagnetism in $Nd_{2-x}Ce_xCuO_{4+y}$ is much less sensitive to doped electrons. The difference causes the precarious and, respectively, robust antiferromagnetic phase of these cuprates. The same sensitivities affect the doping dependence of the incommensurability of density waves, $δ(x)$. In the hole-doped compounds this gives rise to a doping offset for magnetic and charge density waves, $δ_{m,c}^p(x) \propto \sqrt{x-x_{0p}^N}$. Here $x_{0p}^N$ is the doping concentration where the Néel temperature vanishes, $T_N(x_{0p}^N) = 0$. No such doping offset occurs for density waves in the electron-doped compound. Instead, excess oxygen (necessary for stability in crystal growth) of concentration $y$ causes a different doping offset in the latter case, $δ_{m,c}^n(x) \propto \sqrt{x- 2y}$. The square-root formulas result from the assumption of superlattice formation through partitioning of the $CuO_2$ plane by pairs of itinerant charge carriers. Agreement of observed incommensurability $δ(x)$ with the formulas is very good for the hole-doped compounds and reasonable for the electron-doped compound. The deviation in the latter case may be caused by residual excess oxygen.

physics.gen-ph↗
arXiv2013

Superlattice origin of incommensurable density waves in La_{2-x}Ae_xCuO4 (Ae = Ba, Sr)

In line with the Coulomb-oscillator model of superconductivity, loop currents of excited 3s electrons from O^2- ions, passing in the CuO2 plane through nuclei of nearest-neighbor oxygen quartets, create the antiferromagnetic phase of undoped copper oxides. Holes, introduced by alkaline-earth doping of La2CuO4, destroy the loop currents, thereby weakening antiferromagnetism until it disappears at doping x = 0.02. Further doping of La_2-xAe_xCuO4 gives rise to incommensurate free-hole density waves whose wavelength is determined by the spacing of a doping superlattice. Modulating the ordering of the ions' magnetic moments, the charge-density wave, of incommensurability 2 delta, causes a magnetic density wave of incommensurability delta. The formula derived for delta(x) is in excellent agreement with data from X-ray diffraction and neutron scattering.

physics.gen-ph↗
arXiv2013

Coulomb-oscillator origin of superconductivity in p-doped copper oxides

Emergence, development and cessation of superconductivity in three representative compounds of copper oxide families---cation doped Ca_2-xNa_xCuO2Cl2 and La_2-xAe_xCuO4 (Ae = Ba, Sr), as well as oxygen enriched YBa2Cu3O_6+x ---are explained with the Coulomb-oscillator model of superconductivity. By the model, non-resistive current is carried by axial Coulomb oscillations of s electrons through neighbor nuclei---here excited 3s electrons from O^2- ions through next-nearest neighbor oxygen nuclei---if their accompanying lateral oscillation is sufficiently confined to prevent lateral overswing. Cation doping gives rise to a superlattice in the layers that sandwich each CuO2 plane. In Ca_2-xNa_xCuO2Cl2, having one CuO2 plane per unit cell, superconductivity emerges when laterally confined Coulomb oscillators start connecting along 6 x 6 superlattice domains (in units of planar lattice constants) and it peaks at 4 x 4 domains when, at doping x = 1/8, the superlattice is completed. With further doping a new, off-set superlattice grows. Its frustrating effect gradually reduces superconductivity to cessation. The same mechanism holds for La_2-xAe_xCuO4 which has two, staggered CuO2 planes per unit cell. The staggering causes superconducting frustration or boost between adjacent layer sandwiches. This results in a double hump of the transition temperature Tc(x), instead of a dome, with a deep furrow or dip at x = 1/8 for Ae = Ba or Sr, respectively. Oxygen enrichment of YBa2Cu3O_6+x indirectly leads to effective doping in the CuO2 planes themselves (Cu^2+ --> Cu^3+). The ionization of copper ions at the corners of planar unit cells determines whether lateral oscillations between next-nearest neighbor O^2- ions overswing (Tc = 0) or are confined to wide or narrow electron tracks. Their percolating connectivity gives rise to respective plateaus of Tc = 57 K and Tc = 90 K, and intermediate ramps.

physics.gen-ph↗
arXiv2013

Coulomb-oscillator explanation of striped STM images of superconductive copper oxides

Asymmetric scanning tunneling microscopy (STM) of the CuO2 plane of Ca2-xNaxCuO2Cl2, x = 0.125, shows a square domain structure with edge length four times the compound's lattice constant a0 (Cu-O-Cu distance). The domain structure is a direct consequence of the 4a0 by 4a0 superlattice formed by vertical Na+ pairs (oriented parallel to the crystal's c axis) that substitute Ca2+ ions. The surrounding O2- ions are displaced away from, and the Cu2+ ions toward the Na+ pairs. Contrary to the fourfold symmetry of the CuO2 plane, the stable displacement configuration has a twofold symmetry, dominated by large and, respectively, small displacement of opposite O2- ions being nearest neighbors to each vertical Na+ pair. The ion displacements give rise to sufficient squeeze of certain O2- ions that, by the Coulomb-oscillator model of superconductivity, prevents lateral overswing of their excited 3s electrons. The axial 3s oscillations are predominantly oriented in the directions of O2- ion displacements. The observed ladder pattern in the domains provides a direct imaging of the 3s Coulomb oscillators. The 'sidepieces' of the ladders correspond to long unidirectional pathways for 3s electrons in the CuO2 plane. They account for superconductivity. The findings lend support to the validity of the Coulomb-oscillator model of superconductivity.

physics.gen-ph↗
arXiv2013

Road to room-temperature superconductivity: A universal model

In a semiclassical view superconductivity is attributed exclusively to the advance of atoms' outer s electrons through the nuclei of neighbor atoms in a solid. The necessary progression of holes in the opposite direction has the electric and magnetic effect as if two electrons were advancing instead of each actual one. Superconductivity ceases when the associated lateral oscillation of the outer s electrons extends between neighbor atoms. If such overswing occurs already at T = 0, then the material is a normal conductor. Otherwise, lateral overswing can be caused by lattice vibrations at a critical temperature Tc or by a critical magnetic field Bc. Lateral electron oscillations are reduced - and Tc is increased - when the atoms of the outer s electrons are squeezed, be it in the bulk crystal, in a thin film, or under external pressure on the sample. The model is applied to alkali metals and alkali-doped fullerenes. Aluminum serves as an example of a simple metal with superconductivity. Application of the model to transition metals, intertransitional alloys and compounds of transition metals with other elements sheds light on the pattern of their critical temperature. More examples of the squeeze effect are provided by the superconductivity of PdH, MgB2, borocarbides, ferropnictides, and organic charge-transfer salts. The model also provides the superconduction mechanism in the oxide superconductors, exemplified by YBa2Cu3O7. Finally the model suggests which steps to take in order to reach superconductivity at room temperature and above.

physics.gen-ph↗
arXiv2008

Rise and fall of the old quantum theory

The old quantum theory of Bohr and Sommerfeld was abandonned for the wrong reason. Its contradictions were caused not by the orbit concept but by a mental barrier--the inconceivability that an electron might collide with the atomic nucleus. Removing that barrier resolves the theory's main failures--incorrect orbital momenta, He atom, H2+ molecule ion. The inclusion of electron oscillations through the nucleus--a concept called "Coulomb oscillator"--renders the old quantum theory consistent with quantum mechanics (although devoid of wave character). The triple success of the Bohr-Sommerfeld model is its correct description of the H atom (and one-electron ions) concerning (1) the energy levels Enl, (2) the orbital angular momenta Lnl--if corrected as Lnl^2 = l(l+1) hbar^2 and with the Coulomb oscillator included--and (3) the orbits' space quantization--with (Lnl)z = ml hbar. These achievements are succinctly represented by the principal, angular and magnetic quantum numbers (n, l, ml) and visualized by orbital ellipse geometry--major axis, vertex curvature, and tilt angle, respectively. Orbit geometry also accounts for the average orbit size. Moreover, the Coulomb oscillator provides a natural explanation of (isotropic) hyperfine interaction. The shortcomings of the old quantum theory lie in its neglect of three properties of particles--their spin, their wave nature and their quantum statistics. These deficiencies notwithstanding, the visual appeal of the Bohr-Sommerfeld model remains a pedagogical asset to complement the abstract character of quantum mechanics.

physics.hist-ph↗
arXiv2007

Coulomb oscillations as a remedy for the helium atom

The largest failure of the old, Bohr-Sommerfeld quantum theory was with the helium atom. It brought about the theory's demise. I show that this failure does not originate, as commonly believed, with the orbit concept per se. Instead, it was caused by the wrong choice of orbits, compounded by ignorance of the exclusion principle. Choosing semiclassical electron oscillations through the He nucleus, I calculate a singlet ground-state energy that rivals in accuracy with quantum-mechanical results. The same method reveals Bohr's historic energy value as the forbidden triplet ground state--a result beyond the reach of quantum mechanics. At the qualitative level, the concept of Coulomb oscillations visually explains the major features in the He double spectrum in terms of crossed or parallel orbit orientation.

physics.hist-ph↗
arXiv2006

Coulomb oscillation in the hydrogen atom and molecule ion

Semiclassical oscillation of the electron through the nucleus of the H atom yields both the exact energy and the correct orbital angular momentum for l=0 quantum states. Similarly, electron oscillation through the nuclei of H2+ accounts for a stable molecule ion with energy close to the quantum mechanical solution. The small discrepancy arises from the neglect of the electron's wave nature.

physics.hist-ph↗
arXiv2006

Bohr model without quantum jumps

Omission of Bohr's second postulate permits a derivation of spectral intensity. The transition amplitudes serve as upper bounds to quantum mechanical matrix elements. They also provide insight into the latter in terms of Sommerfeld ellipses and transition trajectories. The speed of a nascent photon in the region of the electron transiton is addressed and the orbit concept is reinterpreted.

physics.hist-ph↗