arXiv subjects

David Emin

Publications and source records attributed to David Emin.

13 recordsLinked to original sources

arXiv2023

Large-bipolaron-liquids in cuprate superconductors

Exceptionally displaceable ions, evidenced by huge ratios of materials static to high-frequency dielectric constants, enable short-range electron-phonon interactions to stabilize large planar bipolarons. These large bipolarons are compact and strongly bound. By contrast, very diffuse and weakly bound large bipolarons result without the electron-phonon interactions short-range component. Cuprates superoxygen bipolarons are centered on four inward relaxing oxygen anions circumscribed by four outward relaxing copper cations. Concomitant oxygen to copper charge transfer converts them to spinless Cu1+ cations. The planar d symmetry of superoxygens' groundstate molecular orbital tracks the superoxygens' predominant zeropoint radial vibrations. These large bipolarons distinctive charge transport, absorption, magnetism, local atomic vibrations, condensation into a liquid and subsequent high-temperature superconductivity are consistent with cuprate superconductors' long-established unusual properties.

cond-mat.supr-con↗

arXiv2022

Polarons in condensed matter

Polarons are composite quasiparticles comprising electronic charge carriers taken together with the alterations they induce in surrounding condensed matter. Strong-coupling polarons form when electronic charge carriers become self-trapped: bound within potential wells stabilized by carriers presence. Distinctively, exciting these bound carriers generates broad absorption bands. Strong-coupling polarons are slow and massive since moving them requires atomic motion. Their transport differs qualitatively from that of conventional electronic charge carriers. Large (strong-coupling) polarons move coherently with mobilities that fall with rising temperature. These massive quasiparticles very weak scatterings by phonons produce much lower room-temperature mobilities than those permitted of conventional electronic charge carriers. Moreover, the long scattering times associated with large polarons weak scattering relegates their principal ac (Drude) transport to below phonon frequencies. Small (strong-coupling) polarons move incoherently with even lower thermally assisted mobilities. Strikingly, a magnetic field often deflects small polarons in the opposite sense than it does conventional charge carriers, thereby producing anomalously signed Hall Effects. In exceptional circumstances charge carriers self-trap in pairs thereby forming large and small bipolarons. Some transport features distinguish them from polarons. Interference between carrier-induced atomic displacement patterns produce attractive interactions between like-charged polarons and short-range repulsive interactions between oppositely charged polarons.

cond-mat.str-el↗

arXiv2022

Distinctive ionic transport of freshly excised human epileptogenic brain tissue

Epileptogenic lesions have higher concentrations of sodium than does normal brain tissue. Such lesions are palpably recognized by a surgeon and then excised in order to eliminate epileptic seizures with their associated abnormal electrical behavior. Here we study the frequency-dependent electrical conductivities of lesion-laden tissues excised from the brains of epilepsy patients. The low-frequency (< 1000 Hz) conductivity of biological tissue primarily probes extracellular solvated sodium-cations traveling parallel to membranes within regions bounded by blockages. This conductivity rises monotonically toward saturation as the frequency surpasses the rate with which diffusing solvated sodium cations encounter blockages. We find that saturation occurs at dramatically higher frequencies in excised brain tissue containing epileptogenic lesions than it does in normal brain tissue. By contrast, such an effect is not reported for tumors embedded in other excised biological tissue. All told, epileptogenic lesions generate frequency-dependent conductivities that differ qualitatively from those of both normal brain tissues and tumors.

q-bio.TO↗

arXiv2020

Large (bi)polarons for novel energy conversion and superconductivity

Materials containing high densities of exceptionally displaceable ions (e.g. perovskites) have extremely large ratios of their static to high-frequency dielectric constants, > 2. Large polarons form in such materials as their electronic charge carriers self-trap by displacing surrounding ions. Large polarons are very heavy-massed slow-moving quasi-particles that are very weakly scattered by ambient phonons. Large-polaron mobilities, e.g. 1 cm2/V-sec at 300 K, are much smaller than the minimum possible for conventional electronic charge carriers. The minimum mobility for an itinerant charge carrier of effective mass m, eh/mkT, occurs when its mean-free-path falls to its de Broglie wavelength, e.g. 300 cm2/V-sec at room temperature for m equaling the free-electron mass. Distinctively, large-polarons frequency-dependent conductivities consist of two contributions that separate as the temperature is reduced. Large polarons Drude-like contributions are relegated to frequencies below those of characteristic phonons. Contributions from excitations of large polarons self-trapped electronic carriers occur above those of characteristic phonons. Oppositely charged large polarons repel one another at short range. The resulting suppressed recombination facilitates exceptionally efficient solar cells. Large polarons of the same charge attract one another at short range to enable their real-space pairing into singlet bipolarons. Additional attractions between large bipolarons facilitates their condensation into liquids that can exhibit superconductivity.

cond-mat.supr-con↗

arXiv2019

Proposed high-power beta cells from MgAlB14-type icosahedral-boron semiconductors

Beta cells generate electric power as carrier-producing beta irradiation from incorporated radioisotopes bombard a series of p-n-junctions. However, radiation damage to the semiconductors commonly used in solar cells limits beta cells to extremely weak irradiations that generate concomitantly miniscule electric powers, e.g. micro-Watts. By contrast, beta cells that generate many orders-of-magnitude larger powers are possible with icosahedral boron-rich semiconductors since their bombardment-induced atomic displacements spontaneously self-heal. Furthermore, substitutions for Mg and Al atoms of icosahedral-boron-rich semiconductors based on the MgAlB14 structure can produce p-n junctions as electron transfers from doping-induced interstitial extra-icosahedral atoms convert some normally p-type materials to n-type. Moreover, electron-phonon interactions of the resulting readily displaceable interstitial cations with charge carriers foster their forming large polarons. Oppositely charged polarons repel one another at short range. These repulsions suppress the recombination of n-type with p-type polarons thereby increasing the beta-cell efficiency. All told, use of these icosahedral boron-rich semiconductors could enable beta cells with electric powers that are many orders of magnitude larger than those of existing beta cells. This development opens a new avenue for generating electricity from nuclear decays.

physics.app-ph↗

arXiv2018

Polaron transport of amorphous semiconductors with embedded crystallites

Near-room-temperature electronic transport of annealing induced semiconducting crystallites embedded within its amorphous counterpart is treated within the effective-medium approach. As such, the mixtures transport coefficients become smooth functions of those of its two components. Carrier mobilities in the crystallites are assumed much larger than those of the amorphous phase. Nonetheless, crystallites act as macroscopic traps when their carriers energies lie below those in the amorphous phase. Then the mixtures dc conductivity falls below that of the amorphous phase at low enough carrier concentrations. However, with increasing carrier concentration the shifting chemical potential diminishes this trapping effect enabling crystallites larger mobilities to drive the mixtures electrical conductivity above that of the amorphous phase. Meanwhile the Seebeck coefficient remains insensitive to the annealing-induced introduction and growth of embedded crystallites. These features are qualitatively similar to those reported for an amorphous organic polymer with annealing-induced embedded crystallites.

cond-mat.mes-hall↗

arXiv2017

Barrier to recombination of oppositely charged large polarons

Electronic charge carriers in ionic materials can self-trap to form large polarons. Interference between the ionic displacements associated with oppositely charged large polarons increases as they approach one another. Initially this interference produces an attractive potential that fosters their merger. However, for small enough separations this interference generates a repulsive interaction between oppositely charged large polarons. In suitable circumstances this repulsion can overwhelm their direct Coulomb attraction. Then the resulting net repulsion between oppositely charged large polarons constitutes a potential barrier which impedes their recombination.

cond-mat.other↗

arXiv2017

Anomalous frequency-dependent ionic conductivity of lesion-laden human-brain tissue

We study the effect of lesions on our four-electrode measurements of the ionic conductivity of (1 cm3) samples of human brain excised from patients undergoing pediatric epilepsy surgery. For most (about 94 %) samples the low-frequency ionic conductivity rises upon increasing the applied frequency. We attributed this behavior to the long-range (0.4 mm) diffusion of solvated sodium cations before encountering impenetrable blockages such as cell membranes, blood vessels and cell walls. By contrast, the low-frequency ionic conductivity of some (6 %) brain tissue samples falls with increasing applied frequency. We attribute this unusual frequency-dependence to the electric-field induced liberation of sodium cations from traps introduced by the unusually severe pathology observed in samples from these patients. Thus, the anomalous frequency-dependence of the ionic conductivity indicates trap-producing brain lesions.

physics.med-ph↗

arXiv2017

Dynamic d-symmetry Bose condensate of a planar-large-bipolaron-liquid in cuprate superconductors

Planar large-bipolarons can form if the ratio of the surrounding mediums static to high-frequency dielectric constants is especially large. A large-bipolaron in p-doped La2CuO4 is modeled as two electrons being removed from the out-of-plane orbitals of four oxygen ions circumscribed by four copper ions of a CuO2 layer. These oxygen dianions relax inwardly as they donate electrons to the surrounding outwardly relaxing copper cations. This charge transfer generates the strong in-plane electron-lattice interaction needed to stabilize a large-bipolaron with respect to decomposing into polarons. The lowest-energy radial in-plane optic vibration of a large-bipolarons four core oxygen ions with their associated electronic charges has d-symmetry. Electronic relaxation in response to multiple large-bipolarons atomic vibrations lowers their frequencies to generate a phonon-mediated attraction among them which fosters their condensation into a liquid. This liquid features distinctive transport and optical properties. A large-bipolaron liquids superconductivity can result when it undergoes a Bose condensation yielding macroscopic occupation of its ground-state. The synchronized vibrations of large-bipolarons core-oxygen ions with their electronic charges generate this Bose condensates dynamic global d-symmetry.

cond-mat.supr-con↗

arXiv2016

Determining a hopping polarons bandwidth from its Seebeck coefficient: Measuring the disorder energy of a non-crystalline semiconductor

Charge carriers that execute multi-phonon hopping generally interact strongly enough with phonons to form polarons. A polarons sluggish motion is linked to slowly shifting atomic displacements that severely reduce the intrinsic width of its transport band. Here a means to estimate hopping polarons bandwidths from Seebeck-coefficient measurements is described. The magnitudes of semiconductors Seebeck coefficients are usually quite large (greater than 86 microvolts/K) near room temperature. However, in accord with the third law of thermodynamics, Seebeck coefficients must vanish at absolute zero. Here the transition of the Seebeck coefficient of hopping polarons to its low-temperature regime is investigated. The temperature and sharpness of this transition depends on the concentration of carriers and on the width of their transport band. This feature provides a means of estimating the width of a polarons transport band. Since the intrinsic broadening of polaron bands is very small, less than the characteristic phonon energy, the net widths of polaron transport bands in disordered semiconductors approach the energetic disorder experienced by their hopping carriers, their disorder energy.

cond-mat.mtrl-sci↗

arXiv2016

Large bipolarons and oxide superconductivity

Large-bipolaron superconductivity is plausible with carrier densities well below those of conventional metals. Bipolarons form when carriers self-trap in pairs. Coherently moving large-bipolarons require extremely large ratios of static to optical dielectric-constants. The mutual Coulomb repulsion of a planar large-bipolarons paired carriers drives it to a four-lobed shape. A phonon-mediated attraction among large-bipolarons propels their condensation into a liquid. This liquids excitations move slowly with a huge effective mass. Excitations concomitant weak scattering by phonons produces a moderate low-temperature dc resistivity that increases linearly with rising temperature. With falling temperature an energy gap opens between large-bipolarons excitations and those of their self-trapped electronic carriers.

cond-mat.supr-con↗

arXiv2014

In-plane conductivity of a layered large-bipolaron liquid

Distinctive normal-state properties of cuprate superconductors follow from their charge carriers forming a large-bipolaron liquid. The very weak scattering of the slow-moving heavy-massed excitations of the liquid by acoustic phonons yields a scattering rate that is less than the Debye frequency. The moderate liquid mobility, greater than 1 cm2/V-sec at 300 K, results as the weak scattering of the liquid compensates for its large mass. In resolution of a long-standing dilemma, the dc resistivity resulting from scattering by acoustic phonons remains nearly proportional to temperature to well below the Debye temperature. Above the Debye frequency the frequency-dependent conductivity is dominated by excitation and photo-ionization of the self-trapped electronic carriers of the large-bipolarons. Below the Debye frequency the frequency-dependent conductivity is dominated by the Drude-like collective motion of the large-bipolaron liquid. The gap between these two domains sharpens with decreasing temperature as phonon scattering of the liquid diminishes. The high-frequency electronic excitations survive in the superconducting state.

cond-mat.supr-con↗

arXiv2014

High-density two-dimensional small polaron gas in a delta-doped Mott insulator

Heterointerfaces in complex oxide systems open new arenas in which to test models of strongly correlated material, explore the role of dimensionality in metal-insulator-transitions (MITs) and small polaron formation. Close to the quantum critical point Mott MITs depend on band filling controlled by random disordered substitutional doping. Delta-doped Mott insulators are potentially free of random disorder and introduce a new arena in which to explore the effect of electron correlations and dimensionality. Epitaxial films of the prototypical Mott insulator GdTiO3 are delta-doped by substituting a single (GdO)+1 plane with a monolayer of charge neutral SrO to produce a two-dimensional system with high planar doping density. Unlike metallic SrTiO3 quantum wells in GdTiO3 the single SrO delta-doped layer exhibits thermally activated DC and optical conductivity that agree in a quantitative manner with predictions of small polaron transport but with an extremely high two-dimensional density of polarons, ~ 7E14 cm-2

cond-mat.str-el↗