David Emin

Adjunct Professor of Physics and Astronomy
University of New Mexico
MSC07 4220
1919 Lomas Blvd NE
Albuquerque, NM 87131, USA

Phone: 505 232-2128
Fax: 505-277-1520
Email: emin@unm.edu

David Emin's Invited Publications

Summary of Principal Research

Dr. Emin’s research addressed a very wide range of subjects before they became fashionable. Much of this research is summarized in his 2013 book “Polarons.”

A slow-moving electronic charge carrier in condensed matter becomes self-trapped when it is bound within the potential well formed by displacements of the surrounding atoms. A (strong-coupling) polaron is the quasi-particle comprising a self-trapped electronic charge carrier taken together with the atomic displacements that produces the potential well within which it is bound.  The term polaron arose from initial considerations of a carrier self-trapping in polar materials (e.g. alkali halides).

Whereas electronic charge carriers in conventional semiconductors have effective masses that are less than or comparable to that of a free electron, polaron masses are much larger. As such, polaron mobilities are considerably smaller than the minimum mobility associated with conventional semiconductor transport, eh/mkT (e.g. 300 cm2/V-s for a free-electron mass at room temperature). For example (as exemplified below), room-temperature Hall mobilities of alkali halides and transition-metal oxides are less than 100 cm2/V-s.1-5

  1. HIGH TEMPERATURE MEASUREMENTS OF THE ELECTRON HALL MOBILITY IN THE ALKALI HALIDES, C. H. Seager and David Emin, Physical Review B 2, 3421 (1970).
  2. HIGH MOBILITY N-TYPE CHARGE CARRIERS IN LARGE SINGLE CRYSTALS OF ANATASE (TiO2), L. Forro, O. Chauvet, D. Emin, L. Zuppiroli, H. Berger and F. Levy, Journal of Applied Physics 75, 633-635 (1994).
  3. HIGH DENSITY TWO-DIMENSIONAL SMALL POLARON GAS IN A DELTA-DOPED MOTT INSULATOR, Daniel G. Ouellette, Pouya Moetakef, Tyler A. Cain, Susanne Stemmer, David Emin, S. James Allen, Scientific Reports 3, 3284 (November 21, 2013).
  4. ANISOTROPIC SMALL-POLARON HOPPING IN W:BiVO4 SINGLE CRYSTALS Alexander J. E. Rettie, William D. Chemelewski, Jeffry Lindemuth, John S. McCloy, Luke G. Marshall, Jianshi Zhou, David Emin and C. Buddie Mullins, Applied Physics Letters 106 022106 (2015). 
  5. UNRAVELLING SMALL-POLARON TRANSPORT IN METAL OXIDE PHOTOELECTRODES, Alexander J. E. Rettie, William D. Chemelewski, David Emin and C. Buddie Mullins, Perspective in Journal of Physical Chemistry Letters, 7, 471-479 (2016).

Polaron formation

Polarons formed through the long-range Coulomb interactions of electronic carriers with ions have finite spatial extents and are referred to as being large. However, polaron formation can also occur in covalent materials where carriers have short-range interactions with the atoms they contact. Polaron formation in these instances is dichotomous.1

That is, an electronic charge carrier either does not self-trap or it collapses to a minimal spatial extent and is designated as being small.2

Polaron formation also occurs for an electronic carrier constrained to move in less than three dimensions. Depending on the values of physical parameters, polaron formation on surfaces can produce delayed or spontaneous atomic desorption.3 A polaron formed on a highly anisotropic quasi-one-dimensional covalent chain is generally large.4

  1. ADIABATIC THEORY OF AN ELECTRON IN A DEFORMABLE CONTINUUM, David Emin and T. Holstein, Physical Review Letters 36, 323 (1976).
  2. SMALL POLARONS, David Emin in Physics Today, (American Institute of Physics, New York) 35 6, 34-40 (1982).
  3. STRAIN INDUCED LOCALIZATION AND ELECTRONICALLY STIMULATED DESORPTION/DISSOCIATION, D. R. Jennison and David Emin, Physical Review Letters 51, 1390 (1983).
  4. SELF-TRAPPING IN QUASI-ONE-DIMENSIONAL SOLIDS, David Emin, Physical Review B 33, 3973 (1986).

Phonon-assisted jump rates

Large polarons move coherently while small polarons move incoherently via a series of relatively slow thermally assisted hops. Thermally activated rates for jumps between severely localized states, e.g. small-polaron hops, usually become Arrhenius at temperatures above a significant fraction of that characterizing the associated phonons. The primary component of the associated activation energy arises from the electron-phonon interaction.1 By contrast, at extremely low temperatures the rate for a phonon assisted jump upward in energy also becomes Arrhenius but with an activation energy that is just the difference between the energies of the hop’s initial and final states. The low-temperature activation energy is usually much smaller than that found at high temperatures. Between these two limits the thermally assisted rate is non-Arrhenius.2-4 This non-Arrhenius behavior results from the progressive freezing out of multi-phonon jump processes as the temperature is lowered. This non-Arrhenius feature is a general feature of hopping transport which occurs in crystals as well as in disordered materials. Thus, contrary to a common assumption, its observation does not imply transport dominated by disorder.

Beyond widely utilized oversimplified models, localized carrier not only induces displacements of surrounding atoms’ equilibrium positions but lowers their vibration frequencies.5 This effect can significantly alter carriers’ conductivity and Seebeck coefficient.

  1. SEMICLASSICAL SMALL-POLARON HOPPING IN A GENERALIZED MOLECULAR-CRYSTAL MODEL, David Emin, Physical Review B 43, 11720-11724 (1991).
  2. PHONON-ASSISTED JUMP RATE IN NONCRYSTALLINE SOLIDS, David Emin, Physical Review Letters 32, 303 (1974).
  3. PHONON-ASSISTED TRANSITION RATES I:  OPTICAL-PHONON-ASSISTED HOPS, David Emin, Advances in Physics 24, 305-348 (1975).
  4. PHONON-ASSISTED HOPPING DUE TO INTERACTION WITH BOTH ACOUSTICAL AND OPTICAL PHONONS, E. G. Bergeron and David Emin, Physical Review B 15, 3667 (1977).
  5. FORMATION AND HOPPING MOTION OF MOLECULAR POLARONS, David Emin, Physical Review B 61, 14543-14553 (2000).

Correlated small-polaron hopping

The conventional view of hopping envisions charge carriers making occasional jumps between sites in vibratory equilibrium. However, small-polaron hopping is often too fast to be consistent with this view.1-3 Then hopping occurs in flurries in which a carrier executes rapid hopping interspersed with quiescent periods. Carriers’ rapid back-and-forth inter-site motion lowers the frequencies of the associated atoms. The hopping-induced reduction of atoms’ vibration frequencies enhances the associated vibrational entropy. In accord with the observed Meyer-Neldel compensation rule, the enhanced entropy with classical atomic vibrations is proportional to the activation energy of the high-temperature small-polaron mobility.4 In equilibrium, periods of rapid and slow motion compensate one another. However, injected carriers initially manifest a transient high-mobility which persists until equilibration is achieved.5

  1. CORRELATED SMALL POLARON HOPPING MOTION, David Emin, Physical Review Letters 25, 1751 (1970).
  2. VIBRATIONAL DISPERSION AND SMALL POLARON MOTION:  ENHANCED DIFFUSION, David Emin, Physical Review B 3, 1321 (1971).
  3. LATTICE RELAXATION AND SMALL POLARON HOPPING MOTION, David Emin, Physical Review B 4, 3639 (1971).
  4. GENERALIZED ADIABATIC POLARON HOPPING: MEYER-NELDEL COMPENSATION AND POOLE-FRENKEL BEHAVIOR, David Emin, Physical Review Letters 100, 166602 (2008).
  5. TRANSIENT SMALL-POLARON HOPPING MOTION, David Emin and A. M. Kriman, Physical Review B 34, 7278 (1986).

Semiconductors’ Seebeck coefficients

The Seebeck coefficient, the entropy transported with a charge carrier divided by its charge, depends on carriers’ interactions with surrounding atoms. These effects, neglected in textbook formulae, have been shown to be of qualitative importance in understanding semiconductors’ Seebeck coefficients.

The net vibrational energy transported with a phonon-assisted transition contributes to a carrier’s Seebeck coefficient. For transitions between severely localized states and delocalized states the transported vibrational energy is equal to the energy difference between these states. Thus, the carrier-related energy in the Seebeck is that of the delocalized state rather than the average between the two states.1

Electronic energy levels generally shift with temperature due to thermal expansion and electron-phonon interactions. Incorporation of these significant effects requires explicit consideration of the underlying interactions.2,3 The results differ qualitatively from simply inserting a temperature dependence on textbook formulae.

A charge carrier’s relaxation in response to atoms’ vibrations lowers their frequencies. The vibrational entropy transported with a carrier contributes to its Seebeck coefficient. This contribution is distinguished from standard contributions by its being independent of the carrier concentration.4

Measurements of the Seebeck coefficients of polarons in conducting polymers provides a measure of the widths of their bands of conducting states.5,6 Since the intrinsic width of a polaron band, always less than a vibration frequency, is small its measured width primarily arises from disorder. As the temperature is lowered a semiconductor’s large Seebeck coefficient drops toward zero as its chemical potential moves from outside to inside the band of states that dominate conduction. The temperature at which a semiconductor’s Seebeck coefficient drops toward zero provides a measure of the width of the band of conducting states.7

  1. THERMOELECTRIC POWER DUE TO ELECTRONIC HOPPING MOTION, David Emin, Physical Review Letters 35, 882 (1975).
  2. EFFECT OF TEMPERATURE-DEPENDENT BAND SHIFTS ON SEMICONDUCTOR TRANSPORT PROPERTIES, David Emin, Solid State Communications. 22, 409 (1977).
  3. EFFECT OF TEMPERATURE DEPENDENT ENERGY LEVEL SHIFTS ON A SEMICONDUCTOR'S PELTIER HEAT, David Emin, Physical Review B, 30, 5766 (1984).
  4. ENHANCED SEEBECK EFFECT FROM CARRIER-INDUCED VIBRATIONAL SOFTENING, David Emin, Physical Review B 59, 6205-6210 (1999).
  5. INSIGHT INTO THERMOLECTRIC PROPERTIES OF CONDUCTING POLYMERS: THE CASE OF POLY (3-HEXYL THIOPHENE).  Y. Xuan, X. Liu, S. Desbief, P. Leclère, M. Fahlman, R. Lazzaroni, M. Berggren, J. Cornil, D. Emin and X. Crispin, Physical Review B 82, 115454 (2010) [9 pages].
  6. APPROACHING DISORDER-FREE TRANSPORT IN HIGH MOBILITY CONJUGATED POLYMERS Deepak Venkateshvaran, Mark Nikolka, Aditya Sadhanala, Vincent Lemaur, Mateusz Zelazny, Michal Kepa, Michael Hurhangee, Auke Jisk Kronemeijer, Vincenzo Pecunia, Iyad Nasrallah, Igor Romanov, Katharina Broch, Iain McCulloch, David Emin, Yoann Olivier, Jerome Cornil, David Beljonne and Henning Sirringhaus, Nature Letters 515, 384-388 (2014)
  7. DETERMINING A HOPPING POLARON’S BANDWIDTH FROM ITS SEEBECK COEFFICIENT: MEASURING THE DISORDER ENERGY OF A NON-CRYSTALLINE SEMICONDUCTOR David Emin, Journal of Applied Physics, 119 045101 (2016).

Small-polaron Hall Effect

The Hall mobility measures the deflection of a charge carrier by a magnetic field. The small-polaron Hall Effect results from interference between the amplitudes for a carrier moving between the same initial and final sites via different paths. As such, the small-polaron Hall Effect depends on the geometry of the sites among which electronic carriers hop.1 Within the adiabatic regime electronic carriers readily adjust to atomic positions and the Hall Effect results from atomic motions depending on the magnetic field.2 Unlike the Hall mobility of conventional charge carriers, the small-polaron Hall mobility is usually considerably larger than the mobility which enters into the conductivity. Strikingly, the sign of the Hall Effect is often anomalous with a hole (electron) small-polaron being deflected by a magnetic field in the same sense as a classical negatively (positively) charged particle.3 Observation of these distinctive features facilitates identification of small polarons.

  1. THE HALL MOBILITY OF A SMALL POLARON IN A SQUARE LATTICE, David Emin, Annals of Physics (NY) 64, 336-395 (1971).
  2. STUDIES OF SMALL POLARON MOTION IV: ADIABATIC THEORY OF THE HALL EFFECT, David Emin and T. Holstein, Annals of Physics (N.Y.) 53, 439-520 (1969).
  3. THE SIGN OF THE HALL EFFECT IN HOPPING CONDUCTION, David Emin, Phil. Mag. 35, 1189-1198 (1977).

Thermally induced transport transitions

For an electronic charge carrier to avoid self-trapping and move with high mobility it must not linger near a site long enough to alter the motions of surrounding atoms.1,2 Since the average vibrational velocity of atoms increases with rising temperature, the dynamic stability of a free carrier also rises with increasing temperature. This effect provides a mechanism for a thermally stimulated transition from low-mobility small-polaron transport to higher mobility free-carrier transport.

  1. ENERGY SPECTRUM OF AN ELECTRON IN A PERIODIC DEFORMABLE LATTICE, David Emin, Physical Review Letters 28, 604 (1972).
  2. ON THE EXISTENCE OF FREE AND SELF-TRAPPED CARRIERS IN INSULATORS:  AN ABRUPT TEMPERATURE-DEPENDENT CONDUCTIVITY TRANSITION, David Emin, Advances in Physics 22, 57-116 (1973).

Small polarons in covalent glasses

Even modest disorder of a covalent glass can impede carriers’ motion sufficiently to trigger their collapse into small polarons.1 Indeed, contrary to widespread expectations, the charge transport in covalent glasses agrees with that expected of small polarons.2-6 As such, electrical conduction is by a relatively high density of small polarons that move with very low (<< 1 cm2/V-s) thermally activated mobilities. These mobilities are much lower than even the small anomalous signed Hall mobilities that are measured.7 The broad absorption bands centered near mid-gap of these glasses are associated with exciting self-trapped electronic carriers from the potential wells within which they are bound.8

Increasing the applied emf above a threshold value induces a carrier-density avalanche that precludes small-polaron formation. Switching small polarons into conventional carriers increases the conductivity.9

Annealing progressively introduces embedded crystallites with high-mobility carriers within an amorphous semiconductor with small-polaron conduction.10 The crystallites initially lower the conductivity by serving as extended traps before raising the conductivity as the crystallites grow in size and/or density.

  1. DISORDER-INDUCED SMALL-POLARON FORMATION, David Emin and M.-N. Bussac, Physical Review B 49, 14290-14300 (1994).
  2. SMALL POLARON HOPPING MOTION IN SOME CHALCOGENIDE GLASSES, David Emin, C. H. Seager and R. K. Quinn, Physical Review Letters 28, 813 (1972).
  3. ELECTRICAL TRANSPORT AND STRUCTURAL PROPERTIES OF BULK As-Te-I, As-Te-Ge AND As-Te CHALCOGENIDE GLASSES, C. H. Seager, David Emin and R. K. Quinn, Physical Review B 8, 4746 (1973).
  4. TRANSPORT PROPERTIES OF AMORPHOUS ANTIMONY TELLURIDE, S. A. Baily and David Emin, Physical Review B 73, 165211 (2006) 8 pages.
  5. HALL MOBILITY OF AMORPHOUS Ge2Sb2Te5, S. A. Baily, David Emin and Heng Li, Solid State Communications 139, 161-164 (2006) 4 pages.
  6. AMORPHOUS SEMICONDUCTORS, David Emin, Science 198, 881 (1977).
  7. THE SIGN OF THE HALL EFFECT IN HOPPING CONDUCTION, David Emin, Phil. Mag. 35, 1189-1198 (1977).
  8. OPTICAL PROPERTIES OF LARGE AND SMALL POLARONS AND BIPOLARONS, David Emin, Physical Review B 48 13691-13702 (1993).
  9. CURRENT-DRIVEN THRESHOLD SWITCHING OF A SMALL-POLARON SEMICONDUCTOR TO A METASTABLE CONDUCTOR, David Emin, Physical Review B 74, 035206 (2006) 10 pages.
  10. POLARON TRANSPORT OF AMORPHOUS SEMICONDUCTORS WITH EMBEDDED CRYSTALLITES, David Emin, Philosophical Magazine 99:10 1225-1239 (2019)

Polarons in magnetic semiconductors

Electrons’ transfer between magnetic sites depend on their spins’ alignments.1,2 A carrier in an antiferromagnet forms a magnetic polaron when it becomes bound within the energetic well produced by the spin realignments and ionic displacements its presence induces.3 The change of a magnet’s spin ordering alters the temperature dependence of its Seebeck coefficient.4-6

The collapse of shallow large-radius donors into deep small-polarons in the ferromagnetic insulator EuO is driven by thermally induced spin misalignments and impeded by applying a magnetic field.7-9 The collapse of large-radius donors eliminates conduction through them. A magnetic field suppresses this collapse thereby producing what is now called colossal (negative) magnetic resistance (CMR).

Contrary to the wide-spread presumption that mobile charge carriers induce ferromagnetism in doped LaMnO1, compensating p-type dopants with oxygen vacancies eliminates carriers thereby yielding a ferromagnetic insulator.10-11 Indeed, very early investigators concluded that the strain introduced by dopants, rather than their charge carriers, stabilize this material’s ferromagnetism. Small-polaron formation in doped LaMnO3 is implied by its Hall Effect sign anomalies.12,13 Iconoclastically, Hall Effect sign anomalies in these heavily doped materials are ascribed to carriers hopping between molecular orbitals that encompass Mn sites residing between adjacent dopants.14

  1. DOUBLE EXCHANGE AND SMALL-POLARON HOPPING IN MAGNETIC SEMICONDUCTORS, Nai Li Huang Liu and David Emin, Physical Review Letters 42, 71 (1979).
  2. SMALL-POLARON HOPPING IN MAGNETIC SEMICONDUCTORS, David Emin and N. L. Huang Liu, Physical Review B 24, 4788 (1983).
  3. CONTINUUM STUDIES OF MAGNETIC POLARONS AND BIPOLARONS IN ANTIFERROMAGNETS, David Emin and M. S. Hillery, Physical Review B 37, 4060 (1988).
  4. THERMOELECTRIC POWER OF SMALL-POLARONS IN MAGNETIC SEMICONDUCTORS, Nai-Li H. Liu and David Emin, Physical Review B. 30, 3250 (1984).
  5. PELTIER HEAT OF A SMALL POLARON IN A MAGNETIC SEMICONDUCTOR, N. H. Liu and David Emin, J. Appl. Phys. 57, 3213 (1985).
  6. LOW-TEMPERATURE PELTIER HEAT OF AN ITINERANT ELECTRON IN A FERROMAGNETIC SEMICONDUCTOR, N. L. H. Liu and David Emin, Physical Review B. 32, 2285 (1985).
  7. THERMALLY INDUCED ABRUPT COLLAPSE OF A SHALLOW DONOR STATE IN A FERROMAGNETIC SEMICONDUCTOR, David Emin, M. Hillery and N. L. H. Liu, Physical Review B Rapid Communications, 33, 2933-2936 (1986).
  8. THERMALLY INDUCED ABRUPT SHRINKING OF A DONOR STATE IN A FERROMAGNETIC SEMICONDUCTOR, David Emin, M. Hillery and N. L. H. Liu, Physical Review B 35, 641-652 (1987).
  9. EFFECT OF AN APPLIED MAGNETIC FIELD ON THE ABRUPT DONOR COLLAPSE IN A FERROMAGNETIC SEMICONDUCTOR, M. S. Hillery, D. Emin and N. H. Liu, Physical Review B 38, 9771-9777 (1988).
  10. METAL-SEMICONDUCTOR AND MAGNETIC TRANSITIONS IN COMPENSATED POLYCRYSTALLINE La1-xCaxMnO3-d WITH x = 0.20, 0.25, T. L. Aselage, D. Emin, S. S. McCready, E. L. Venturini, M.A. Rodriguez, J. A. Voigt and T. J. Headley, Physical Review B 68, 134448 (2003) 8 pages.
  11. EFFECT OF OXYGEN REDUCTION ON THE OPTICAL CONDUCTIVITY OF La0.75Ca0.25MnO3, A. Nucara, A. Perucchi, P. Calvani, T. Aselage, and D. Emin, Physical Review B 68, 174432 (2003) 7 pages.
  12. Jaime, H. T. Hardner, M. B. Salamon, M. Rubinstein, P. Dorsey and D. Emin, Physical Review Letters 78, 951-954 (1997).
  13. ANOMALOUS HALL EFFECT IN Gd-DOPED La2/3Ca1/3MnO3, M. Jaime, H. T. Hardner, M. B. Salamon, M. Rubinstein, P. Dorsey and D. Emin, J. of Applied Phys. 81 (8), 4958 (1997).
  14. POLARONS, David Emin, Cambridge University Press ISBN 978-0-521-51906-9 Hardback (2013) Sec. 5.2.

Icosahedral boron-rich solids

Boron-rich icosahedral solids have distinctive structures and bonding.1,2 Boron atoms residing at the twelve vertices of icosahedra are bound together by electrons that reside on its twenty faces. These B12 icosahedra generally garner two electrons to fill their bonding molecular orbitals.3,5 Boron atoms knocked from these structures during atomic bombardments exit as very small cations that are attracted to the enhanced negative charge on residual structurally stable “degraded” icosahedra. The spontaneous recombination of these constituents explains these solid’s extraordinary resistance to radiation damage.6,7 Solar-cell–like devices with doped icosahedral boron-rich semiconductors powered by nuclear decay products can survive while those utilizing conventional semiconductors cannot.8 Simple substitutional schemes suggest how icosahedral boron-rich insulators can be doped.9

Boron carbides, B12+xC3-x with 0.1 < x < 1.7, are mixed crystals formed as carbon and boron atoms substitute for one another within and between icosahedra.10,11

Increasing x introduces p-type electronic carriers that pair as singlets on icosahedra.12-15 These singlet bipolarons break apart as individual carriers execute high-temperature thermally activated inter-icosahedral hops.16,17 Low-temperature dielectric measurements also indicate phonon-assisted hopping with a covalent solid.18,19 Bipolaron formation among the four-fold degenerate frontier orbitals of icosahedra produces an exceptionally large softening of the associated atoms’ vibrations.20 Despite boron carbides being very hard with very high sound velocities, their thermal diffusivities fall dramatically to very low values as x is increased.21 This carrier-induced softening of atomic vibrations also generates a large distinctive x-independent contribution to the Seebeck coefficient.22,23 These unusual features combine to make boron carbides’ thermoelectric figure-of-merits orders of magnitude greater than those expected of conventional semiconductors. In addition, a small solid-state neutron detector was designed which exploits boron carbides’ remarkable properties.24

  1. ICOSAHEDRAL BORON-RICH SOLIDS, David Emin in Physics Today (American Institute of Physics, New York) 40, pp. 55-62, January 1987.
  2. ISOTOPE DEPENDENCIES OF RAMAN SPECTRA OF B12As2, B12P2, B12O2, and B12+x C3-x: BONDING OF INTERICOSAHEDRAL CHAINS, T. L. Aselage, D. R. Tallant and D. Emin, Physical Review B 56, 3122-3129 (1997).
  3. BIPOLARONS IN BORON ICOSAHEDRA, I. A. Howard, C. L. Beckel and David Emin, Physical Review B 35, 2929 (1987).
  4. BIPOLARONS IN BORON ICOSAHEDRA: EFFECTS OF CARBON SUBSTITUTION, I. A. Howard, C. L. Beckel and David Emin, Physical Review B 35, 9265 (1987).
  5. AB INITIO SCF CALCULATIONS ON BORANE ICOSAHEDRA WITH ZERO, ONE OR TWO SUBSTITUTED CARBON ATOMS, T. A. Green, A. C. Switendick and David Emin, Journal of Chemical Physics 89, 6815 (1988).
  6. DEFECT CLUSTERING AND SELF-HEALING OF ELECTRON-IRRADIATED BORON-RICH SOLIDS, M. Carrard, D. Emin and L. Zuppiroli, Physical Review B 51, 11270-11274 (1995).
  7. UNUSUAL PROPERTIES OF ICOSAHEDRAL BORON-RICH SOLIDS, David Emin, Journal of Solid State Chemistry 179, 2791-2798 (2006).
  8. BETA CELL DEVICE USING ICOSAHEDRAL BORIDE COMPOUNDS, Terrence L. Aselage and David Emin, Patent # 6,479,919 (November 12, 2002).
  9. BONDING AND DOPING OF SIMPLE ICOSAHEDRAL-BORIDE SEMICONDUCTORS, David Emin, Journal of Solid State Chemistry 177/4-5, 1619-1623 (2004).
  10. STRUCTURE AND SINGLE-PHASE REGIME OF BORON CARBIDES, David Emin, Physical Review B 38, 6041 (1988).
  11. BORON CARBIDE STRUCTURE BY RAMAN SPECTROSCOPY, D. R. Tallant, T. L. Aselage, A. N. Campbell and David Emin, Physical Review B 40, 5649 (1989).
  12. CONDUCTION MECHANISM IN BORON CARBIDES, C. Wood and David Emin, Physical Review B. 29, 4582 (1984).
  13. PRESSURE AND TEMPERATURE DEPENDENCES OF THE ELECTRONIC CONDUCTIVITY OF BORON CARBIDES, G. A. Samara, David Emin and C. Wood, Physical Review B. 32, 2315 (1985).
  14. MAGNETIC SUSCEPTIBILITY STUDY OF BORON CARBIDES, J. Azevedo, E. L. Venturini, D. Emin and C. Wood, Physical Review B. 32, 7970 (1985).
  15. PHOSPHORUS DOPING OF BORON CARBIDES, T. L. Aselage, D. Emin, G. A. Samara, D. R. Tallant, S. B. Van Deusen. M. O. Eatough, H. L. Tardy and E. L. Venturini, Physical Review B 48, 11759-11766 (1993)
  16. PAIR BREAKING IN SEMICLASSICAL SINGLET SMALL-BIPOLARON HOPPING, David Emin, Physical Review B, 53, 1260-1268 (1996).
  17. SPIN SUSCEPTIBILITY OF BORON CARBIDES: DISSOCIATION OF SINGLET SMALL-BIPOLARONS, O. Chauvet, D. Emin, L. Forro, L. Zuppiroli and T. L. Aselage, Physical Review B 53, 14450-14457 (1996).
  18. LOW-TEMPERATURE AC CONDUCTIVITY OF ADIABATIC SMALL-POLARONIC HOPPING IN DISORDERED SYSTEMS, David Emin, Physical Review B 46, 9419-9427 (1992).
  19. ac HOPPING CONDUCTIVITIES, DIELECTRIC CONSTANTS AND REFLECTIVITIES OF BORON CARBIDES, G. A. Samara, H. L. Tardy, E. L. Venturini, T. L. Aselage and D. Emin, Physical Review B 48, 1468-1477 (1993).
  20. SINGLET BIPOLARON FORMATION AMONG DEGENERATE ELECTRONIC ORBITALS: “SOFTENING” BIPOLARONS, David Emin, Physical Review B 61, 6069-6085 (2000).
  21. THERMAL CONDUCTIVITY OF BORON CARBIDES, C. Wood, David Emin and P. E. Gray, Physical Review B, 31, 6811 (1985).
  22. LARGE ENHANCEMENT OF BORON CARBIDES’ SEEBECK COEFFICIENTS THROUGH VIBRATIONAL SOFTENING, T. L Aselage, D. Emin, S. S. McCready and R. V. Duncan, Physical Review Letters 81, 2316-2319 (1998).
  23. CONDUCTIVITIES AND SEEBECK COEFFICIENTS OF BORON CARBIDES: “SOFTENING BIPOLARON” HOPPING, T. L. Aselage, D. Emin and S. S. McCready, Physical Review B 64, 054302 (2001) 8 pages.
  24. A PROPOSED BORON CARBIDE BASED SOLID-STATE NEUTRON DETECTOR, D. Emin and T. L. Aselage, Journal of Applied Physics 97, 013529 (2005).

Large-bipolarons’ formation and novel superconductivity

The atomic displacements induced by two like-charged carriers drives them toward merger. By itself, the long-range component of an ionic material’s electron-phonon interaction can offset most of a pair’s mutual Coulomb repulsion.1,2 The residual repulsion can be overcome with the addition of a sufficiently strong short-range component of the electron-phonon interaction. It is then energetically favorable for two charge carriers to form a singlet bipolaron. The bipolaron will be large unless the short-range electron-phonon interaction is strong enough to trigger its collapse into a small bipolaron. Large-bipolaron formation is promoted by the ratio of the material’s static to high-frequency dielectric constants rising above 2.1,2 Such large ratios of dielectric constants occur for materials with exceptionally displaceable ions, such as cuprate superconductors. Electron-correlation effects, neglected above, play a minor role in large-bipolaron formation.3,4

Large bipolarons have very large effective masses since they only move when the associated atoms move. These very large effective masses generate some unusual transport properties.5

Large-bipolarons’ polarizabilities lower the vibration frequencies of associated atoms,6,7 This effect generates a phonon-mediated attraction between large-bipolarons that facilitates their condensation into a liquid.8,9 This Bose liquid will be superconducting unless it solidifies upon further cooling. Large-bipolarons’ solidification commensurate with the underlying lattice will occur at carrier concentrations of 2/(5´5), 2/(4´4), and 2/(3´3) for orthorhombic Sr-doped La2CuO4.8,9 Atoms of the superconducting bipolaron liquid will execute synchronous zero-point vibrations.10 In particular, the lowest-energy and largest-amplitude zero-point vibrations for bipolaron holes on the out-of-plane non-bonding oxygen orbitals of a CuO2-plane will have d-symmetry.10

Distinctively, large bipolarons’ absorption has two separate contributions.11 The weak scattering of a large-bipolaron compresses its temperature-dependent Drude contribution to frequencies below those of associated phonons. A broad absorption band arising from exciting large-bipolarons’ self-trapped electronic carriers occurs above these phonon frequencies.

  1. FORMATION, MOTION AND HIGH-TEMPERATURE SUPERCONDUCTIVITY OF LARGE BIPOLARONS, David Emin, Physical Review Letters 62, 1544 (1989).
  2. FORMATION OF A LARGE SINGLET BIPOLARON, APPLICATION TO HIGH-TEMPERATURE BIPOLARONIC SUPERCONDUCTIVITY, David Emin and M. S. Hillery, Physical Review B 39, 6575-6593 (1989).
  3. ELECTRON CORRELATION EFFECTS IN ONE-DIMENSIONAL LARGE-BIPOLARON FORMATION, David Emin, Jun Ye and Charles L. Beckel, Physical Review B 46, 10710-10720 (1992).
  4. EFFECT OF ELECTRONIC CORRELATION ON THE SHAPE OF A LARGE BIPOLARON: FOUR-LOBED PLANAR LARGE BIPOLARON IN AN IONIC MEDIUM, David Emin, Physical Review B 52, 13874-13882 (1995).
  5. LARGE BIPOLARON TRANSPORT AND CUPRATE SUPERCONDUCTORS, David Emin, Physical Review B 45, 5525-5529 (1992).
  6. ORBITAL MAGNETISM OF SINGLET LARGE BIPOLARONS, David Emin, Physical Review B 43, 2633-2636 (1991).
  7. ADDITIONAL SHORT-WAVELENGTH VIBRATORY MODES OF A LARGE (BI)POLARON, David Emin, Physical Review B 43, 8610-8612 (1991).
  8. PHONON-MEDIATED ATTRACTION BETWEEN LARGE BIPOLARONS: CONDENSATION TO A LIQUID, David Emin, Physical Review Letters, 72 1052-1055 (1994).
  9. PHONON-MEDIATED ATTRACTION BETWEEN LARGE BIPOLARONS: CONDENSATION TO A LIQUID, David Emin, Physical Review B 49, 9157-9167 (1994).
  10. DYNAMIC d-SYMMETRY BOSE CONDENSATE OF A PLANAR-LARGE-BIPOLARON-LIQUID IN CUPRATE SUPERCONDUCTORS, David Emin, Philosophical Magazine 97, 2931-2945 (2017).
  11. IN-PLANE CONDUCTIVITY OF A LAYERED LARGE-BIPOLARON LIQUID David Emin, Philosophical Magazine 95, 918-934 (2015).

Repulsion between oppositely-charged large-polarons enhance solar-cell efficiencies

Materials with readily displaceable ions foster short-range repulsions between oppositely charged large polarons.1 The resulting suppression of the recombination of photo-induced oppositely charged large polarons offers the promise of novel high-efficiency solar cells.

  1. BARRIER TO RECOMBINATION OF OPPOSITELY CHARGED LARGE POLARONS, David Emin, Journal of Applied Physics 123, 055105 (2018).

Wannier-Stark localization and negative differential conductivity

Wannier argued that applying an electric field lifts the translational degeneracy of a crystal’s states thereby producing a series of localized states centered on the crystal’s sites. The energy differences between these non-degenerate states just equals the potential-energy differences generated by the applied electric field. Wannier’s argument only considered a singlet electronic band. However, these Wannier-Stark states survive for an infinite crystal (no edge effects) when all energy bands are considered.1,2 For strong enough electric fields phonon-assisted carrier transport between Wannier-Stark states reduces to small-polaron hopping.3 Negative differential conductivity, like that from phonon-bottlenecks in shallow-impurity conduction, occurs for extremely large electric fields.4

  • EXISTENCE OF WANNIER-STARK LOCALIZATION, David Emin and C. F. Hart, Physical Review B 36, 7353-7359 (1987).
  • TIME EVOLUTION OF A BLOCH ELECTRON IN A CONSTANT ELECTRIC FIELD, C. F. Hart and David Emin, Physical Review B 37, 6100-6104 (1988).
  • PHONON-ASSISTED HOPPING OF AN ELECTRON ON A WANNIER-STARK LADDER IN A STRONG ELECTRIC FIELD, David Emin and C. F. Hart, Physical Review B, 36, 2530-2546 (1987).
  • NEGATIVE DIFFERENTIAL CONDUCTIVITY IN SHALLOW IMPURITY HOPPING, David Emin and C. F. Hart, Physical Review B. 32, 6503 (1985).

Light interstitial diffusion in metals

The diffusion of light-interstitial atoms, such as hydrogen and its isotopes, in a metal differs from that of small polarons.1,2 In particular,

  1. The number of bound states supported within an interstitial potential well is much greater for a light atom than for an electron and
  2. The tunneling between states of adjacent interstitial potential wells is much less likely for a light atom than for an electron.

As a result, at all but the lowest temperatures, the diffusion constant for hydrogen in bcc metals falls as their isotopic mass increases. By contrast, the thermally activated small-polaron diffusion constant rises with increasing carrier mass as the analogue of the electron-phonon interaction falls.

  1. SMALL POLARONIC DIFFUSION OF LIGHT INTERSTITIALS IN bcc METALS, David Emin, M. I. Baskes and W. D. Wilson, Physical Review Letters 42, 791 (1979).
  2. POLARONS, David Emin, Cambridge University Press ISBN 978-0-521-51906-9 Hardback (2013) Chap. 17.

Solvated cation transport in excised brain tissue

Charge transport in living matter is often that of cations solvated by the alignment of surrounding water molecules. Usually the conductivity associated with such charge transport increases slowly with the frequency of an applied electric field.1,2 By contrast, the conductivities of lesion-laden samples excised from human brain decrease with applied frequency.3 This behavior is consistent with freeing cations from lesions, e.g. tumors, that trap cations.3

  1. IONIC CHARGE TRANSPORT BETWEEN BLOCKAGES: SODIUM CATION CONDUCTION IN FRESHLY EXCISED BULK BRAIN TISSUE David Emin, Massoud Akhtari, B. M. Ellingson and G. W. Mathern, AIP Advances 5, Issue 8, 087133 (2015).
  2. MEASURING THE LOCAL ELECTRICAL CONDUCTIVITY OF HUMAN BRAIN TISSUE Massoud Akhtari, David Emin, Benjamin M Ellingson, David Woodworth, Andrew Frew and Gary W. Mathern, Journal of Applied Physics 119, 064701(2016).
  3. ANOMALOUS FREQUENCY-DEPENDENT IONIC CONDUCTIVITY OF LESION-LADEN HUMAN-BRAIN TISSUE, David Emin, Massoud Akhtari, Aria Fallah, Harry Vinters, and Gary W. Mathern, Journal of Applied Physics 122, 154701 (2017).