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1.
Phys Rev Lett ; 125(23): 236803, 2020 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-33337202

RESUMO

We report on transport signatures of hidden quantum Hall stripe (hQHS) phases in high (N>2) half-filled Landau levels of Al_{x}Ga_{1-x}As/Al_{0.24}Ga_{0.76}As quantum wells with varying Al mole fraction x<10^{-3}. Residing between the conventional stripe phases (lower N) and the isotropic liquid phases (higher N), where resistivity decreases as 1/N, these hQHS phases exhibit isotropic and N-independent resistivity. Using the experimental phase diagram, we establish that the stripe phases are more robust than theoretically predicted, calling for improved theoretical treatment. We also show that, unlike conventional stripe phases, the hQHS phases do not occur in ultrahigh mobility GaAs quantum wells but are likely to be found in other systems.

2.
Nano Lett ; 17(8): 4634-4642, 2017 08 09.
Artigo em Inglês | MEDLINE | ID: mdl-28704060

RESUMO

Networks of ligand-free semiconductor nanocrystals (NCs) offer a valuable combination of high carrier mobility and optoelectronic properties tunable via quantum confinement. In principle, maximizing carrier mobility entails crossing the insulator-metal transition (IMT), where carriers become delocalized. A recent theoretical study predicted that this transition occurs at nρ3 ≈ 0.3, where n is the carrier density and ρ is the interparticle contact radius. In this work, we satisfy this criterion in networks of plasma-synthesized ZnO NCs by using intense pulsed light (IPL) annealing to tune n and ρ independently. IPL applied to as-deposited NCs increases ρ by inducing sintering, and IPL applied after the NCs are coated with Al2O3 by atomic layer deposition increases n by removing electron-trapping surface hydroxyls. This procedure does not substantially alter NC size or composition and is potentially applicable to a wide variety of nanomaterials. As we increase nρ3 to at least twice the predicted critical value, we observe conductivity scaling consistent with arrival at the critical region of a continuous quantum phase transition. This allows us to determine the critical behavior of the dielectric constant and electron localization length at the IMT. However, our samples remain on the insulating side of the critical region, which suggests that the critical value of nρ3 may in fact be significantly higher than 0.3.

3.
Phys Rev Lett ; 118(10): 106801, 2017 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-28339226

RESUMO

Stimulated by experimental advances in electrolyte gating methods, we investigate theoretically percolation in thin films of inhomogeneous complex oxides, such as La_{1-x}Sr_{x}CoO_{3} (LSCO), induced by a combination of bulk chemical and surface electrostatic doping. Using numerical and analytical methods, we identify two mechanisms that describe how bulk dopants reduce the amount of electrostatic surface charge required to reach percolation: (i) bulk-assisted surface percolation and (ii) surface-assisted bulk percolation. We show that the critical surface charge strongly depends on the film thickness when the film is close to the chemical percolation threshold. In particular, thin films can be driven across the percolation transition by modest surface charge densities. If percolation is associated with the onset of ferromagnetism, as in LSCO, we further demonstrate that the presence of critical magnetic clusters extending from the film surface into the bulk results in considerable enhancement of the saturation magnetization, with pronounced experimental consequences. These results should significantly guide experimental work seeking to verify gate-induced percolation transitions in such materials.

4.
Nat Mater ; 15(3): 299-303, 2016 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26618885

RESUMO

To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.


Assuntos
Condutividade Elétrica , Nanopartículas Metálicas/química , Semicondutores , Membranas Artificiais , Nanotecnologia/métodos , Tamanho da Partícula , Fósforo/química , Silício/química
5.
Phys Rev Lett ; 109(17): 176801, 2012 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-23215212

RESUMO

As-grown topological insulators (TIs) are typically heavily doped n-type crystals. Compensation by acceptors is used to move the Fermi level to the middle of the band gap, but even then TIs have a frustratingly small bulk resistivity. We show that this small resistivity is the result of band bending by poorly screened fluctuations in the random Coulomb potential. Using numerical simulations of a completely compensated TI, we find that the bulk resistivity has an activation energy of just 0.15 times the band gap, in good agreement with experimental data. At lower temperatures activated transport crosses over to variable range hopping with a relatively large localization length.

6.
Phys Rev Lett ; 109(12): 126805, 2012 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-23005975

RESUMO

The Coulomb gap in the single-particle density of states (DOS) is a universal consequence of electron-electron interaction in disordered systems with localized electron states. Here we show that in arrays of monodisperse metallic nanocrystals, there is not one but three identical adjacent Coulomb gaps, which together form a structure that we call a "Coulomb gap triptych." We calculate the DOS and the conductivity in two- and three-dimensional arrays using a computer simulation. Unlike in the conventional Coulomb glass models, in nanocrystal arrays the DOS has a fixed width in the limit of large disorder. The Coulomb gap triptych can be studied via tunneling experiments.

7.
Phys Rev E Stat Nonlin Soft Matter Phys ; 83(5 Pt 2): 056102, 2011 May.
Artigo em Inglês | MEDLINE | ID: mdl-21728600

RESUMO

Electric double-layer supercapacitors are a fast-rising class of high-power energy storage devices based on porous electrodes immersed in a concentrated electrolyte or ionic liquid. As yet there is no microscopic theory to describe their surprisingly large capacitance per unit volume (volumetric capacitance) of ~100 F/cm(3), nor is there a good understanding of the fundamental limits on volumetric capacitance. In this paper we present a non-mean-field theory of the volumetric capacitance of a supercapacitor that captures the discrete nature of the ions and the exponential screening of their repulsive interaction by the electrode. We consider analytically and via Monte Carlo simulations the case of an electrode made from a good metal and show that in this case the volumetric capacitance can reach the record values. We also study how the capacitance is reduced when the electrode is an imperfect metal characterized by some finite screening radius. Finally, we argue that a carbon electrode, despite its relatively large linear screening radius, can be approximated as a perfect metal because of its strong nonlinear screening. In this way the experimentally measured capacitance values of ~100 F/cm(3) may be understood.

8.
Phys Rev E Stat Nonlin Soft Matter Phys ; 82(1 Pt 2): 016107, 2010 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-20866689

RESUMO

Mean-field theories claim that the capacitance of the double layer formed at a metal/ionic conductor interface cannot be larger than that of the Helmholtz capacitor, whose width is equal to the radius of an ion. However, in some experiments the apparent width of the double layer capacitor is substantially smaller. We propose an alternate non-mean-field theory of the ionic double layer to explain such large capacitance values. Our theory allows for the binding of discrete ions to their image charges in the metal, which results in the formation of interface dipoles. We focus primarily on the case where only small cations are mobile and other ions form an oppositely charged background. In this case, at small temperature and zero applied voltage dipoles form a correlated liquid on both contacts. We show that at small voltages the capacitance of the double layer is determined by the transfer of dipoles from one electrode to the other and is therefore limited only by the weak dipole-dipole repulsion between bound ions so that the capacitance is very large. At large voltages the depletion of bound ions from one of the capacitor electrodes triggers a collapse of the capacitance to the much smaller mean-field value, as seen in experimental data. We test our analytical predictions with a Monte Carlo simulation and find good agreement. We further argue that our "one-component plasma" model should work well for strongly asymmetric ion liquids. We believe that this work also suggests an improved theory of pseudocapacitance.

9.
Phys Rev Lett ; 104(12): 128302, 2010 Mar 26.
Artigo em Inglês | MEDLINE | ID: mdl-20366568

RESUMO

The capacitance of the double layer formed at a metal/ionic-conductor interface can be remarkably large, so that the apparent width of the double layer is as small as 0.3 A. Mean-field theories fail to explain such large capacitance. We propose an alternate theory of the ionic double layer which allows for the binding of discrete ions to their image charges in the metal. We show that at small voltages the capacitance of the double layer is limited only by the weak dipole-dipole repulsion between bound ions, and is therefore very large. At large voltages the depletion of bound ions from one of the capacitor electrodes triggers a collapse of the capacitance to the mean-field value.

10.
Phys Rev E Stat Nonlin Soft Matter Phys ; 82(5 Pt 2): 056102, 2010 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-21230540

RESUMO

We use Monte Carlo simulations to examine the simplest model of a room-temperature ionic liquid (RTIL), called the "restricted primitive model," at a metal surface. We find that at moderately low temperatures the capacitance of the metal-RTIL interface is so large that the effective thickness of the electrostatic double layer is up to three times smaller than the ion radius. To interpret these results we suggest an approach which is based on the interaction between discrete ions and their image charges in the metal surface and which therefore goes beyond the mean-field approximation. When a voltage is applied across the interface, the strong image attraction causes counterions to condense onto the metal surface to form compact ion-image dipoles. These dipoles repel each other to form a correlated liquid. When the surface density of these dipoles is low, the insertion of an additional dipole does not require much energy. This leads to a large capacitance C that decreases monotonically with voltage V, producing a "bell-shaped" curve C(V). We also consider what happens when the electrode is made from a semimetal rather than a perfect metal. In this case, the finite screening radius of the electrode shifts the reflection plane for image charges to the interior of the electrode, and we arrive at a "camel-shaped" C(V). These predictions seem to be in qualitative agreement with experiment.

11.
J Phys Condens Matter ; 21(42): 424104, 2009 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-21715839

RESUMO

Screening of a strongly charged macroion by its multivalent counterions cannot be described in the framework of a mean-field Poisson-Boltzmann (PB) theory because multivalent counterions form a strongly correlated liquid (SCL) on the surface of the macroion. It was predicted that a distant counterion polarizes the SCL as if it were a metallic surface and creates an electrostatic image. The attractive potential energy of the image is the reason why the charge density of counterions decreases faster with distance from the charged surface than in PB theory. Using the Monte Carlo method to find the equilibrium distribution of counterions around the macroion, we confirm the existence of the image potential energy. It is also shown that, due to the negative screening length of the SCL, -2ξ, the effective metallic surface is actually above the SCL by |ξ|.

12.
Phys Rev E Stat Nonlin Soft Matter Phys ; 78(3 Pt 1): 032901, 2008 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-18851091

RESUMO

Translocation of a single-stranded DNA molecule through genetically engineered alpha -hemolysin channels with positively charged walls is studied. It is predicted that transport properties of such channels are dramatically different from neutral wild-type alpha -hemolysin channels. We assume that the wall charges compensate a fraction x of the bare charge q_{b} of the DNA piece residing in the channel. Our predictions are as follows. (i) At small concentration of salt the blocked ion current decreases with x . (ii) The effective charge q_{s} of the DNA piece, which is very small at x=0 (neutral channel) grows with x and at x=1 reaches q_{b} . (iii) The rate of DNA capture by the channel grows exponentially with x . Our theory is also applicable to translocation of a double-stranded DNA molecular in narrow solid state nanopores with positively charged walls.


Assuntos
Biofísica/métodos , DNA/química , Nanoestruturas/química , Nanotecnologia/métodos , DNA de Cadeia Simples/química , Eletroquímica/métodos , Canais Iônicos/química , Íons , Modelos Biológicos , Eletricidade Estática
13.
Phys Rev E Stat Nonlin Soft Matter Phys ; 75(5 Pt 1): 051901, 2007 May.
Artigo em Inglês | MEDLINE | ID: mdl-17677092

RESUMO

Many viruses self-assemble from a large number of identical capsid proteins with long flexible N-terminal tails and single-stranded (ss) RNA. We study the role of the strong Coulomb interaction of positive N-terminal tails with ssRNA in the kinetics of in vitro virus self-assembly. Capsid proteins stick to the unassembled chain of ssRNA (which we call an "antenna") and slide on it toward the assembly site. We show that at excess of capsid proteins such one-dimensional diffusion accelerates self-assembly more than ten times. On the other hand at excess of ssRNA, the antenna slows self-assembly down. Several experiments are proposed to verify the role of the ssRNA antenna.


Assuntos
Modelos Biológicos , Modelos Químicos , RNA Viral/química , RNA Viral/fisiologia , Vírion/química , Vírion/fisiologia , Montagem de Vírus/fisiologia , Simulação por Computador , Modelos Moleculares , RNA Viral/ultraestrutura
14.
Phys Rev E Stat Nonlin Soft Matter Phys ; 75(2 Pt 1): 021906, 2007 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-17358366

RESUMO

Translocation of a single stranded DNA (ssDNA) through an alpha -hemolysin channel in a lipid membrane driven by applied transmembrane voltage V was extensively studied recently. While the bare charge of the ssDNA piece inside the channel is approximately 12 (in units of electron charge) measurements of different effective charges resulted in values between one and two. We explain these challenging observations by a large self-energy of a charge in the narrow water filled gap between ssDNA and channel walls, related to large difference between dielectric constants of water and lipid, and calculate effective charges of ssDNA. We start from the most fundamental stall charge q(s), which determines the force F(s)=q(s)V/L stalling DNA against the voltage V ( L is the length of the channel). We show that the stall charge q(s) is proportional to the ion current blocked by DNA, which is small due to the self-energy barrier. Large voltage V reduces the capture barrier which DNA molecule should overcome in order to enter the channel by /q(c)/V, where q(c) is the effective capture charge. We expressed it through the stall charge q(s). We also relate the stall charge q(s) to two other effective charges measured for ssDNA with a hairpin in the back end: the charge q(u) responsible for reduction of the barrier for unzipping of the hairpin and the charge q(e) responsible for DNA escape in the direction of hairpin against the voltage. At small V we explain reduction of the capture barrier with the salt concentration.


Assuntos
DNA/química , Proteínas de Escherichia coli/química , Proteínas Hemolisinas/química , Lipídeos de Membrana/química , Potenciais da Membrana , Modelos Químicos , Modelos Moleculares , Simulação por Computador , DNA/ultraestrutura , Campos Eletromagnéticos , Transferência de Energia , Proteínas de Escherichia coli/ultraestrutura , Proteínas Hemolisinas/ultraestrutura , Ativação do Canal Iônico , Movimento (Física) , Eletricidade Estática
15.
Phys Rev E Stat Nonlin Soft Matter Phys ; 76(5 Pt 1): 051909, 2007 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-18233689

RESUMO

Proteins are known to locate their specific targets on DNA up to two orders of magnitude faster than predicted by the Smoluchowski three-dimensional diffusion rate. One of the mechanisms proposed to resolve this discrepancy is termed "intersegment transfer." Many proteins have two DNA binding sites and can transfer from one DNA segment to another without dissociation to water. We calculate the target search rate for such proteins in a dense globular DNA, taking into account intersegment transfer working in conjunction with DNA motion and protein sliding along DNA. We show that intersegment transfer plays a very important role in cases where the protein spends most of its time adsorbed on DNA.


Assuntos
Proteínas de Ligação a DNA/química , DNA/química , Modelos Químicos , Sítios de Ligação , Simulação por Computador , Ligação Proteica
16.
Phys Rev E Stat Nonlin Soft Matter Phys ; 74(2 Pt 1): 021903, 2006 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-17025468

RESUMO

Proteins can locate their specific targets on DNA up to two orders of magnitude faster than the Smoluchowski three-dimensional diffusion rate. This happens due to nonspecific adsorption of proteins to DNA and subsequent one-dimensional sliding along DNA. We call such a one-dimensional route towards the target an "antenna." We studied the role of the dispersion of nonspecific binding energies within the antenna due to a quasirandom sequence of natural DNA. A random energy profile for sliding proteins slows the searching rate for the target. We show that this slowdown is different for macroscopic and mesoscopic antennas.


Assuntos
Proteínas de Ligação a DNA/química , DNA/química , Modelos Químicos , Modelos Moleculares , Adsorção , Sítios de Ligação , Simulação por Computador , Ligação Proteica
17.
Phys Rev E Stat Nonlin Soft Matter Phys ; 73(5 Pt 1): 051205, 2006 May.
Artigo em Inglês | MEDLINE | ID: mdl-16802926

RESUMO

Ion transport through narrow water-filled channels is impeded by a high electrostatic barrier. The latter originates from the large ratio of the dielectric constants of the water and the surrounding media. We show that "doping," i.e., immobile charges attached to the walls of the channel, substantially reduces the barrier. This explains why most of the biological ion channels are "doped." We show that at rather generic conditions the channels may undergo ion exchange phase transitions (typically of the first order). Upon such a transition a finite latent concentration of ions may either enter or leave the channel, or be exchanged between the ions of different valences. We discuss possible implications of these transitions for the Ca-vs-Na selectivity of biological Ca channels. We also show that transport of divalent Ca ions is assisted by their fractionalization into two separate excitations.

18.
Biophys J ; 90(8): 2731-44, 2006 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-16461402

RESUMO

It is known since the early days of molecular biology that proteins locate their specific targets on DNA up to two orders-of-magnitude faster than the Smoluchowski three-dimensional diffusion rate. An accepted explanation of this fact is that proteins are nonspecifically adsorbed on DNA, and sliding along DNA provides for the faster one-dimensional search. Surprisingly, the role of DNA conformation was never considered in this context. In this article, we explicitly address the relative role of three-dimensional diffusion and one-dimensional sliding along coiled or globular DNA and the possibility of correlated readsorption of desorbed proteins. We have identified a wealth of new different scaling regimes. We also found the maximal possible acceleration of the reaction due to sliding. We found that the maximum on the rate-versus-ionic strength curve is asymmetric, and that sliding can lead not only to acceleration, but also in some regimes to dramatic deceleration of the reaction.


Assuntos
Proteínas de Ligação a DNA/metabolismo , DNA/metabolismo , Modelos Biológicos , Conformação de Ácido Nucleico , DNA/química , Proteínas de Ligação a DNA/química , Eletricidade Estática , Água/química
19.
Phys Rev E Stat Nonlin Soft Matter Phys ; 72(2 Pt 1): 021405, 2005 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-16196563

RESUMO

It is known that in a water solution with multivalent counterions (Z-ions) two likely charged macroions can attract each other due to correlations of Z-ions adsorbed on their surfaces. This "correlation" attraction is short ranged and decays exponentially with increasing distance between macroions at characteristic distance A/2pi , where A is the average distance between Z -ions on the surfaces of macroions. In this work, we show that an additional long-range "polarization" attraction exists when the bare surface charge densities of the two macroions have the same sign, but are different in absolute values. The key idea is that with adsorbed Z -ions, two insulating macroions can be considered as conductors with fixed but different electric potentials. Each potential is determined by the difference between the entropic bulk chemical potential of a Z -ion and its correlation chemical potential at the surface of the macroion determined by its bare surface charge density. When the two macroions are close enough, they get polarized in such a way that their adjacent spots form a charged capacitor, which leads to attraction. In a salt-free solution this polarization attractive force is long ranged: it decays as a power of the distance between the surfaces of two macroions, d. The polarization force decays slower than the van der Waals attraction and therefore is much larger than it in a large range of distances. In the presence of large amount of monovalent salt, the polarization attraction decays exponentially at d larger than the Debye-Hückel screening radius r(s) . Still, when A/2pi<

20.
Phys Rev Lett ; 95(14): 148101, 2005 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-16241693

RESUMO

We consider ion transport through protein ion channels in lipid membranes and water-filled nanopores in silicon films. It is known that, due to the large ratio of dielectric constants of water and the surrounding material, an ion placed inside the channel faces a large electrostatic self-energy barrier. The barrier leads to an exponentially large resistance of the channel. We study reduction of the electrostatic barrier by immobile charges located on the internal walls of the channel. We show that the barrier practically vanishes already at relatively small concentration of wall charges.


Assuntos
Biofísica/métodos , Canais Iônicos/química , Cátions , Modelos Estatísticos , Modelos Teóricos , Nanotecnologia/métodos , Proteínas , Eletricidade Estática
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