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Philip Phillips

Publications and source records attributed to Philip Phillips.

At least 73 records · Page 4Linked to original sources
arXiv2003

Nonlinear Transport Near a Quantum Phase Transition in Two Dimensions

The problem of non-linear transport near a quantum phase transition is solved within the Landau theory for the dissipative insulator-superconductor phase transition in two dimensions. Using the non-equilibrium Schwinger round-trip Green function formalism, we obtain the scaling function for the non-linear conductivity in the quantum disordered regime. We find that the conductivity scales as $E^2$ at low field but crosses over at large fields to a universal constant on the order of $e^2/h$. The crossover between these two regimes obtains when the length scale for the quantum fluctuations becomes comparable to that of the electric field within logarithmic accuracy.

cond-mat.supr-con↗
arXiv2003

Absence of Phase Stiffness in the Quantum Rotor Phase Glass

We analyze here the consequence of local rotational-symmetry breaking in the quantum spin (or phase) glass state of the quantum random rotor model. By coupling the spin glass order parameter directly to a vector potential, we are able to compute whether the system is resilient (that is, possesses a phase stiffness) to a uniform rotation in the presence of random anisotropy. We show explicitly that the O(2) vector spin glass has no electromagnetic response indicative of a superconductor at mean-field and beyond, suggesting the absence of phase stiffness. This result confirms our earlier finding (PRL, {\bf 89}, 27001 (2002)) that the phase glass is metallic, due to the main contribution to the conductivity arising from fluctuations of the superconducting order parameter. In addition, our finding that the spin stiffness vanishes in the quantum rotor glass is consistent with the absence of a transverse stiffness in the Heisenberg spin glass found by Feigelman and Tsvelik (Sov. Phys. JETP, {\bf 50}, 1222 (1979).

cond-mat.supr-con↗
arXiv2003

Pseudogap in Doped Mott Insulators is the Near-neighbour Analogue of the Mott Gap

We show that the strong coupling physics inherent to the insulating Mott state in 2D leads to a jump in the chemical potential upon doping and the emergence of a pseudogap in the single particle spectrum below a characteristic temperature. The pseudogap arises because any singly-occupied site not immediately neighbouring a hole experiences a maximum energy barrier for transport equal to $t^2/U$, where $t$ is the nearest-neighbour hopping integral and $U$ the on-site repulsion. The resultant pseudogap cannot vanish before each lattice site, on average, has at least one hole as a near neighbour. The ubiquituity of this effect in all doped Mott insulators suggests that the pseudogap in the cuprates has a simple origin.

cond-mat.str-el↗
arXiv2002

Hall Conductivity near the z=2 Superconductor-Insulator Transition in 2D

We analyze here the behavior of the Hall conductivity $σ_{xy}$ near a $z=2$ insulator-superconductor quantum critical point in a perpendicular magnetic field. We show that the form of the conductivity is sensitive to the presence of dissipation $η$, and depends non-monotonically on $H$ once $η$ is weak enough. $σ_{xy}$ passes through a maximum at $H \sim ηT$ in the quantum critical regime, suggesting that the limits $H \to 0$ and $η\to 0$ do not commute.

cond-mat.supr-con↗
arXiv2002

A Phase Glass is a Bose Metal: New Conducting State in 2D

In the quantum rotor model with random exchange interactions having a non-zero mean, three phases, a 1) phase (Bose) glass, 2) superfluid, and 3) Mott insulator, meet at a bi-critical point. We demonstrate that proximity to the bi-critical point and the coupling between the energy landscape and the dissipative degrees of freedom of the phase glass lead to a metallic state at T=0. Consequently, the phase glass is unique in that it represents a concrete example of a metallic state that is mediated by disorder, even in 2D. We propose that the experimentally observed metallic phase which intervenes between the insulator and the superconductor in a wide range of thin films is in actuality a phase glass.

cond-mat.str-el↗
arXiv2002

The Kondo effect and weak localization

The word Kondo means battle in Swahili. This coincidence is fortuitous because in the Kondo effect, a battle inevitably ensues anytime a magnetic impurity is placed in a non-magnetic metal. Below some energy scale, the Kondo temperature ($T_k$), a lone magnetic impurity is robbed of its spin. Above the Kondo temperature, rapid spin-flip scattering produces a temperature-dependent correction to the resistivity of the form, $B_k\ln T$. Until recently, both the Kondo resistivity and $T_k$ were thought to be determined solely by the host metal and the magnetic impurity. However, numerous presentations in this volume attest, there is now overwhelming evidence that both are affected by the size of the sample as well as non-magnetic random scattering. In this paper, I will focus on the theoretical work we have performed on the experiments revealing that non-magnetic scattering suppppresses the Kondo resistivity in thin Kondo alloys.

cond-mat.dis-nn↗
arXiv2001

Deconstruction of the Trap Model for the New Conducting State in 2D

A key prediction of the trap model for the new conducting state in 2D is that the resistivity turns upwards below some characteristic temperature, $T_{\rm min}$. Altshuler, Maslov, and Pudalov have argued that the reason why no upturn has been observed for the low density conducting samples is that the temperature was not low enough in the experiments. We show here that $T_{\rm min}$ within the Altshuler, Maslov, and Pudalov trap model actually increases with decreasing density, contrary to their claim. Consequently, the trap model is not consistent with the experimental trends.

cond-mat.str-el↗
arXiv2001

Electron Quasiparticles Drive the Superconductor-to-Insulator Transition in Homogeneously Disordered Thin Films

Transport data on Bi, MoGe, and PbBi/Ge homogeneously-disordered thin films demonstrate that the critical resistivity, $R_c$, at the nominal insulator-superconductor transition is linearly proportional to the normal sheet resistance, $R_N$. In addition, the critical magnetic field scales linearly with the superconducting energy gap and is well-approximated by $H_{c2}$. Because $R_N$ is determined at high temperatures and $H_{c2}$ is the pair-breaking field, the two immediate consequences are: 1) electron-quasiparticles populate the insulating side of the transition and 2) standard phase-only models are incapable of describing the destruction of the superconducting state. As gapless electronic excitations populate the insulating state, the universality class is no longer the 3D XY model. The lack of a unique critical resistance in homogeneously disordered films can be understood in this context. In light of the recent experiments which observe an intervening metallic state separating the insulator from the superconductor in homogeneously disordered MoGe thin films, we argue that the two transitions that accompany the destruction of superconductivity are 1) superconductor to Bose metal in which phase coherence is lost and 2) Bose metal to localized electron insulator via pair-breaking.

cond-mat↗
arXiv2001

Can Short-Range Interactions Mediate a Bose Metal Phase in 2D?

We show here based on a 1-loop scaling analysis that short-range interactions are strongly irrelevant perturbations near the insulator-superconductor (IST) quantum critical point. The lack of any proof that short-range interactions mediate physics which is present only in strong coupling leads us to conclude that short-range interactions are strictly irrelevant near the IST quantum critical point. Hence, we argue that no new physics, such as the formation of a uniform Bose metal phase can arise from an interplay between on-site and nearest-neighbour interactions.

cond-mat↗
arXiv2001

Local Dynamics and Strong Correlation Physics I: 1D and 2D Half-filled Hubbard Models

We report on a non-perturbative approach to the 1D and 2D Hubbard models that is capable of recovering both strong and weak-coupling limits. We first show that even when the on-site Coulomb repulsion, U, is much smaller than the bandwith, the Mott-Hubbard gap never closes at half-filling in both 1D and 2D. Consequently, the Hubbard model at half-filling is always in the strong-coupling non-perturbative regime. For both large and small U, we find that the population of nearest-neighbour singlet states approaches a value of order unity as $T\to 0$ as would be expected for antiferromagnetic order. We also find that the double occupancy is a smooth monotonic function of U and approaches the anticipated non-interacting limit and large U limits. Finally, in our results for the heat capacity in 1D differ by no more than 1% from the Bethe ansatz predictions. In addition, we find that in 2D, the heat capacity vs T for different values of U exhibits a universal crossing point at two characteristic temperatures as is seen experimentally in a wide range of strongly-correlated systems such as $^3He$, $UBe_3$, and $CeCu_{6-x}Al_x$. The success of this method in recovering well-established results that stem fundamentally from the Coulomb interaction suggests that local dynamics are at the heart of the physics of strongly correlated systems.

cond-mat↗
arXiv2001

Transport Properties near the z=2 Insulator-Superconductor Transition

We consider here the fluctuation conductivity near the point of the insulator-superconductor transition in a system of regular Josephson junction arrays in the presence of particle-hole asymmetry or equivalently homogeneous charge frustration. The transition is characterised by the dynamic critical exponent $z=2$, opening the possibility of the perturbative renormalization-group (RG) treatment. The quartic interaction in the Ginzburg-Landau action and the coupling to the Ohmic heat bath, giving the finite quasiparticle life-time, lead to the non-monotonic behavior of the dc conductivity as a function of temperature in the leading logarithmic approximation.

cond-mat↗
arXiv2001

Interaction-induced Bose Metal in 2D

We show here that the regularization of the conductivity resulting from the bosonic interactions on the `insulating' (quantum disordered) side of an insulator-superconductor transition in 2D gives rise to a metal with a finite conductivity, $σ=(2/π) 4 e^2/h$, as temperature tends to zero. The Bose metal is stable to weak disorder and hence represents a concrete example of an interaction-induced metallic phase. Phenomenological inclusion of dissipation reinstates the anticipated insulating behaviour in the quantum-disordered regime. Hence, we conclude that the traditionally-studied insulator-superconductor transition, which is driven solely by quantum fluctuations, corresponds to a superconductor-metal transition. The possible relationship to experiments on superconducting thin films in which a low-temperature metallic phase has been observed is discussed.

cond-mat↗
arXiv2001

Magneto-transport Properties near the Superconductor-Insulator Transition in 2D

We analyze here the behavior near the 2D insulator-superconductor quantum critical point in the presence of a perpendicular magnetic field. We show that with increasing field $H$, the quantum disordered and quantum critical regimes, in which vortex degrees of freedom are suppressed, crossover to a new magnetically activated (MA) regime, where the correlation length $ξ\sim 1/\sqrt{H}$. In this regime, we show that the conductivity decreases monotonically as opposed to the anticipated saturation predicted from hyperuniversality arguments. This discrepancy arises from the lack of commutativity of the frequency and temperature tending to zero limits of the conductivity. In the low-field regime such that $\sqrt{H}\llΔ$, and in the absence of Ohmic dissipation, where $Δ$ is a measure of the distance from the quantum critical point, the resistivity saturates to the Bose metal value found previously for Cooper pairs lacking phase coherence.

cond-mat↗
arXiv2001

Nearest-neighbour Attraction Stabilizes Staggered Currents in the 2D Hubbard Model

Using a strong-coupling approach, we show that staggered current vorticity does not obtain in the repulsive 2D Hubbard model for large on-site Coulomb interactions, as in the case of the copper oxide superconductors. This trend also persists even when nearest-neighbour repulsions are present. However, staggered flux ordering emerges {\bf only} when attractive nearest-neighbour Coulomb interactions are included. Such ordering opens a gap along the $(π,0)-(0,π)$ direction and persists over a reasonable range of doping.

cond-mat.str-el↗
arXiv2000

d_{x^2-y^2} pairing of composite excitations in the 2D Hubbard model

We report on a strong coupling approach (on-site Coulomb repulsion, U larger than the nearest-neighbour hopping energy |t|) to the Hubbard model. Starting from the Hubbard operators which diagonalize the interaction term, we generate a hierarchy of composite operators from the equations of motion. Using the Hubbard operators as a basis, we are able to compute the associated Green functions including the anomalous Green functions which describe pair formation. We show explicitly that these anomalous Green functions are non-zero in the d_{x^2-y^2} channel; however, the entities that pair up are not single electron-like particles but rather composite excitations (which we call cexons) made out of an electron and a hole on nearest-neighbour sites. Cexons are fermionic in nature as they have spin 1/2 and also have unit charge. Our calculations of the chemical potential reveal that negative compressibility in the 2D Hubbard model and composite excitation pairing are intimately connected, namely, the larger the negative compressibility, the larger the pairing amplitude. Our observation of negative compressibility in the under-doped regime is consistent with phase segregation or stripe formation in the normal state. While pairing ameliorates the negative compresssibility, it does not eliminate it entirely. In addition, we find that the anomalous correlation functions are particle-hole symmetric and exhibit a maximum at a doping level of roughly 10% as measured from half-filling. For U=8|t|, the onset temperature for pair formation is 0.02|t|.

cond-mat↗
arXiv2000

Quantum Conductors in a Plane

When electrons are confined to move in a plane, strange things happen. For example, under normal circumstances, they are not expected to conduct electricity at low temperatures. However, recent experiments on a dilute electron gas confined to move at the interface between two semiconductors pose a distinct counterexample to the standard view. Transport measurements reveal that as the temperature is lowered, the resistivity drops without any signature of the anticipated up-turn as required by the standard account. It is the possible existence of a new conducting state in two dimensions that is the primary focus of this session.

cond-mat.str-el↗
arXiv2000

Single and Paired Point Defects in a 2D Wigner Crystal

Using the path-integral Monte Carlo method, we calculate the energy to form single and pair vacancies and interstitials in a two-dimensional Wigner crystal of electrons. We confirm that the lowest-lying energy defects of a 2D electron Wigner crystal are interstitials, with a creation energy roughly 2/3 that of a vacancy. The formation energy of the defects goes to zero near melting, suggesting that point defects might mediate the melting process. In addition, we find that the interaction between defects is strongly attractive, so that most defects will exist as bound pairs.

cond-mat↗