Coulomb drag

Coulomb drag is a transport phenomenon whereby long-range Coulomb interaction between charge carriers in two closely spaced but electrically isolated conductors induces a voltage (or, in a closed circuit, a current) in one of the conductors when an electrical current is passed through the other. The magnitude of the effect depends on the exact nature of the charge carriers and microscopic, many-body structure of the electronic systems in the two conductors. Drag measurements have become part of the standard toolbox in condensed matter physics that can be used to study fundamental properties of diverse physical systems including semiconductor heterostructures, graphene, quantum wires, quantum dots, and optical cavities.Comment: Review article, 59 pages, 35 figures, lots of references (pages 52-59); submitted to Reviews of Modern Physic

Quantum-fluctuation effects in transport properties of superconductors above the paramagnetic limit

We study the transport in ultrathin disordered film near the quantum critical point induced by the Zeeman field. We calculate corrections to the normal state conductivity due to quantum pairing fluctuations. The fluctuation-induced transport is mediated by virtual rather than real quasi-particles. We find that at zero temperature, where the corrections come from purely quantum fluctuations, the Aslamazov-Larkin paraconductivity term, the Maki-Thompson interference contribution and the density of states effects are all of the same order. The total correction leads to the negative magnetoresistance. This result is in qualitative agreement with the recent transport observations in the parallel magnetic field of the homogeneously disordered amorphous films and superconducting two-dimensional electron gas realized at the oxide interfaces.Comment: 4+ pages, 1 figur

Hydrodynamic flows of non-Fermi liquids: magnetotransport and bilayer drag

We consider a hydrodynamic description of transport for generic two dimensional electron systems that lack Galilean invariance and do not fall into the category of Fermi liquids. We study magnetoresistance and show that it is governed only by the electronic viscosity provided that the wavelength of the underlying disorder potential is large compared to the microscopic equilibration length. We also derive the Coulomb drag transresistance for double-layer non-Fermi liquid systems in the hydrodynamic regime. As an example, we consider frictional drag between two quantum Hall states with half-filled lowest Landau levels, each described by a Fermi surface of composite fermions coupled to a $U(1)$ gauge field. We contrast our results to prior calculations of drag of Chern-Simons composite particles and place our findings in the context of available experimental data.Comment: 4 pages + references + supplementary information, 1 figur

The Evolution of Comparative Advantage: Measurement and Welfare Implications

Using an industry-level dataset of production and trade spanning 75 countries and 5 decades, and a fully speciÞed multi-sector Ricardian model, we estimate productivities at sector level and examine how they evolve over time in both developed and developing countries. We find that in both country groups, comparative advantage has become weaker: productivity grew systematically faster in sectors that were initially at the greater comparative disadvantage. The global welfare implications of this phenomenon are significant. Relative to the counterfactual scenario in which an individual countryÕs comparative advantage remained the same as in the 1960s, and technology in all sectors grew at the same country-specific average rate, welfare today is 1.9% lower at the median. The welfare impact varies greatly across countries, ranging from -0.5% to 6% among OECD countries, and from -9% to 27% among non-OECD countries. Remarkably, for the OECD countries, nearly all of the welfare impact is driven by changes in technology in OECD countries, and for the non-OECD countries, nearly all of the welfare impact is driven by changes in technology in non-OECD countries.evolution of comparative advantage, welfare, Ricardian models of trade

Incoherent pair tunneling in the pseudogap phase of cuprates

Motivated by a recent experiment by Bergeal et al., we reconsider incoherent pair tunneling in a cuprate junction formed from an optimally doped superconducting lead and an underdoped normal metallic lead. We study the impact of the pseudogap on the pair tunneling by describing fermions in the underdoped lead with a model self-energy that has been developed to reproduce photoemission data. We find that the pseudogap causes an additional temperature dependent suppression of the pair contribution to the tunneling current. We discuss consistency with available experimental data and propose future experimental directions.Comment: 5 pages, 3 figure

Brownian scattering of a spinon in a Luttinger liquid

We consider strongly interacting one-dimensional electron liquids where elementary excitations carry either spin or charge. At small temperatures a spinon created at the bottom of its band scatters off low-energy spin- and charge-excitations and follows the diffusive motion of a Brownian particle in momentum space. We calculate the mobility characterizing these processes, and show that the resulting diffusion coefficient of the spinon is parametrically enhanced at low temperatures compared to that of a mobile impurity in a spinless Luttinger liquid. We briefly discuss that this hints at the relevance of spin in the process of equilibration of strongly interacting one-dimensional electrons, and comment on implications for transport in clean single channel quantum wires