85 research outputs found
Recommended from our members
Effect of interorbital scattering on superconductivity in doped Dirac semimetals
Unconventional superconductivity has been discovered in a variety of doped quantum materials, including topological insulators and semimetals. A unifying property of these systems is strong orbital hybridization, which leads to pairing of states with nontrivial Bloch wave functions. In contrast to naive expectation, however, many of these superconductors are relatively resilient to disorder. Here we study the interplay of superconductivity and disorder in doped three-dimensional Dirac systems, which serve as a paradigmatic dispersion in quantum materials, using Abrikosov-Gor'kov theory. In this way, the role of disorder is captured by a single parameter Γ, the pair scattering rate. In contrast to previous studies, we argue that interorbital scattering cannot be neglected due to the strong orbital hybridization in Dirac systems. We find that the robustness of different pairing states highly depends on the relative strength of the different interorbital scattering channels. In particular, we find that the "nematic"superconducting state, which is argued to be the ground state in many Bi2Se3-related compounds, is not protected from disorder in any way. The pair scattering rate in this case is at best smaller by a factor of 3 compared to systems without spin-orbit coupling. We also find that the odd-parity pairing state with total angular momentum zero (the B phase of superfluid He3) is protected against certain types of disorder, which include a family of time-reversal odd (magnetic) impurities. Namely, this odd-parity state is a singlet of partners under CT symmetry (rather than T symmetry in the standard Anderson's theory), where C and T are chiral and time-reversal symmetries, respectively. As a result, it is protected against any disorder potential that respects CT symmetry. Our procedure is very general and can be readily applied to different band structures and disorder configurations
Phonon Squeezed States Generated by Second Order Raman Scattering
We study squeezed states of phonons, which allow a reduction in the quantum
fluctuations of the atomic displacements to below the zero-point quantum noise
level of coherent phonon states. We investigate the generation of squeezed
phonon states using a second order Raman scattering process. We calculate the
expectation values and fluctuations of both the atomic displacement and the
lattice amplitude operators, as well as the effects of the phonon squeezed
states on macroscopically measurable quantities, such as changes in the
dielectric constant. These results are compared with recent experiments.Comment: 4 pages, REVTE
Quantum Phonon Optics: Coherent and Squeezed Atomic Displacements
In this paper we investigate coherent and squeezed quantum states of phonons.
The latter allow the possibility of modulating the quantum fluctuations of
atomic displacements below the zero-point quantum noise level of coherent
states. The expectation values and quantum fluctuations of both the atomic
displacement and the lattice amplitude operators are calculated in these
states---in some cases analytically. We also study the possibility of squeezing
quantum noise in the atomic displacement using a polariton-based approach.Comment: 6 pages, RevTe
A Gate-tunable Polarized Phase of Two-Dimensional Electrons at the LaAlO3/SrTiO3 Interface
Controlling the coupling between localized spins and itinerant electrons can
lead to exotic magnetic states. A novel system featuring local magnetic moments
and extended 2D electrons is the interface between LaAlO3 and SrTiO3. The
magnetism of the interface, however, was observed to be insensitive to the
presence of these electrons and is believed to arise solely from extrinsic
sources like oxygen vacancies and strain. Here we show the existence of
unconventional electronic phases in the LaAlO3/SrTiO3 system pointing to an
underlying tunable coupling between itinerant electrons and localized moments.
Using anisotropic magnetoresistance and anomalous Hall effect measurements in a
unique in-plane configuration, we identify two distinct phases in the space of
carrier density and magnetic field. At high densities and fields, the
electronic system is strongly polarized and shows a response, which is highly
anisotropic along the crystalline directions. Surprisingly, below a
density-dependent critical field, the polarization and anisotropy vanish
whereas the resistivity sharply rises. The unprecedented vanishing of the easy
axes below a critical field is in sharp contrast with other coupled magnetic
systems and indicates strong coupling with the moments that depends on the
symmetry of the itinerant electrons. The observed interplay between the two
phases indicates the nature of magnetism at the LaAlO3/SrTiO3 interface as both
having an intrinsic origin and being tunable.Comment: Finalized version containing modifications introduced after
peer-review. The results are completely unchange
Recommended from our members
Superconductivity near a Ferroelectric Quantum Critical Point in Ultralow-Density Dirac Materials
The experimental observation of superconductivity in doped semimetals and semiconductors, where the Fermi energy is comparable to or smaller than the characteristic phonon frequencies, is not captured by the conventional theory. In this paper, we propose a mechanism for superconductivity in ultralow-density three-dimensional Dirac materials based on the proximity to a ferroelectric quantum critical point. We derive a low-energy theory that takes into account both the strong Coulomb interaction and the direct coupling between the electrons and the soft phonon modes. We show that the Coulomb repulsion is strongly screened by the lattice polarization near the critical point even in the case of a vanishing carrier density. Using a renormalization group analysis, we demonstrate that the effective electron-electron interaction is dominantly mediated by the transverse phonon mode. We find that the system generically flows towards strong electron-phonon coupling. Hence, we propose a new mechanism to simultaneously produce an attractive interaction and suppress strong Coulomb repulsion, which does not require retardation. For comparison, we perform the same analysis for covalent crystals, where lattice polarization is negligible. We obtain qualitatively similar results, though the screening of the Coulomb repulsion is much weaker. We then apply our results to study superconductivity in the low-density limit. We find a strong enhancement of the transition temperature upon approaching the quantum critical point. Finally, we also discuss scenarios to realize a topological p-wave superconducting state in covalent crystals close to the critical point
Dynamics of entanglement and transport in one-dimensional systems with quenched randomness
Quenched randomness can have a dramatic effect on the dynamics of isolated 1D quantum many-body systems, even for systems that thermalize. This is because transport, entanglement, and operator spreading can be hindered by “Griffiths” rare regions, which locally resemble the many-body-localized phase and thus act as weak links. We propose coarse-grained models for entanglement growth and for the spreading of quantum operators in the presence of such weak links. We also examine entanglement growth across a single weak link numerically. We show that these weak links have a stronger effect on entanglement growth than previously assumed: entanglement growth is subballistic whenever such weak links have a power-law probability distribution at low couplings, i.e., throughout the entire thermal Griffiths phase. We argue that the probability distribution of the entanglement entropy across a cut can be understood from a simple picture in terms of a classical surface growth model. We also discuss spreading of operators and conserved quantities. Surprisingly, the four length scales associated with (i) production of entanglement, (ii) spreading of conserved quantities, (iii) spreading of operators, and (iv) the width of the “front” of a spreading operator, are characterized by dynamical exponents that in general are all distinct. Our numerical analysis of entanglement growth between weakly coupled systems may be of independent interest
Recommended from our members
Effect of interorbital scattering on superconductivity in doped Dirac semimetals
Unconventional superconductivity has been discovered in a variety of doped quantum materials, including topological insulators and semimetals. A unifying property of these systems is strong orbital hybridization, which leads to pairing of states with nontrivial Bloch wave functions. In contrast to naive expectation, however, many of these superconductors are relatively resilient to disorder. Here we study the interplay of superconductivity and disorder in doped three-dimensional Dirac systems, which serve as a paradigmatic dispersion in quantum materials, using Abrikosov-Gor'kov theory. In this way, the role of disorder is captured by a single parameter Γ, the pair scattering rate. In contrast to previous studies, we argue that interorbital scattering cannot be neglected due to the strong orbital hybridization in Dirac systems. We find that the robustness of different pairing states highly depends on the relative strength of the different interorbital scattering channels. In particular, we find that the "nematic"superconducting state, which is argued to be the ground state in many Bi2Se3-related compounds, is not protected from disorder in any way. The pair scattering rate in this case is at best smaller by a factor of 3 compared to systems without spin-orbit coupling. We also find that the odd-parity pairing state with total angular momentum zero (the B phase of superfluid He3) is protected against certain types of disorder, which include a family of time-reversal odd (magnetic) impurities. Namely, this odd-parity state is a singlet of partners under CT symmetry (rather than T symmetry in the standard Anderson's theory), where C and T are chiral and time-reversal symmetries, respectively. As a result, it is protected against any disorder potential that respects CT symmetry. Our procedure is very general and can be readily applied to different band structures and disorder configurations
- …