232 research outputs found
Universal scaling relation in high-temperature superconductors
Scaling laws express a systematic and universal simplicity among complex
systems in nature. For example, such laws are of enormous significance in
biology. Scaling relations are also important in the physical sciences. The
seminal 1986 discovery of high transition-temperature (high-T_c)
superconductivity in cuprate materials has sparked an intensive investigation
of these and related complex oxides, yet the mechanism for superconductivity is
still not agreed upon. In addition, no universal scaling law involving such
fundamental properties as T_c and the superfluid density \rho_s, a quantity
indicative of the number of charge carriers in the superconducting state, has
been discovered. Here we demonstrate that the scaling relation \rho_s \propto
\sigma_{dc} T_c, where the conductivity \sigma_{dc} characterizes the
unidirectional, constant flow of electric charge carriers just above T_c,
universally holds for a wide variety of materials and doping levels. This
surprising unifying observation is likely to have important consequences for
theories of high-T_c superconductivity.Comment: 11 pages, 2 figures, 2 table
Recommended from our members
Soliton superlattices in twisted hexagonal boron nitride.
Properties of atomic van der Waals heterostructures are profoundly influenced by interlayer coupling, which critically depends on stacking of the proximal layers. Rotational misalignment or lattice mismatch of the layers gives rise to a periodic modulation of the stacking, the moiré superlattice. Provided the superlattice period extends over many unit cells, the coupled layers undergo lattice relaxation, leading to the concentration of strain at line defects - solitons - separating large area commensurate domains. We visualize such long-range periodic superstructures in thin crystals of hexagonal boron nitride using atomic-force microscopy and nano-infrared spectroscopy. The solitons form sub-surface hexagonal networks with periods of a few hundred nanometers. We analyze the topography and infrared contrast of these networks to obtain spatial distribution of local strain and its effect on the infrared-active phonons of hBN
Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe1-xCoxAs
Although the origin of high temperature superconductivity in the iron
pnictides is still under debate, it is widely believed that magnetic
interactions or fluctuations play an important role in triggering Cooper
pairing. Because of the relevance of magnetism to pairing, the question of
whether long range spin magnetic order can coexist with superconductivity
microscopically has attracted strong interests. The available experimental
methods used to answer this question are either bulk probes or local ones
without control of probing position, thus the answers range from mutual
exclusion to homogeneous coexistence. To definitively answer this question,
here we use scanning tunneling microscopy to investigate the local electronic
structure of an underdoped NaFe1-xCoxAs near the spin density wave (SDW) and
superconducting (SC) phase boundary. Spatially resolved spectroscopy directly
reveal both the SDW and SC gap features at the same atomic location, providing
compelling evidence for the microscopic coexistence of the two phases. The
strengths of the SDW and SC features are shown to anti correlate with each
other, indicating the competition of the two orders. The microscopic
coexistence clearly indicates that Cooper pairing occurs when portions of the
Fermi surface (FS) are already gapped by the SDW order. The regime TC < T <
TSDW thus show a strong resemblance to the pseudogap phase of the cuprates
where growing experimental evidences suggest a FS reconstruction due to certain
density wave order. In this phase of the pnictides, the residual FS has a
favorable topology for magnetically mediated pairing when the ordering moment
of the SDW is small.Comment: 18 pages, 4 figure
Chemical potential oscillations from a single nodal pocket in the underdoped high-Tc superconductor YBa2Cu3O6+x
The mystery of the normal state in the underdoped cuprates has deepened with
the use of newer and complementary experimental probes. While photoemission
studies have revealed solely `Fermi arcs' centered on nodal points in the
Brillouin zone at which holes aggregate upon doping, more recent quantum
oscillation experiments have been interpreted in terms of an ambipolar Fermi
surface, that includes sections containing electron carriers located at the
antinodal region. To address the question of whether an ambipolar Fermi surface
truly exists, here we utilize measurements of the second harmonic quantum
oscillations, which reveal that the amplitude of these oscillations arises
mainly from oscillations in the chemical potential, providing crucial
information on the nature of the Fermi surface in underdoped YBa2Cu3O6+x. In
particular, the detailed relationship between the second harmonic amplitude and
the fundamental amplitude of the quantum oscillations leads us to the
conclusion that there exists only a single underlying quasi-two dimensional
Fermi surface pocket giving rise to the multiple frequency components observed
via the effects of warping, bilayer splitting and magnetic breakdown. A range
of studies suggest that the pocket is most likely associated with states near
the nodal region of the Brillouin zone of underdoped YBa2Cu3O6+x at high
magnetic fields.Comment: 7 pages, 4 figure
Electronic correlations in the iron pnictides
In correlated metals derived from Mott insulators, the motion of an electron
is impeded by Coulomb repulsion due to other electrons. This phenomenon causes
a substantial reduction in the electron's kinetic energy leading to remarkable
experimental manifestations in optical spectroscopy. The high-Tc
superconducting cuprates are perhaps the most studied examples of such
correlated metals. The occurrence of high-Tc superconductivity in the iron
pnictides puts a spotlight on the relevance of correlation effects in these
materials. Here we present an infrared and optical study on single crystals of
the iron pnictide superconductor LaFePO. We find clear evidence of electronic
correlations in metallic LaFePO with the kinetic energy of the electrons
reduced to half of that predicted by band theory of nearly free electrons.
Hallmarks of strong electronic many-body effects reported here are important
because the iron pnictides expose a new pathway towards a correlated electron
state that does not explicitly involve the Mott transition.Comment: 10 page
Holographic Superconductors from Einstein-Maxwell-Dilaton Gravity
We construct holographic superconductors from Einstein-Maxwell-dilaton
gravity in 3+1 dimensions with two adjustable couplings and the charge
carried by the scalar field. For the values of and we
consider, there is always a critical temperature at which a second order phase
transition occurs between a hairy black hole and the AdS RN black hole in the
canonical ensemble, which can be identified with the superconducting phase
transition of the dual field theory. We calculate the electric conductivity of
the dual superconductor and find that for the values of and where
is small the dual superconductor has similar properties to the
minimal model, while for the values of and where is
large enough, the electric conductivity of the dual superconductor exhibits
novel properties at low frequencies where it shows a "Drude Peak" in the real
part of the conductivity.Comment: 25 pages, 13 figures; v2, typos corrected; v3, refs added, to appear
in JHE
Intrinsic Plasmon-Phonon Interactions in Highly Doped Graphene: A Near-Field Imaging Study.
Author's accepted versionFinal version available from ACS via the DOI in this recordAs a two-dimensional semimetal, graphene offers clear advantages for plasmonic applications over conventional metals, such as stronger optical field confinement, in situ tunability, and relatively low intrinsic losses. However, the operational frequencies at which plasmons can be excited in graphene are limited by the Fermi energy EF, which in practice can be controlled electrostatically only up to a few tenths of an electronvolt. Higher Fermi energies open the door to novel plasmonic devices with unprecedented capabilities, particularly at mid-infrared and shorter-wave infrared frequencies. In addition, this grants us a better understanding of the interaction physics of intrinsic graphene phonons with graphene plasmons. Here, we present FeCl3-intercalated graphene as a new plasmonic material with high stability under environmental conditions and carrier concentrations corresponding to EF > 1 eV. Near-field imaging of this highly doped form of graphene allows us to characterize plasmons, including their corresponding lifetimes, over a broad frequency range. For bilayer graphene, in contrast to the monolayer system, a phonon-induced dipole moment results in increased plasmon damping around the intrinsic phonon frequency. Strong coupling between intrinsic graphene phonons and plasmons is found, supported by ab initio calculations of the coupling strength, which are in good agreement with the experimental data.FJGA and PA-G acknowledge support from the Spanish Ministry of Economy and Competitiveness through the national programs MAT2014-59096-P and FIS2014-60195-JIN, respectively. MFC and SR acknowledge support from EPSRC (Grant no. EP/J000396/1, 281 EP/K017160/1, EP/K010050/1, EPG036101/1, EP/M001024/1, EPM- 002438/1), from Royal Society International Exchanges Scheme 2012/R3 and 2013/R2 and from European Commission (FP7-ICT-2013-613024-GRASP). SD, DNB and MF acknowledge support of ONR N00014-15-1-2671. DNB is the Moore Investigator in Quantum Materials funded by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4533
The connection between superconducting phase correlations and spin excitations in YBaCuO: A magnetic field study
One of the most striking universal properties of the
high-transition-temperature (high-) superconductors is that they are all
derived from the hole-doping of their insulating antiferromagnetic (AF) parent
compounds. From the outset, the intimate relationship between magnetism and
superconductivity in these copper-oxides has intrigued researchers. Evidence
for this link comes from neutron scattering experiments that show the
unambiguous presence of short-range AF correlations (excitations) in cuprate
superconductors. Even so, the role of such excitations in the pairing mechanism
and superconductivity is still a subject of controversy. For
YBaCuO, where controls the hole-doping level, the most
prominent feature in the magnetic excitations spectra is the ``resonance''.
Here we show that for underdoped YBaCuO, where and
are below the optimal values, modest magnetic fields suppress the resonance
significantly, much more so for fields approximately perpendicular rather than
parallel to the CuO planes. Our results indicate that the resonance
measures pairing and phase coherence, suggesting that magnetism plays an
important role in the superconductivity of cuprates. The persistence of a field
effect above favors mechanisms with preformed pairs in the normal state
of underdoped cuprates.Comment: 12 pages, 4 figures, Nature (in press
Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates
In strongly-correlated systems the electronic properties at the Fermi energy (EF) are intertwined with those at high energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high energy scale physics associated with Mott-like excitations (|E-EF|>1 eV) is involved in the condensate formation. Here we show the interplay between the many-body high-energy CuO2 excitations at 1.5 and 2 eV and the onset of HTSC. This is revealed by a novel optical pump supercontinuum-probe technique, which provides access to the dynamics of the dielectric function in Y-Bi2212 over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (Tc)
Dzyaloshinskii-Moriya Interaction and Spiral Order in Spin-orbit Coupled Optical Lattices
We show that the recent experimental realization of spin-orbit coupling in
ultracold atomic gases can be used to study different types of spin spiral
order and resulting multiferroic effects. Spin-orbit coupling in optical
lattices can give rise to the Dzyaloshinskii-Moriya (DM) spin interaction which
is essential for spin spiral order. By taking into account spin-orbit coupling
and an external Zeeman field, we derive an effective spin model in the Mott
insulator regime at half filling and demonstrate that the DM interaction in
optical lattices can be made extremely strong with realistic experimental
parameters. The rich finite temperature phase diagrams of the effective spin
models for fermions and bosons are obtained via classical Monte Carlo
simulations.Comment: 7 pages, 5 figure
- …