14,401 research outputs found
Optical conductivity near finite-wavelength quantum criticality
We study the optical conductivity sigma(Omega) of an electron system near a
quantum-critical point with finite-wavelength ordering. sigma(Omega) vanishes
in clean Galilean-invariant systems, unless electrons are coupled to dynamical
collective modes, which dissipate the current. This coupling introduces a
nonuniversal energy scale. Depending on the parameters of each specific system,
a variety of responses arise near criticality: scaling peaks at a temperature-
and doping-dependent frequency, peaks at a fixed frequency, or no peaks to be
associated with criticality. Therefore the lack of scaling in the far-infrared
conductivity in cuprates does not necessarily call for new concepts of quantum
criticality.Comment: 4 pages, 4 figures; version as publishe
Bayesian optimization approach to input shaper design for flexible beam vibration suppression
This paper tackles the problem of suppressing vibrations of a flexible beam mounted on a mobile robot for inspection purposes. The adopted approach is an input shaper design along with Bayesian optimization. The latter methodology is employed to find out the optimal shaping parameter, taking into account non-ideal behaviors as controller hysteresis and time delays. Experimental results bolster the performance of the proposed approach
Weak-coupling phase diagrams of bond-aligned and diagonal doped Hubbard ladders
We study, using a perturbative renormalization group technique, the phase
diagrams of bond-aligned and diagonal Hubbard ladders defined as sections of a
square lattice with nearest-neighbor and next-nearest-neighbor hopping. We find
that for not too large hole doping and small next-nearest-neighbor hopping the
bond-aligned systems exhibit a fully spin-gapped phase while the diagonal
systems remain gapless. Increasing the next-nearest-neighbor hopping typically
leads to a decrease of the gap in the bond-aligned ladders, and to a transition
into a gapped phase in the diagonal ladders. Embedding the ladders in an
antiferromagnetic environment can lead to a reduction in the extent of the
gapped phases. These findings suggest a relation between the orientation of
hole-rich stripes and superconductivity as observed in LSCO.Comment: Published version. The set of RG equations in the presence of
magnetization was corrected and two figures were replace
Electrodynamics near the Metal-to-Insulator Transition in V3O5
The electrodynamics near the metal-to-insulator transitions (MIT) induced, in
V3O5 single crystals, by both temperature (T) and pressure (P) has been studied
by infrared spectroscopy. The T- and P-dependence of the optical conductivity
may be explained within a polaronic scenario. The insulating phase at ambient T
and P corresponds to strongly localized small polarons. Meanwhile the T-induced
metallic phase at ambient pressure is related to a liquid of polarons showing
incoherent dc transport, in the P-induced metallic phase at room T strongly
localized polarons coexist with partially delocalized ones. The electronic
spectral weight is almost recovered, in both the T and P induced metallization
processes, on an energy scale of 1 eV, thus supporting the key-role of
electron-lattice interaction in the V3O5 metal-to-insulator transition.Comment: 7 pages, 5 figure
Modelling and control of a variable-length flexible beam on inspection ground robot
Stabilising an inverted pendulum on a cart is a well-known control problem. This paper proposes the mechanical and control design for solving the oscillation problem of a variable-length flexible beam mounted on a mobile robot. The system under consideration is the robot PovRob, used at the European Organization for Nuclear Research (CERN) for visual and remote inspection tasks of particle accelerators. The flexible beam mounted on the robot houses cameras and sensors. The innovative aspect of the approach concerns the use of actuated masses mounted at the end of the rod, which induces an impulsive moment due to their inertia and angular acceleration. The modelling of the flexible rod has been suitably simplified in a lumped-parameter system, with dynamic parameters related to the rod’s flexibility. A linearisation of the dynamic model allows a linear-quadratic control to stabilise the system. Experimental results support the identification and the validation of the dynamic model, while simulation results evaluate the performances of the designed control law
Universal observation of multiple order parameters in cuprate superconductors
The temperature dependence of the London penetration depth \lambda was
measured for an untwined single crystal of YBa_2Cu_3O_{7-\delta} along the
three principal crystallographic directions (a, b, and c). Both in-plane
components (\lambda_a and \lambda_b) show an inflection point in their
temperature dependence which is absent in the component along the c-direction
(\lambda_c). The data provide convincing evidence that the in-plane
superconducting order parameter is a mixture of s+d-wave symmetry whereas it is
exclusively s-wave along the c-direction. In conjunction with previous results
it is concluded that coupled s+d-order parameters are universal and intrinsic
to cuprate superconductors.Comment: 5 pages, 3 figure
Conservation laws and bosonization in integrable Luttinger liquids
We examine and explain the Luttinger-liquid character of models solvable by
the Bethe ansatz by introducing a suitable bosonic operator algebra. In the
case of the Hubbard chain, this involves two bosonic algebras which apply to
{\it all} values of , electronic density, and magnetization. Only at zero
magnetization does this lead to the usual charge - spin separation. We show
that our ``pseudoparticle'' operator approach clarifies, unifies, and extends
several recent results, including the existence of independent right and left
equations of motion and the concept of ``pseudoparticle'' (also known as
``Bethe quasiparticle'').Comment: 12 pages, RevTeX, preprint CSI
Visual control through narrow passages for an omnidirectional wheeled robot
Robotic systems are gradually replacing human intervention in dangerous facilities to improve human safety and prevent risky situations. In this domain, our work addresses the problem of autonomous crossing narrow passages in a semi-structured (i.e., partially-known) environment. In particular, we focus on the CERN’s Super Proton Synchrotron particle accelerator, where a mobile robot platform is equipped with a lightweight arm to perform measurements, inspection, and maintenance operations. The proposed approach leverages an image-based visual servoing strategy that exploits computer vision to detect and track known geometries defining narrow passage gates. The effectiveness of the proposed approach has been demonstrated in a realistic mock-up
Ward identity and optical-conductivity sum rule in the d-density wave state
We consider the role of the Ward identity in dealing with the transport
properties of an interacting system forming a d-wave modulated charge-density
wave or staggered flux phase. In particular, we address this issue from the
point of view of the restricted optical-conductivity sum rule. Our aim is to
provide a controlled approximation for the current-current correlation function
which allows us also to determine analytically the corresponding sum rule. By
analyzing the role of the vertex functions in both the microscopic interacting
model and in the effective mean-field Hamiltonian, we propose a non-standard
low-energy sum-rule for this system. We also discuss the possible applicability
of these results for the description of cuprate superconductors in the
pseudogap regime.Comment: Revised version, accepted for publication in Phys. Rev.
Anomalous optical absorption in overdoped cuprates near the charge-ordering instability
We propose an interpretation for the hump observed in the optical
conductivity at or below a few hundreds of cm, in overdoped cuprates
like the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\gtrsim 0.15 and the
hole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on
the direct excitation of charge collective modes, which become nearly critical
in the proximity to a charge-ordering instability. The nearly critical
character of these excitations entails a peculiar temperature dependence and a
pseudo-scaling form of the lineshapes, which are in agreement with the
experimental data.Comment: 5 pages, 3 figure
- …