Alberto Rojo - Academia.edu (original) (raw)
Papers by Alberto Rojo
American Journal of Physics, 2005
Physical Review A, 2001
We consider a system of two species of bosons of equal mass, with interactions U a (|x|) and U x ... more We consider a system of two species of bosons of equal mass, with interactions U a (|x|) and U x (|x|) for bosons of the same and different species respectively. We present a rigorous proof-valid when the Hamiltonian does not include a species switching term-showing that, when U x (|x|) > U a (|x|), the ground state is fully "polarized" (consists of atoms of one kind only). In the unpolarized phase the low energy excitation spectrum corresponds to two linearly dispersing modes that are even and odd under species exchange. The polarization instability is signaled by the vanishing of the velocity of the odd modes.
Physical Review B, 1996
A proof is presented for the absence of gap for spin 1/2 ladders with an odd number of legs, in t... more A proof is presented for the absence of gap for spin 1/2 ladders with an odd number of legs, in the infinite leg length limit. This result is relevant to the current discussion of coupled one-dimensional spin systems, a physical realization of which are vanadyl pyrophosphate, (VO) 2 P 2 O 7 , and stoichiometric Sr n−1 Cu n+1 O 2n (with n = 3, 5, 7, 9,. . .).
Physical review. E, 2016
In this paper we establish a connection between particle trajectories subject to a nonholonomic c... more In this paper we establish a connection between particle trajectories subject to a nonholonomic constraint and light ray trajectories in a variable index of refraction. In particular, we extend the analysis of systems with linear nonholonomic constraints to the dynamics of particles in a potential subject to nonlinear velocity constraints. We contrast the long time behavior of particles subject to a constant kinetic energy constraint (a thermostat) to particles with the constraint of parallel velocities. We show that, while in the former case the velocities of each particle equalize in the limit, in the latter case all the kinetic energies of each particle remain the same.
We study an ultra-cold atomic Fermi Gas with the atom-atom interaction modulated periodically in ... more We study an ultra-cold atomic Fermi Gas with the atom-atom interaction modulated periodically in space. A novel ground state with cooper pairs occupying non-zero center of mass momenta is found. Such a state is closely related to the state proposed by Fulde, Ferrell, Larkin, and Ovchinnikov(FFLO). The resultant single particle excitations with momenta along the direction of the modulation shows multiple-gap structures. Such a system can be realized in experiment with a spatially modulated Feshbach resonance. Experimental signatures of such a state are discussed.
Physical Review B, 1992
We discuss ellipsometric experiments seeking evidence of broken time-reversal symmetry in the hig... more We discuss ellipsometric experiments seeking evidence of broken time-reversal symmetry in the hightemperature copper oxide superconductors. We use both a generalized symmetry analysis, and a magnetoelectric model suggested by Dzyaloshinskii, to argue that the hypothesis that best fits the various experimental results is the P'T-invariance hypothesiswhich assumes broken '7 symmetry in each plane, and antiferromagnetic (alternating) order of the broken symmetry in the c direction. We suggest two experimental tests of the PV:invariant model; one of these is sufficient to rule out any other brokensymmetry state and so has the potential to be extremely useful.
Physical Review B, 1997
We have studied thermal conductivity, thermopower, and resistivity of bulk polycrystalline La 0.8... more We have studied thermal conductivity, thermopower, and resistivity of bulk polycrystalline La 0.8 Ca 0.2 MnO 3. Large changes in the magnetothermal conductivity of about 15% and the thermopower as large as 80% were found in addition to a large magnetoresistivity of 80% around the magnetic transition temperature at 240 K. The temperature dependence of the thermal conductivity displays a striking dip, while the resistivity and the thermopower have a peak near 240 K. The temperature and field dependences of the thermal conductivity are attributed to the scattering of the phonons by spin fluctuations.
European Journal of Control, 2004
In this paper we analyze the quantum control of squeezed states of harmonic oscillators as well a... more In this paper we analyze the quantum control of squeezed states of harmonic oscillators as well as spin squeezing and its relationship to quadrature squeezed states of other bosonic fields. Squeezing provides a method for reducing noise below the quantum limit and provides an example of the control of under-actuated control systems in the stochastic and quantum context. We consider also the interaction of a squeezed quantum oscillator with an external heat bath and the problem of cancellation of squeezed states. Our controls consist of single or multiple pulses.
Physical Review E, 2002
We present a model that explains the origin and predicts the statistical properties of columnar q... more We present a model that explains the origin and predicts the statistical properties of columnar quasi-hexagonal crack patterns, as observed in the columnar jointing of basaltic lava flows. Irregular fractures appear at the surface of the material, induced by temperature gradients during cooling. At later times fractures penetrate into the material, and tend to form polygonal patterns. We show that this ordering can be described as a tendency to minimize an energy functional. Atomistic simulations confirm this interpretation. Numerical simulations based on a phenomenological implementation of this principle generate patterns that have remarkably good statistical agreement with real ones.
We analyze the stability of a gyroscopic oscillator interacting with a finite- and infinite-dimen... more We analyze the stability of a gyroscopic oscillator interacting with a finite- and infinite-dimensional heat bath in both the classical and quantum cases. We consider a finite gyroscopic oscillator model of a particle in a magnetic field and examine the stability before and after coupling to a heat bath. It is shown that if the oscillator is gyroscopically stable, coupling
Science, 1997
Femtosecond laser pulses and coherent two-phonon Raman scattering were used to excite KTaO 3 into... more Femtosecond laser pulses and coherent two-phonon Raman scattering were used to excite KTaO 3 into a squeezed state, nearly periodic in time, in which the variance of the atomic displacements dips below the standard quantum limit for half of a cycle. This nonclassical state involves a continuum of transverse acoustic modes that leads to oscillations in the refractive index associated with the frequency of a van Hove singularity in the phonon density of states.
Physical Review Letters, 1997
Physical Review Letters, 2000
Physical Review Letters, 1991
We consider the coupling between two T-violating systems by a non-T-violating interaction. We sho... more We consider the coupling between two T-violating systems by a non-T-violating interaction. We show that the configuration in which the signs of the T breaking in the two systems are opposite generally has lower energy. In the context of those theories of the cuprate superconductors which postulate breaking of T invariance in the Cu02 planes, a crude estimate of the Coulomb-induced "antiferromagnetic" coupling for reasonable domain sizes indicates that it is an "edge effect but at least competitive with the "ferromagnetic" one due to 3osephson and magnetic eAects.
Physical Review E, 2013
It is well known that there is an analogy between optics and mechanics that prompted much of the ... more It is well known that there is an analogy between optics and mechanics that prompted much of the classical theory of mechanics and indeed extended to the theory of quantum mechanics. We develop here an optical mechanical analogy for a prototypical non-holonomic mechanical system, a knife-edge moving on a plane under the influence of a potential. We show that this approach is related but different from the classical theory of Hamiltonization of nonholonomic systems.
American Journal of Physics, 2005
Physical Review A, 2001
We consider a system of two species of bosons of equal mass, with interactions U a (|x|) and U x ... more We consider a system of two species of bosons of equal mass, with interactions U a (|x|) and U x (|x|) for bosons of the same and different species respectively. We present a rigorous proof-valid when the Hamiltonian does not include a species switching term-showing that, when U x (|x|) > U a (|x|), the ground state is fully "polarized" (consists of atoms of one kind only). In the unpolarized phase the low energy excitation spectrum corresponds to two linearly dispersing modes that are even and odd under species exchange. The polarization instability is signaled by the vanishing of the velocity of the odd modes.
Physical Review B, 1996
A proof is presented for the absence of gap for spin 1/2 ladders with an odd number of legs, in t... more A proof is presented for the absence of gap for spin 1/2 ladders with an odd number of legs, in the infinite leg length limit. This result is relevant to the current discussion of coupled one-dimensional spin systems, a physical realization of which are vanadyl pyrophosphate, (VO) 2 P 2 O 7 , and stoichiometric Sr n−1 Cu n+1 O 2n (with n = 3, 5, 7, 9,. . .).
Physical review. E, 2016
In this paper we establish a connection between particle trajectories subject to a nonholonomic c... more In this paper we establish a connection between particle trajectories subject to a nonholonomic constraint and light ray trajectories in a variable index of refraction. In particular, we extend the analysis of systems with linear nonholonomic constraints to the dynamics of particles in a potential subject to nonlinear velocity constraints. We contrast the long time behavior of particles subject to a constant kinetic energy constraint (a thermostat) to particles with the constraint of parallel velocities. We show that, while in the former case the velocities of each particle equalize in the limit, in the latter case all the kinetic energies of each particle remain the same.
We study an ultra-cold atomic Fermi Gas with the atom-atom interaction modulated periodically in ... more We study an ultra-cold atomic Fermi Gas with the atom-atom interaction modulated periodically in space. A novel ground state with cooper pairs occupying non-zero center of mass momenta is found. Such a state is closely related to the state proposed by Fulde, Ferrell, Larkin, and Ovchinnikov(FFLO). The resultant single particle excitations with momenta along the direction of the modulation shows multiple-gap structures. Such a system can be realized in experiment with a spatially modulated Feshbach resonance. Experimental signatures of such a state are discussed.
Physical Review B, 1992
We discuss ellipsometric experiments seeking evidence of broken time-reversal symmetry in the hig... more We discuss ellipsometric experiments seeking evidence of broken time-reversal symmetry in the hightemperature copper oxide superconductors. We use both a generalized symmetry analysis, and a magnetoelectric model suggested by Dzyaloshinskii, to argue that the hypothesis that best fits the various experimental results is the P'T-invariance hypothesiswhich assumes broken '7 symmetry in each plane, and antiferromagnetic (alternating) order of the broken symmetry in the c direction. We suggest two experimental tests of the PV:invariant model; one of these is sufficient to rule out any other brokensymmetry state and so has the potential to be extremely useful.
Physical Review B, 1997
We have studied thermal conductivity, thermopower, and resistivity of bulk polycrystalline La 0.8... more We have studied thermal conductivity, thermopower, and resistivity of bulk polycrystalline La 0.8 Ca 0.2 MnO 3. Large changes in the magnetothermal conductivity of about 15% and the thermopower as large as 80% were found in addition to a large magnetoresistivity of 80% around the magnetic transition temperature at 240 K. The temperature dependence of the thermal conductivity displays a striking dip, while the resistivity and the thermopower have a peak near 240 K. The temperature and field dependences of the thermal conductivity are attributed to the scattering of the phonons by spin fluctuations.
European Journal of Control, 2004
In this paper we analyze the quantum control of squeezed states of harmonic oscillators as well a... more In this paper we analyze the quantum control of squeezed states of harmonic oscillators as well as spin squeezing and its relationship to quadrature squeezed states of other bosonic fields. Squeezing provides a method for reducing noise below the quantum limit and provides an example of the control of under-actuated control systems in the stochastic and quantum context. We consider also the interaction of a squeezed quantum oscillator with an external heat bath and the problem of cancellation of squeezed states. Our controls consist of single or multiple pulses.
Physical Review E, 2002
We present a model that explains the origin and predicts the statistical properties of columnar q... more We present a model that explains the origin and predicts the statistical properties of columnar quasi-hexagonal crack patterns, as observed in the columnar jointing of basaltic lava flows. Irregular fractures appear at the surface of the material, induced by temperature gradients during cooling. At later times fractures penetrate into the material, and tend to form polygonal patterns. We show that this ordering can be described as a tendency to minimize an energy functional. Atomistic simulations confirm this interpretation. Numerical simulations based on a phenomenological implementation of this principle generate patterns that have remarkably good statistical agreement with real ones.
We analyze the stability of a gyroscopic oscillator interacting with a finite- and infinite-dimen... more We analyze the stability of a gyroscopic oscillator interacting with a finite- and infinite-dimensional heat bath in both the classical and quantum cases. We consider a finite gyroscopic oscillator model of a particle in a magnetic field and examine the stability before and after coupling to a heat bath. It is shown that if the oscillator is gyroscopically stable, coupling
Science, 1997
Femtosecond laser pulses and coherent two-phonon Raman scattering were used to excite KTaO 3 into... more Femtosecond laser pulses and coherent two-phonon Raman scattering were used to excite KTaO 3 into a squeezed state, nearly periodic in time, in which the variance of the atomic displacements dips below the standard quantum limit for half of a cycle. This nonclassical state involves a continuum of transverse acoustic modes that leads to oscillations in the refractive index associated with the frequency of a van Hove singularity in the phonon density of states.
Physical Review Letters, 1997
Physical Review Letters, 2000
Physical Review Letters, 1991
We consider the coupling between two T-violating systems by a non-T-violating interaction. We sho... more We consider the coupling between two T-violating systems by a non-T-violating interaction. We show that the configuration in which the signs of the T breaking in the two systems are opposite generally has lower energy. In the context of those theories of the cuprate superconductors which postulate breaking of T invariance in the Cu02 planes, a crude estimate of the Coulomb-induced "antiferromagnetic" coupling for reasonable domain sizes indicates that it is an "edge effect but at least competitive with the "ferromagnetic" one due to 3osephson and magnetic eAects.
Physical Review E, 2013
It is well known that there is an analogy between optics and mechanics that prompted much of the ... more It is well known that there is an analogy between optics and mechanics that prompted much of the classical theory of mechanics and indeed extended to the theory of quantum mechanics. We develop here an optical mechanical analogy for a prototypical non-holonomic mechanical system, a knife-edge moving on a plane under the influence of a potential. We show that this approach is related but different from the classical theory of Hamiltonization of nonholonomic systems.