arXiv subjects

A. Cano

Publications and source records attributed to A. Cano.

At least 55 records · Page 3Linked to original sources

arXiv2011

Multi-domain ferroelectricity as a limiting factor for voltage amplification in ferroelectric field-effect transistors

We revise the possibility of having an amplified surface potential in ferroelectric field-effect transistors pointed out by [S. Salahuddin and S. Datta, Nano Lett. 8, 405 (2008)]. We show that the negative-capacitance regime that allows for such an amplification is actually bounded by the appearance of multi-domain ferroelectricity. This imposes a severe limit to the maximum step-up of the surface potential obtainable in the device. We indicate new device design rules taking into account this scenario.

cond-mat.mtrl-sci↗

arXiv2011

High impact of spin-nematic order on the lattice domains in thin films of iron-based superconductors

The spin-nematic scenario proposed for the structural instability of iron-based superconductors is studied theoretically within a Ginzburg-Landau approach in epitaxial thin films. The transition temperature and the domains that appear due the interface with the substrate are computed analytically by following a variatonal procedure. It is predicted a cross-over thickness $l_*$ that separates the Kittel regime, in which the size of the domains goes as the square root of the film thickness $l$, from a new ultrathin regime in which the system tends to reach the single-domain state as $l \to 0$. The experimental observation of this cross-over is therefore proposed as a direct probe of spin nematiciy.

cond-mat.str-el↗

arXiv2011

Magnetoelastic Effects in Iron Telluride

Iron telluride doped lightly with selenium is known to undergo a first order magneto-structural transition before turning superconducting at higher doping. We study the effects of magneto-elastic couplings on this transition using symmetry considerations. We find that the magnetic order parameters are coupled to the uniform monoclinic strain of the unit cell with one iron per cell, as well as to the phonons at high symmetry points of the Brillouin zone. In the magnetic phase the former gives rise to monoclinic distortion while the latter induces dimerization of the ferromagnetic iron chains due to alternate lengthening and shortening of the nearest-neighbour iron-iron bonds. We compare this system with the iron arsenides and propose a microscopic magneto-elastic Hamiltonian which is relevant for all the iron based superconductors. We argue that this describes electron-lattice coupling in a system where electron-electron interaction is crucial.

cond-mat.str-el↗

arXiv2010

Interplay of magnetic and structural transitions in Fe-based pnictide superconductors

The interplay between the structural and magnetic phase transitions occurring in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau approach. We show that the magnetoelastic coupling between the corresponding order parameters is behind the salient features observed in the phase diagram of these systems. This naturally explains the coincidence of transition temperatures observed in some cases as well as the character (first or second-order) of the transitions. We also show that magnetoelastic coupling is the key ingredient determining the collinearity of the magnetic ordering, and we propose an experimental criterion to distinguish between a pure elastic from a spin-nematic-driven structural transition.

cond-mat.str-el↗

arXiv2010

Pseudo-proper ferroelectricity in thin films

We study ferroelectricity in thin films of pseudo-proper ferroelectrics such as the so-called spiral multiferroics. We find that this type of ferroelectricity stands better against depolarizing fields than conventional one. Its single-domain state can be easily preserved by metallic electrodes even in ultrathin films. In fact, single-domain ferroelectricty can be generated as a metastable state in the absence of electrodes. We also find a new regime of small thickness where unscreened films develop unusual multi-domain states with properties determined by non-electrostatic boundary conditions.

cond-mat.str-el↗

arXiv2009

Aharonov-Bohm oscillations in the local density of states

The scattering of electrons with inhomogeneities produces modulations in the local density of states of a metal. We show that electron interference contributions to these modulations are affected by the magnetic field via the Aharonov-Bohm effect. This can be exploited in a simple STM setup that serves as an Aharonov-Bohm interferometer at the nanometer scale.

cond-mat.mes-hall↗

arXiv2009

Electromagnon Resonances in the Optical Response of Spiral Magnets

The optical response of spiral magnets is studied, with special attention to its electromagnon features. We show that these features trace back to the resonant magnetoelectric response resulting from the spiral ordering (irrespective of any concomitant ferroelectricity). This response, being magnetoelectric in nature, not always can be reduced to an effective electric permittivity. We argue that electromagnons in spiral magnets can produce, in addition to the observed peaks in the optical absorption of multiferroics, a (dynamically enhanced) optical rotation and a negative refractive index behavior.

cond-mat.str-el↗

arXiv2008

Koshino-Taylor effect in graphene

We discuss the phonon-assisted scattering of electrons by defects, i.e., the so-called Koshino-Taylor effect, in graphene. The two-dimensional character of graphene implies that the strength of the Koshino-Taylor effect can be considerably larger than in ordinary metals. We show that at finite temperatures the defect-induced resistivity formally diverges in the thermodynamic limit, having a non-analytic $T\ln T$ component when finite size effects are taken into account.

cond-mat.mes-hall↗

arXiv2008

Electromagnon excitations in modulated multiferroics

The phenomenological theory of ferroelectricity in spiral magnets presented in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe consistently states with both uniform and modulated-in-space ferroelectric polarizations. A key point in this description is the symmetric part of the magnetoelectric coupling since, although being irrelevant for the uniform component, it plays an essential role for the non-uniform part of the polarization. We illustrate this importance in generic examples of modulated magnetic systems: longitudinal and transverse spin-density wave states and planar cycloidal phase. We show that even in the cases with no uniform ferroelectricity induced, polarization correlation functions follow to the soft magnetic behavior of the system due to the magnetoelectric effect. Our results can be easily generalized for more complicated types of magnetic ordering, and the applications may concern various natural and artificial systems in condensed matter physics (e.g., magnon properties could be extracted from dynamic dielectric response measurements).

cond-mat.mtrl-sci↗

arXiv2007

A glass-like behavior in the low-temperature specific heat is a natural property of any real crystal

We provide a rigorous calculation of the free energy of a non-metallic crystal containing a small concentration of defects. The low-temperature leading contribution is found to be $\propto T^2$. This further gives a linear-in-$T$ low-temperature specific heat as that exhibited by glasses. These results also show that, similarly to what happens in glasses, the long-wavelength spectrum of a nearly perfect crystal does not suffice to determine its low-temperature behavior.

cond-mat.stat-mech↗

arXiv2007

Quasi-normal mode analysis in BEC acoustic black holes

We perform a quasi-normal mode analysis of black hole configurations in Bose-Einstein condensates (BEC). In this analysis we use the full Bogoliubov dispersion relation, not just the hydrodynamic or geometric approximation. We restrict our attention to one-dimensional flows in BEC with step-like discontinuities. For this case we show that in the hydrodynamic approximation quasi-normal modes do not exist. The full dispersion relation, however, allows the existence of quasi-normal modes. Remarkably, the spectrum of these modes is not discrete but continuous.

gr-qc↗

arXiv2006

Universal features of the defect-induced damping of lattice vibrations

It is shown that any defect gives an Ohmic contribution to the damping of any normal mode of the crystal lattice with nonzero wavevector which does not vanish at zero temperature. This explains the large phason damping observed at low temperatures in incommensurate phases, and might be a key factor to understand the linear-in-$T$ specific heat observed in a number of real dielectrics at low enough temperatures.

cond-mat.stat-mech↗

arXiv2006

On low-temperature structural phase transitions

We comment on zero- and low-temperature structural phase transitions, expecting that these comments might be relevant not only for this structural case. We first consider a textbook model whose classical version is the only model for which the Landau theory of phase transitions and the concept of ``soft mode'' introduced by Ginzburg are exact. Within this model, we reveal the effects of quantum fluctuations and thermal ones at low temperatures. To do so, the knowledge of the dynamics of the model is needed. However, as already was emphasized by Ginzburg et al. in eighties, a realistic theory for such a dynamics at high temperatures is lacking, what also seems to be the case in the low temperature regime. Consequently, some theoretical conclusions turn out to be dependent on the assumptions on this dynamics. We illustrate this point with the low-temperature phase diagram, and discuss some unexpected shortcomings of the continuous medium approaches.

cond-mat.stat-mech↗

arXiv2006

Stability analysis of sonic horizons in Bose-Einstein condensates

We examine the linear stability of various configurations in Bose-Einstein condensates with sonic horizons. These configurations are chosen in analogy with gravitational systems with a black hole horizon, a white hole horizon and a combination of both. We discuss the role of different boundary conditions in this stability analysis, paying special attention to their meaning in gravitational terms. We highlight that the stability of a given configuration, not only depends on its specific geometry, but especially on these boundary conditions. Under boundary conditions directly extrapolated from those in standard General Relativity, black hole configurations, white hole configurations and the combination of both into a black hole--white hole configuration are shown to be stable. However, we show that under other (less stringent) boundary conditions, configurations with a single black hole horizon remain stable, whereas white hole and black hole--white hole configurations develop instabilities associated to the presence of the sonic horizons.

gr-qc↗

arXiv2005

Low-temperature specific heat of real crystals: Possibility of leading contribution of optical and short-wavelength acoustical vibrations

We point out that the repeatedly reported glass-like properties of crystalline materials are not necessarily associated with localized (or quasilocalized) excitations. In real crystals, optical and short-wavelength acoustical vibrations remain damped due to defects down to zero temperature. If such a damping is frequency-independent, e.g. due to planar defects or charged defects, these optical and short-wavelength acoustical vibrations yield a linear-in-$T$ contribution to the low-temperature specific heat of the crystal lattices. At low enough temperatures such a contribution will prevail over that of the long-wavelength acoustical vibrations (Debye contribution). The crossover between the linear and the Debye regime takes place at $T^* \propto \sqrt N$, where $N$ is the concentration of the defects responsible for the damping. Estimates show that this crossover could be observable.

cond-mat.stat-mech↗

arXiv2005

Strong effect of surfaces on resolution limit of negative-index "superlens"

We show that subwavelength imaging by negative index materials (NIM), related to their "soft" electromagnetic response, is very (and non trivially) sensitive to the surface properties. A minute deviation of dielectric permittivity epsilon or magnetic permeability mu from the ideal values epsilon = mu = -1 in thin surface layer(s) results in drastic reduction of the resolution limit of a NIM slab. There may be a gap in the polariton spectrum and this would allow establishment of a stationary regime even without losses.

cond-mat.mtrl-sci↗

arXiv2004

Universal mechanism of discontinuity of commensurate-incommensurate transitions in three-dimensional solids: Strain dependence of soliton self-energy

We show that there exists a universal mechanism of long-range soliton attraction in three-dimensional solids and, therefore, of discontinuity of any commensurate-incommensurate (C-IC) phase transition. This mechanism is due to the strain dependence of the soliton self-energy and specific features of the solid-state elasticity. The role of this mechanism is studied in detail for a class of C-IC transitions where the IC modulation is one-dimensional, the anisotropy in the order parameter space is small, and the symmetry of the systems allows the existence of the Lifshitz invariant. Two other mechanisms of soliton attraction are operative here but the universal mechanism considered in this paper is found to be the most important one in some cases. Comparison with the most extensively studied C-IC transition in $\rm K_2SeO_4$ shows that the experimentally observed thermal anomalies can be understood as a result of the smearing of the theoretically predicted discontinuous transition.

cond-mat.stat-mech↗

arXiv2004

An explanation for the glass-like anomaly in the low-temperature specific heat of incommensurate phases

An explanation for the glass-like anomaly observed in the low-temperature specific heat of incommensurate phases is proposed. The key point of this explanation is the proper account for the phason damping when computing the thermodynamic magnitudes. The low-temperature specific heat of the incommensurate phases is discussed within three possible scenarios for the phason dynamics: no phason gap, static phason gap and a phason gap of dynamical origin. Existing NMR and inelastic scattering data indicate that these scenarios are possible in biphenyl, blue bronze $\rm K_{0.30}MoO_3$ and BCPS respectively. Estimates of the corresponding low-temperature specific heat are in reasonable agreement with the experiments.

cond-mat.stat-mech↗