arXiv subjects

D. V. Anghel

Publications and source records attributed to D. V. Anghel.

13 recordsLinked to original sources

arXiv2023

Resonance phenomena in a nanomagnet coupled to a Josephson junction under external periodic drive

We investigate resonance phenomena in a system consisting of a nanomagnet coupled to a Josephson junction under external periodic drive. The coupling in the system leads to appearance of additional resonance peaks whose properties depend on the periodic signal and Josephson junction dynamics. In the linear regime, we derive an analytical description of the resonance phenomena which is then confirmed by numerical simulations. This analytical method is universal and can be also applied to Josephson junctions with anomalous phase shift in current phase relation. This work provides a new method of controlling the resonances of hybrid structures, which may be interesting for applications.

cond-mat.supr-con↗

arXiv2023

Controlling the electron-phonon heat exchange in a metallic film by its position in a dielectric slab

We theoretically study the heat flux between electrons and phonons in a thin metallic film embedded in a suspended dielectric slab (called a \textit{membrane}, in accordance with the established nomenclature), forming a layered structure. The thickness of the membrane is much smaller than the other two dimensions and, in the considered temperature range, is comparable to the dominant phonon wavelength. The thickness of the metallic layer is an order of magnitude smaller than the thickness of the membrane. While the dependence of the heat exchange on the thicknesses of the film and of the membrane has been studied before, it is not yet known how this depends on the position of the film inside the membrane. Here we show that the position strongly influences the heat exchange. If we denote by $T_e$ the effective temperature of the electrons in the metal and by $T_{ph}$ the effective temperature of the phonons (assumed to be uniform in the entire system), then we may write in general the heat power as $P \equiv P^{(0)}(T_e) - P^{(0)}(T_{ph})$, where $P^{(0)}(T) \equiv P_s^{(0)}(T) + P_a^{(0)}(T)$, with $P_s^{(0)}(T)$ and $P_a^{(0)}(T)$ being the contributions of the symmetric and antisymmetric Lamb modes, respectively. In the low-temperature limit, we may write $P_s^{(0)}(T) \equiv C_s T^4$ and $P_a^{(0)}(T) \equiv C_a T^{3.5}$, where $C_s$ is independent of the position of the film inside the membrane, whereas $C_a$ increases with the distance between the mid-plane of the film and the mid-plane of the membrane, being zero when the film is at the center of the membrane. Our examples show that by changing the position of the film inside the membrane one may change the electron-phonon heat power by orders of magnitude, depending on the dimensions and the temperature range.

cond-mat.mes-hall↗

arXiv2022

Kapitsa pendulum effects in Josephson junction + nanomagnet under external periodic drive

We investigate reorientation effects under external periodic drive in the nanomagnet dynamics coupled to a Josephson junction. The Kapitsa pendulum is introduced as a mechanical analog to this system and we demonstrate the reorientation of the easy axis of the nanomagnet. The magnetic field generated by the Josephson junction and external drive plays the role of the oscillating force of the suspension point in the Kapitsa pendulum. The high frequency oscillations change the orientation of the magnetic moment. The magnetic field of the quasiparticle current determines the frequency dependence of the magnetic moment's orientation. We obtain simple analytical formulas for the stable position of the magnetic moment, both under the external periodic drive and without it. The influence of external periodic drive on the voltage of complete reorientation have been demonstrated.

cond-mat.other↗

arXiv2019

Cold-electron bolometer, as a 1 cm wavelength photon counter

We investigate theoretically the possibility of using the cold-electron bolometer (CEB) as a counter for 1 cm wavelength (30 GHz) photons. To reduce the flux of photons from the environment, which interact with the detector, the bath temperature is assumed to be below 50 mK. At such temperatures, the time interval between two subsequent photons of 30 GHz that hit the detector is more than 100 hours, on average, for a frequency window of 1 MHz. Such temperatures allow the observation of the physically significant photons produced in rare events, like the axions conversion (or Primakoff conversion) in magnetic field. We present the general formalism for the detector's response and noise, together with numerical calculations for proper experimental setups. We observe that the current-biased regime is favorable, due to lower noise, and allows for the photons counting at least below 50 mK. For the experimental setups investigated here, the voltage-biased CEBs may also work as photons counters, but with less accuracy and eventually at bath temperatures below 40 mK. The voltage-biased setups also require smaller volumes of the normal metal island of the detector.

cond-mat.mes-hall↗

arXiv2019

Electromagnetic radiation detectors based on Josephson junctions: Effective Hamiltonian

We theoretically analyze two setups of low energy single-photon counters based on Josephson junctions (JJ). For this, we propose two simple and general models, based on the macroscopic quantum tunneling formalism (MQT). The first setup is similar to the photon counter based on the "cold electron bolometer" (CEB), where the JJ replaces the CEB in the center of the superconducting antenna. In the second setup, the JJ is capacitively coupled to the antenna. We derive the Hamiltonians for the two setups and we write the Schrödinger equations taking into account both, the antenna and the JJ. The quantum particles of the MQT models are in two-dimensional potential landscapes, which are parabolic along one direction and may have the form of a washboard potential along another direction. Such a potential landscape has a series of local minima, separated by saddle points. If the particle is prepared in the initial state in the metastable "ground state" of a local minimum, then the photon absorption causes it to jump into an excited state. If the excitation energy is bigger than the potential barrier seen by the quantum particle (the difference between the "ground state" and the saddle point), the photon is detected. The models are simple and allow us to do mostly analytical calculations. We show that the two setups are equivalent form the MQT point of view, since one Hamiltonian can be transformed into the other by changes of variables. For typical values of the JJ and antenna parameters, the setups may work as counters of photons of wavelengths up to 1 cm, at least.

cond-mat.mes-hall↗

arXiv2016

Low-temperature electron-phonon heat transfer in metal films

We consider the deformation potential mechanism of the electron-phonon coupling in metal films and investigate the intensity of the associated heat transfer between the electron and phonon subsystems. The focus is on the temperature region below dimensional crossover $T<T^{\ast}$ where the thermally relevant vibrations are described in terms of a quasi-two-dimensional elastic medium, while electron excitations behave as a three-dimensional Fermi gas. We derive an explicit expression for the power $P\left( T\right) $ of the electron-phonon heat transfer which explains the behavior observed in some experiments including the case of metallic film supported by an insulating membrane with different acoustic properties. It is shown that at low temperatures the main contribution is due to the coupling with Lamb's dilatational and flexural acoustic modes.

cond-mat.mes-hall↗

arXiv2007

Interaction of two level systems in amorphous materials with arbitrary phonon fields

To describe the interaction of the two level systems (TLSs) of an amorphous solid with arbitrary strain fields, we introduce a generalization of the standard interaction Hamiltonian. In this new model, the interaction strength depends on the orientation of the TLS with respect to the strain field through a $6\times 6$ symmetric tensor of deformation potential parameters, $[R]$. Taking into account the isotropy of the amorphous solid, we deduce that $[R]$ has only two independent parameters. We show how these two parameters can be calculated from experimental data and we prove that for any amorphous bulk material the average coupling of TLSs with longitudinal phonons is always stronger than the average coupling with transversal phonons (in standard notations, $γ_l>γ_t$).

cond-mat.dis-nn↗

arXiv2007

The tensor of interaction of a two-level system with an arbitrary strain field

The interaction between two-level systems (TLS) and strain fields in a solid is contained in the diagonal matrix element of the interaction hamiltonian, $δ$, which, in general, has the expression $δ=2[γ]:[S]$, with the tensor $[γ]$ describing the TLS ``deformability'' and $[S]$ being the symmetric strain tensor. We construct $[γ]$ on very general grounds, by associating to the TLS two objects: a direction, $\hat\bt$, and a forth rank tensor of coupling constants, $[[R]]$. Based on the method of construction and on the invariance of the expression of $δ$ with respect to the symmetry transformation of the solid, we conclude that $[[R]]$ has the same structure as the tensor of stiffness constants, $[[c]]$, from elasticity theory. In particular, if the solid is isotropic, $[[R]]$ has only two independent parameters, which are the equivalent of the Lamé constants. Employing this model we calculate the absorption and emission rates of phonons on TLSs and show that in isotropic solids, on average, the longitudinal phonons interact stronger with the TLSs than the transversal ones, as it is observed in experiments. We also show that in isotropic solids, a transversal wave leaves unperturbed all the TLSs with the direction contained in one of the two planes that are perpendicular either to the wave propagation direction or to the polarization direction and that a longitudinal strain applied to the solid polarises the TLS ensemble.

cond-mat.dis-nn↗

arXiv2007

Interaction of Lamb modes with two-level systems in amorphous nanoscopic membranes

Using a generalized model of interaction between a two-level system (TLS) and an arbitrary deformation of the material, we calculate the interaction of Lamb modes with TLSs in amorphous nanoscopic membranes. We compare the mean free paths of the Lamb modes with different symmetries and calculate the heat conductivity $κ$. In the limit of an infinitely wide membrane, the heat conductivity is divergent. Nevertheless, the finite size of the membrane imposes a lower cut-off for the phonons frequencies, which leads to the temperature dependence $κ\propto T(a+b\ln T)$. This temperature dependence is a hallmark of the TLS-limited heat conductance at low temperature.

cond-mat.mes-hall↗

arXiv2006

Quantization of the elastic modes in an isotropic plate

We quantize the elastic modes in a plate. For this, we find a complete, orthogonal set of eigenfunctions of the elastic equations and we normalize them. These are the phonon modes in the plate and their specific forms and dispersion relations are manifested in low temperature experiments in ultra-thin membranes.

cond-mat.mtrl-sci↗

arXiv2004

Heat transport in ultra-thin dielectric membranes and bridges

Phonon modes and their dispersion relations in ultrathin homogenous dielectric membranes are calculated using elasticity theory. The approach differs from the previous ones by a rigorous account of the effect of the film surfaces on the modes with different polarizations. We compute the heat capacity of membranes and the heat conductivity of narrow bridges cut out of such membranes, in a temperature range where the dimensions have a strong influence on the results. In the high temperature regime we recover the three-dimensional bulk results. However, in the low temperature limit the heat capacity, $C_V$, is proportional with $T$ (temperature), while the heat conductivity, $κ$, of narrow bridges is proportional to $T^{3/2}$, leading to a thermal cut-off frequency $f_c=κ/C_V\propto T^{1/2}$.

cond-mat.mes-hall↗

arXiv2000

Performance of cryogenic microbolometers and calorimeters with on-chip coolers

Astronomical observations of cosmic sources in the far-infrared and X-ray bands require extreme sensitivity. The most sensitive detectors are cryogenic bolometers and calorimeters operating typically at about 100 mK. The last stage of cooling (from 300 mK to 100 mK) often poses significant difficulties in space-borne experiments, both in system complexity and reliability. We address the possibility of using refrigeration based on normal metal/insulator/superconductor (NIS) tunnel junctions as the last stage cooler for cryogenic thermal detectors. We compare two possible schemes: the direct cooling of the electron gas of the detector with the aid of NIS tunnel junctions and the indirect cooling method, when the detector lattice is cooled by the refrigerating system, while the electron gas temperature is decreased by electron-phonon interaction. The latter method is found to allow at least an order of magnitude improvement in detector noise equivalent power, when compared to the direct electron cooling.

cond-mat.mes-hall↗

arXiv2000

Noise in refrigerating tunnel junctions and in microbolometers

Microrefrigerators based on normal metal-insulator-superconductor (NIS) junctions represent a very attractive alternative to cool the microbolometers and calorimeters for astrophysical observations in space-borne experiments. The performance in such measurements requires a good knowledge of the noise sources in the detectors. In this paper we present detailed calculations of the thermal fluctuations and of the noise equivalent power due to the heat transfer through the NIS junctions or due to the thermal contact between different subsystems of the detector. The influence of the background radiation will also be evaluated. Analytical approximations, valid at low temperatures, are given.

cond-mat.mes-hall↗