Intrinsic Thermal Fluctuations Research Articles

Learning the non-equilibrium dynamics of Brownian movies

Time-lapse microscopy imaging provides direct access to the dynamics of soft and living systems. At mesoscopic scales, such microscopy experiments reveal intrinsic thermal and non-equilibrium fluctuations. These fluctuations, together with measurement noise, pose a challenge for the dynamical analysis of these Brownian movies. Traditionally, methods to analyze such experimental data rely on tracking embedded or endogenous probes. However, it is in general unclear, especially in complex many-body systems, which degrees of freedom are the most informative about their non-equilibrium nature. Here, we introduce an alternative, tracking-free approach that overcomes these difficulties via an unsupervised analysis of the Brownian movie. We develop a dimensional reduction scheme selecting a basis of modes based on dissipation. Subsequently, we learn the non-equilibrium dynamics, thereby estimating the entropy production rate and time-resolved force maps. After benchmarking our method against a minimal model, we illustrate its broader applicability with an example inspired by active biopolymer gels.

Optical-lattice-based Cs active clock with a continual superradiant lasing signal

We demonstrate state-of-the-art technique of an active clock to provide a continuous superradiant lasing signal using an ensemble of trapped Cs atoms in the optical lattice. A magic wavelength of the proposed |7S1/2; F = 4, MF = 0> - |6P3/2; F = 3, MF = 0> clock transition in Cs atom is identified at 1181 nm for constructing the optical lattice. Pertinent optical lines are also found for pumping and repumping atoms from their ground states. A fractional uncertainty about 10-15 level to the clock frequency has been predicted by carrying out rigorous calculations of several atomic properties. The bad-cavity operational mode of the active clock is anticipated to improve its short-term stability remarkably by suppressing intrinsic thermal fluctuations. Thus, a composite clock system with better in both short-term and long-term stabilities can be built by combining the above proposed active clock with another high-accuracy passive optical clock.

Graphene-based Josephson junction microwave bolometer.

Sensitive microwave detectors are essential in radioastronomy1, dark-matter axion searches2 and superconducting quantum information science3,4. The conventional strategy to obtain higher-sensitivity bolometry is the nanofabrication of ever smaller devices to augment the thermal response5-7. However, it is difficult to obtain efficient photon coupling and tomaintain the material properties in a device with a large surface-to-volume ratio owing to surface contamination. Here we present an ultimately thin bolometric sensor based on monolayer graphene. To utilize the minute electronic specific heat and thermal conductivity of graphene, we develop a superconductor-graphene-superconductor Josephson junction8-13 bolometer embedded in a microwave resonator with a resonance frequency of 7.9 gigahertz and over 99 per cent coupling efficiency. The dependence of the Josephson switching current on the operating temperature, charge density, input power and frequency shows a noise-equivalent power of 7×10-19 watts per square-root hertz, which corresponds to an energy resolution of a single 32-gigahertz photon14, reaching the fundamental limit imposed by intrinsic thermal fluctuations at 0.19 kelvin. Our results establish that two-dimensional materials could enable the development of bolometers with the highest sensitivity allowed by the laws of thermodynamics.

Experimental study on molecular fluctuations of biomolecules by force spectroscopy

Atomic force microscopy (AFM) measurements of biomolecules, such as proteins, are often affected by molecular fluctuations. Not only due to the intrinsic thermal fluctuation, deformation and displacement of a part or the whole molecule are induced by the AFM tip, especially when the molecules are weakly fixed to a substrate. Here we have studied the effect of the molecular fluctuations on the topography of the streptavidin (SA) protein molecules bound to a DNA origami in two different configurations by force spectroscopy experiments. While the experimental and theoretical height histograms of a tightly-fixed SA molecule were in good agreement, the experimental height histogram of a weakly-fixed SA molecule was distorted. We discussed the origins of the discrepancy of the histogram for the latter and suggested that it was due to the tip-induced deformation or the displacement of the molecule. The methodology presented here is useful for assessment of the appropriate fixation conditions for high-resolution topographic and functional imaging experiments of a single molecule by AFM.

Quantum nonlinear mixing of thermal photons to surpass the blackbody limit.

Nearly all thermal radiation phenomena involving materials with linear response can be accurately described via semi-classical theories of light. Here, we go beyond these traditional paradigms to study a nonlinear system that, as we show, requires quantum theory of damping. Specifically, we analyze thermal radiation from a resonant system containing a χ(2) nonlinear medium and supporting resonances at frequencies ω1 and ω2 ≈ 2ω1, where both resonators are driven only by intrinsic thermal fluctuations. Within our quantum formalism, we reveal new possibilities for shaping the thermal radiation. We show that the resonantly enhanced nonlinear interaction allows frequency-selective enhancement of thermal emission through upconversion, surpassing the well-known blackbody limits associated with linear media. Surprisingly, we also find that the emitted thermal light exhibits non-trivial statistics (g(2)(0) ≠ ~2) and biphoton intensity correlations (at two distinct frequencies). We highlight that these features can be observed in the near future by heating a properly designed nonlinear system, without the need for any external signal. Our work motivates new interdisciplinary inquiries combining the fields of nonlinear photonics, quantum optics and thermal science.

Persistent and Nonpersistent Error Optimization for STT-RAM Cell Design

Rapidly increasing demands for memory capacity and severe technical scaling challenges of conventional memory technologies motivated recent investments on next-generation nonvolatile memory technologies. As a promising candidate, spin-transfer torque random access memory (STT-RAM) has demonstrated many attractive properties, such as nanosecond access time, high integration density, nonvolatility, and excellent CMOS integration compatibility. However, similar to all other nano-devices, the performance and reliability of STT-RAM cells are greatly affected by process variations, device operating uncertainties, and environmental fluctuations. As a result, the read and write operations of STT-RAM demonstrate some variabilities and errors. In this paper, we systematically analyze the impacts of CMOS and magnetic tunneling junction (MTJ) process variations, MTJ resistance switching randomness that are induced by intrinsic thermal fluctuations, and working temperature changes on STT-RAM cell designs. The STT-RAM cell reliability issues in both read and write operations are first investigated. A combined circuit and magnetic simulation platform is then established to quantitatively study the persistent and nonpersistent errors in STT-RAM cell operations. Our analysis proved the importance of a full statistical design method in STT-RAM designs for design pessimism minimization.

Proposal for an optomechanical microwave sensor at the subphoton level.

Because of their low energy content, microwave signals at the single-photon level are extremely challenging to measure. Guided by recent progress in single-photon optomechanics and hybrid optomechanical systems, we propose a multimode optomechanical transducer that can detect intensities significantly below the single-photon level via adiabatic transfer of the microwave signal to the optical frequency domain where the measurement is then performed. The influence of intrinsic quantum and thermal fluctuations is also discussed.

Superconducting properties in heavily overdoped Ba(Fe0.86Co0.14)2As2 single crystals

We report the intrinsic superconducting parameters in a heavily overdoped Ba(Fe1−xCox)2As2 (x=0.14) single crystal and their influence in the resulting vortex dynamics. We find a bulk superconducting critical temperature of 9.8K, magnetic penetration depth λab (0)=660±50nm, coherence length ξab (0)=6.4±0.2nm, and the upper critical field anisotropy γT→Tc≈3.7. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. The intrinsic pinning energy plays an important role in the resulting vortex dynamics as compared with similar pinning landscape and comparable intrinsic thermal fluctuations.

The origin of wrinkles on transferred graphene

When two-dimensional graphene is exfoliated from three-dimensional highly oriented pyrolytic graphite (HOPG), ripples or corrugations always exist due to the intrinsic thermal fluctuations. Surface-grown graphenes also exhibit wrinkles, which are larger in dimension and are thought to be caused by the difference in thermal expansion coefficients between graphene and the underlying substrate in the cooling process after high temperature growth. For further characterization and applications, it is necessary to transfer the surface-grown graphenes onto dielectric substrates, and other wrinkles are generated during this process. Here, we focus on the wrinkles of transferred graphene and demonstrate that the surface morphology of the growth substrate is the origin of the new wrinkles which arise in the surface-to-surface transfer process; we call these morphology-induced wrinkles. Based on a careful statistical analysis of thousands of atomic force microscopy (AFM) topographic data, we have concluded that these wrinkles on transferred few-layer graphene (typically 1–3 layers) are determined by both the growth substrate morphology and the transfer process. Depending on the transfer medium and conditions, most of the wrinkles can be either erased or preserved. Our work suggests a new route for graphene engineering involving structuring the growth substrate and tailoring the transfer process. Open image in new window

Energy resolution of terahertz single-photon-sensitive bolometric detectors

We report measurements of the energy resolution of ultrasensitive superconducting bolometric detectors. The device is a superconducting titanium nanobridge with niobium contacts. A fast microwave pulse is used to simulate a single higher-frequency photon, where the absorbed energy of the pulse is equal to the photon energy. This technique allows precise calibration of the input coupling and avoids problems with unwanted background photons. Present devices have an intrinsic full-width at half-maximum energy resolution of approximately 23 THz, near the predicted value due to intrinsic thermal fluctuation noise.

Niobium direct detectors for fast and sensitive terahertz spectroscopy

We report the performance of a niobium hot-electron bolometer designed for laboratory terahertz spectroscopy. The antenna-coupled detector can operate above 4.2 K and has fast (subnanosecond) response. Detailed microwave measurements of performance over a wide range of operating conditions were correlated with quantitative terahertz measurements. The maximum responsivity is 4 x 10(4) VW with a noise equivalent power at the detector of 2 x 10(-14) W/Hz(12), approaching the intrinsic thermal fluctuation limit for the device. This detector enables a variety of novel laboratory spectroscopy measurements.

High-frequency noise measurements in spin-valve devices

High-frequency magnetic noise in magnetoresistive devices being developed for read-sensor and magnetic random access memory applications may present fundamental limitations on the performance of submicrometer magnetic devices. High-frequency magnetic noise (HFN) arises from intrinsic thermal fluctuations of the device magnetization. High-frequency noise spectroscopy provides a powerful tool to characterize the dynamics and response of small multilayer magnetic devices. In this study, the noise characteristics of micrometer-dimension spin valves have been investigated at frequencies in the range 0.1–6 GHz. At frequencies below this range 1/f noise dominates. HFN measurements, as a function of bias current and longitudinal magnetic field, are obtained for IrMn exchange-biased spin valves using a 50 GHz spectrum analyzer, low-noise amplifier, and a microwave probing system. The magnetic noise is obtained by taking the difference between the noise spectrum of the device in a saturated and unsaturated state. The data can be fit to simple models that predict the noise power to be proportional to the imaginary part of the free-layer magnetic susceptibility. There are some important differences between the high-frequency noise measurements and direct measurements of the device susceptibility (both at the device and wafer level). The noise measurements show a smaller damping parameter (a smaller ferromagnetic resonance linewidth) and additional features due to the presence of nonuniform modes.

Thermodynamic fluctuations in [formula omitted]-phase SmLa 0.8Sr 0.2CuO 4− δ single crystal

Series of M– T measurements have been conducted on the T ∗ -phase SmLa 0.8Sr 0.2CuO 4− δ single crystal ( T c on≈24 K) in a broad range of magnetic field (1000 Oe–10 kOe) applied along the crystal c-axis. The result clearly establishes the existence of a crossing point (M ∗ , T ∗) , signifying an intrinsic thermal fluctuations effect in this system. The effective CuO 2 interlayer distance deduced from this analysis is s≈15.50 Å, in a good agreement with the XRD refinement result. The effective dimensionality of fluctuations associated with the thermodynamic scaling properties of magnetization is analyzed on the basis of Ginzburg–Landau (GL) theory with the order parameter restricted to the lowest Landau level (GL-LLL) as well as the XY fluctuations model. It is shown that the data for the field above or equal to 8000 Oe exhibit distinct scaling behavior characteristic of the 2D GL-LLL fluctuation model. It also shows a clear departure from this and the corresponding 3D model in the lower field regime considered in this work.

The influence of thermal noise on the onset of travelling-wave convection in binary fluid mixtures: an experimental investigation

When dealing with systems showing a Hopf bifurcation as the first instability from a conductive state leading to travelling waves, the distinction between convective and absolute instability becomes significant. The convectively unstable regime is characterized by the fact that a homogeneous disturbance may have a positive growth rate, while a single localized perturbation cannot trigger the onset of nonlinear convection. In this paper the convective instability occurring in binary fluid mixtures for a negative separation ratio is utilized for amplifying intrinsic thermal fluctuations, which in this way become accessible to quantitative measurements. The experiments are performed in a quasi-one-dimensional convection channel which, by means of subcritical ramps, effectively prevents the reflection of the travelling waves from the sidewalls. Thus, that range of the convective instability within which linear waves can be observed is strongly enhanced. The temperature variations involved in the observed travelling-wave states are quantified by using the shadowgraph method. By resonantly stimulating the system with its linear Hopf frequency, the reflection ability and some coefficients of the amplitude equation appropriate for describing the convection features near onset can be determined. Without stimulation, travelling-wave states of very small amplitudes showing an erratic spatio-temporal behaviour occur spontaneously inside the convectively unstable regime. The temporal correlation function calculated from the measured light intensity caused by these states is compared with a theoretical expression obtained from a Ginzburg—Landau equation containing a noise term. A very good agreement is found for the amplitude if thermal noise is assumed to be the reason for these fluctuating convection rolls, thus supporting the idea that the response of the system to thermal fluctuations is observed.

Dissipation in Gravitationally Driven Saturated Superfluid 4He Films

A new method utilizing a traditional two beaker geometry is used to study dissipation in moving 4He films. Data are obtained at temperatures between 1.5 K and Tλ, and the static film thickness is varied by a factor of two. Our results are analyzed using the intrinsic thermal fluctuation theory [1]. We find that the fluctuation frequency is essentially constant across the investigated temperature range, while the barrier velocity is proportional to ρs/ρT above 1.9 K. We also observe for the first time in a gravitational flow experiment the expected [2] decrease in the critical velocity as the lambda transition is approached. This decrease is found to depend on the geometry of the experiment.

On the streaming phenomenon—II. Dynamical considerations

The dynamical aspects of streaming are considered. It is found that the calculated characteristic times resulting from intrinsic thermal fluctuations in the CuAu and Fe-32 at.% Ni systems differ by nine to ten orders of magnitude. This is in sharp contrast to the reported experimental observations which have found the characteristic times to be similar. It is proposed that streaming observed in Fe-32% Ni is caused by extrinsic excitations perhaps associated with oscillating thermal stresses resulting from fluctuations in the beam current of the electron microscope. Streaming observed in CuAu may, however, result from intrinsic thermal fluctuations as the calculated characteristic time is in reasonable agreement with reported experimental results.