Direction Of Heat Current Research Articles

Thermal conductivity of nonunitary triplet superconductors: application to UTe2

Considerable evidence shows that the heavy fermion material UTe2 is a spin-triplet superconductor, possibly manifesting time-reversal symmetry breaking, as measured by Kerr effect below the critical temperature, in some samples. Such signals can arise due to a chiral orbital state or possible nonunitary pairing. Although experiments at low temperatures appear to be consistent with point nodes in the spectral gap, the detailed form of the order parameter and even the nodal positions are not yet determined. Thermal conductivity measurements can extend to quite low temperatures, and varying the heat current direction should be able to provide information on the order parameter structure. Here, we derive a general expression for the thermal conductivity of a spin-triplet superconductor and use it to compare the low-temperature behavior of various states proposed for UTe2.

Heat Modulation on Target Thermal Bath via Coherent Auxiliary Bath

We study a scheme of thermal management where a three-qubit system assisted with a coherent auxiliary bath (CAB) is employed to implement heat management on a target thermal bath (TTB). We consider the CAB/TTB being ensemble of coherent/thermal two-level atoms (TLAs), and within the framework of collision model investigate the characteristics of steady heat current (also called target heat current (THC)) between the system and the TTB. It demonstrates that with the help of the quantum coherence of ancillae the magnitude and direction of heat current can be controlled only by adjusting the coupling strength of system-CAB. Meanwhile, we also show that the influences of quantum coherence of ancillae on the heat current strongly depend on the coupling strength of system—CAB, and the THC becomes positively/negatively correlated with the coherence magnitude of ancillae when the coupling strength below/over some critical value. Besides, the system with the CAB could serve as a multifunctional device integrating the thermal functions of heat amplifier, suppressor, switcher and refrigerator, while with thermal auxiliary bath it can only work as a thermal suppressor. Our work provides a new perspective for the design of multifunctional thermal device utilizing the resource of quantum coherence from the CAB.

Magneto-optical painting of heat current

Active control of heat flow is crucial for the thermal management of increasingly complex electronic and spintronic devices. In addition to conventional heat transport engineering, spin caloritronics has received extensive attention as a heat control principle owing to its high controllability and unique thermal energy conversion symmetry. Here we demonstrate that the direction of heat currents generated by spin-caloritronic phenomena can be changed simply by illuminating magnetic materials with visible light. The optical control of heat currents is realized through a combination of the spin-driven thermoelectric conversion called an anomalous Ettingshausen effect and all-optical helicity-dependent switching of magnetization. This approach enables not only pinpoint manipulation and flexible design of the heat current distribution by patterning the illuminating light but also on/off control of the resulting temperature modulation by tuning the light polarization. These versatile heat control functionalities will open up a pathway for nanoscale thermal energy engineering.

Strain-induced switching of heat current direction generated by magneto-thermoelectric effects

Since the charge current plays a major role in information processing and Joule heating is inevitable in electronic devices, thermal management, i.e., designing heat flows, is required. Here, we report that strain application can change a direction of a heat current generated by magneto-thermoelectric effects. For demonstration, we used metallic magnets in a thin-film form, wherein the anomalous Ettingshausen effect mainly determines the direction of the heat flow. Strain application can alter the magnetization direction owing to the magnetoelastic effect. As a result, the heat current, which is in the direction of the cross product of the charge current and the magnetization vector, can be switched or rotated simply by applying a tensile strain to the metallic magnets. We demonstrate 180° switching and 90° rotation of the heat currents in an in-plane magnetized Ni sample on a rigid sapphire substrate and a perpendicularly magnetized TbFeCo film on a flexible substrate, respectively. An active thermography technique was used to capture the strain-induced change in the heat current direction. The method presented here provides a novel method for controlling thermal energy in electronic devices.

Anisotropic Thermal Magnetoresistance for an Active Control of Radiative Heat Transfer

The discovery that the near-field radiative heat transfer enables to overcome the limit set by Planck’s law holds the promise to have an impact in different nanotechnologies that make use of thermal radiation, and the challenge now is to find strategies to actively control and manipulate this near-field thermal radiation. Here, we predict a huge anisotropic thermal magnetoresistance (ATMR) in the near-field radiative heat transfer between magneto-optical particles when the direction of an external magnetic field is changed with respect to the heat current direction. We illustrate this effect with the case of two InSb particles where we find that the ATMR amplitude can reach values of up to 800% for a magnetic field of 5 T, which is many orders of magnitude larger than its spintronic analogue. This thermomagnetic effect could find broad applications in the field of ultrafast thermal management as well as magnetic and thermal remote sensing.

Controlling heat and particle currents in nanodevices by quantum observation

We demonstrate that in a standard thermo-electric nanodevice the current and heat flows are not only dictated by the temperature and potential gradient, but also by the external action of a local quantum observer that controls the coherence of the device. Depending on how and where the observation takes place, the direction of heat and particle currents can be independently controlled. In fact, we show that the current and heat flow in a quantum material can go against the natural temperature and voltage gradients. Dynamical quantum observation offers new possibilities for the control of quantum transport far beyond classical thermal reservoirs. Through the concept of local projections, we illustrate how we can create and directionality control the injection of currents (electronic and heat) in nanodevices. This scheme provides novel strategies to construct quantum devices with application in thermoelectrics, spintronic injection, phononics, and sensing among others. In particular, highly efficient and selective spin injection might be achieved by local spin projection techniques.

Domain formation mechanism of the Si(110) “ 16×2 ” reconstruction

The main factor that determines which of the two domains form upon reconstruction of the Si(110)"16 x 2" surface has been investigated. LEED and STM images showed that the domain orientation was independent of the heating current direction used to induce the Si(110)"16 x 2" reconstruction. Reciprocal-space lattice models of the reconstruction allowed for the correct identification of the domain orientations in the LEED images and confirm that the reconstruction is 2D-chiral. It is proposed that the domain orientation upon surface reconstruction is determined by the direction of monoatomic steps present on the Si(110) plane. This is in turn determined by the direction at which the surface is polished off-axis from the (110) plane.

Ballistic induced pumping of hypersonic heat current in DNA nano wire

Heat shuttling properties of DNA nano-wire driven by an external force against the spontaneous heat current direction in non-zero temperature bias (non averaged) have been studied. We examined the valid region of driving amplitude and frequency to have pumping state in terms of temperature bias and the system size. It was shown that DNA could act as a high efficiency thermal pump in the hypersonic region. Amplitude-dependent resonance frequencies of DNA indicating intrinsic base pair internal vibrations have been detected. Nonlinearity implies that by increasing the driven amplitude new vibration modes are detected. To verify the results, an analytical parallel investigation based on multifractal concept has been done. By using the geometric properties of the strange attractor of the system, the threshold value to transition to the pumping state for given external amplitude has been identified. It was shown that the system undergoes a phase transition in sliding point to the pumping state. Fractal dimension demonstrates that the ballistic transport is responsible for energy pumping in the system. In the forbidden band gap, DNA could transmit the energy by exceeding the threshold amplitude. Despite of success in energy pumping, in this framework, DNA could not act as a real cooler.

Thermoelectric study of dissipative quantum-dot heat engines

This paper examines the thermoelectric response of a dissipative quantum dot heat engine based on the Anderson-Holstein model in two relevant operating limits: (i) when the dot phonon modes are out of equilibrium, and (ii) when the dot phonon modes are strongly coupled to a heat bath. In the first case, a detailed analysis of the physics related to the interplay between the quantum dot level quantization, the on-site Coulomb interaction and the electron-phonon coupling on the thermoelectric performance reveals that an n-type heat engine performs better than a p-type heat engine. In the second case, with the aid of the dot temperature estimated by incorporating a {\it{thermometer bath}}, it is shown that the dot temperature deviates from the bath temperature as electron-phonon interaction becomes stronger. Consequently, it is demonstrated that the dot temperature controls the direction of phonon heat currents, thereby influencing the thermoelectric performance. Finally, the conditions on the maximum efficiency with varying phonon couplings between the dot and all the other macroscopic bodies are analyzed in order to reveal the nature of the optimum junction.

Thermodynamic magnon recoil for domain wall motion

We predict a thermodynamic magnon recoil effect for domain wall motions in the presence of temperature gradients. All current thermodynamic theories assert that a magnetic domain wall must move toward the hotter side, based on equilibrium thermodynamic arguments. Microscopic calculations on the other hand show that a domain wall can move either along or against the direction of heat currents, depending on how strong the heat currents are reflected by the domain wall. We have resolved the inconsistency between these two approaches by augmenting the theory in the presence of thermal gradients by incorporating in the free energy of domain walls by a heat current term present in nonequilibrium steady states. The condition to observe a domain wall propagation toward the colder regime is derived analytically and can be tested by future experiments.

Existence of anisotropic spin fluctuations at low temperature in the normal phase of the superconducting ferromagnet UCoGe

Thermal conductivity measurements have been performed on the superconducting ferromagnet UCoGe down to very low temperature and under magnetic field. In addition to the electronic quasiparticle thermal conductivity, additional contributions to the thermal transport are detected: they are sensitive to the amplitude and direction of the magnetic field, and at low temperature, they display a strong anisotropy with the heat current direction. We identify these contributions as arising from magnetic fluctuations. Detection of such fluctuations on the thermal transport in 3D weak ferromagnets is very rare if not unique, and pledges for a strongly itinerant character of the magnetism of UCoGe.

Tuning magnetic anisotropies of Fe films on Si(111) substrate via direction variation of heating current

We adopted a novel method to tune the terrace width of Si(111) substrate by varying the direction of heating current. It was observed that the uniaxial magnetic anisotropy (UMA) of Fe films grown on the Si(111) substrate enhanced with decreasing the terrace width and superimposed on the weak six-fold magnetocrystalline anisotropy. Furthermore, on the basis of the scanning tunneling microscopy (STM) images, self-correlation function calculations confirmed that the UMA was attributed mainly from the long-range dipolar interaction between the spins on the surface. Our work opens a new avenue to manipulate the magnetic anisotropy of magnetic structures on the stepped substrate by the decoration of its atomic steps.

Heating asymmetry induced by tunneling current flow in magnetic tunnel junctions

In this work, exchange bias was used as a probe to characterise the temperature profile induced by the inelastic relaxation of electrons tunnelling across a MgO barrier. Thermally assisted magnetic random access memory (TA-MRAM) cells comprising a magnetic tunnel junction (MTJ) with a reference pinned layer and a FeMn exchange biased storage layer were used. The pinning direction of the ferromagnetic storage layer is reversed when heated above the blocking temperature of the antiferromagnetic layer (FeMn). The power density required to reach this blocking temperature in the FeMn layer depends on the current polarity, indicating that the heat source term associated with the current flowing through the barrier depends itself on the current direction in contrast to simple Joule heating. This effect is due to the mechanism of energy dissipation in tunnelling. The tunnelling itself is ballistic i.e., without dissipation. However, after tunnelling, the hot electrons very quickly relax to the Fermi energy thereby loosing their excess energy in the receiving electrode. Therefore, the heat is essentially generated on one side of the barrier so that the whole profile of temperature throughout the pillar depends on the current direction. Full 3D thermal simulations also confirmed the temperature profile asymmetry. The proper choice of heating current direction (i.e., voltage polarity applied to the MTJ) can yield a reduction of about 10% in the heating power density required to enable writing in thermally assisted MRAM cells.

Hydrogen-bond enhanced thermal energy transport at functionalized, hydrophobic and hydrophilic silica–water interfaces

In this Letter, we investigate the nanoscale heat transfer across hydrophilic silanol and hydrophobic silane interfaces. We calculate the thermal conductance at the interface via the vibrational relaxation of silica in aqueous solutions. Additionally, we directly determine the heat flux across the interface by non-equilibrium molecular dynamics. The results indicate a temperature and time dependence of the thermal conductance across the hydrophilic interface, whereas the conductance at the hydrophobic one stays constant, emphasizing the importance of hydrogen-bonded networks. Most importantly, we observe a rectifying effect of the hydrophilic silanol to the bulk substrate depending on the heat current direction.

Separation of Micron-Scale Particles by Size Using Helium II

Here we report on a new technique we are developing to classify micron scale particles, i.e., separate them by size, using superfluid 4He (He II). The technique is based on the unique property of He II, that is its two fluid nature which includes the viscous normal fluid component that flows in the direction of a heat current. The gravitational settling of small particles of a particular size can be balanced by the viscous drag from the normal fluid in a bath of He II. Further a non-uniform heat flux can allow a range of particle sizes to be suspended and collected at different locations within a He II counterflow channel. To demonstrate this principle, we have built a prototype particle separator and tested the process with particle diameters in the range 1 μm to 10 μm. The motion of the particles in this separator was measured using the Particle Image Velocimetry technique. Separation of a mixture of particles into two different size categories was achieved demonstrating the validity of this concept to classify particles. Particle agglomeration was also observed and is an issue particularly for particles with a diameter smaller than a few microns.

Study of step instability on Si surfaces

Step instabilities on Si(1 1 1) vicinal surface and Si(1 1 1) vicinal surface induced by Au adsorption were observed by reflection electron microscopy. On the Si(0 0 1) vicinal surface faceting of (0 0 1) surface due to surface reconstruction takes place and surface steps are bunching. The kinetics of mass transport of Si depends on the substrate temperature and steps are impermeable for Si adatoms at lower temperature and they are permeable at higher temperature. On the Si(1 1 1) vicinal surface the permeability of steps depends on the heating current direction. The steps are permeable for step-up current heating and are impermeable for step-down current heating. Above a critical coverage of Au (≈0.3 ML) steps are bunching irrespective of heating current direction and periodic array of extremely straight step bands is formed for the step-down current heating.

Thermal conductivity tensor inYBa2Cu3O7−x: Effects of a planar magnetic field

We have measured the thermal conductivity tensor of a twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ single crystal as a function of angle $\ensuremath{\theta}$ between the magnetic field applied parallel to the ${\mathrm{CuO}}_{2}$ planes and the heat current direction, at different magnetic fields and at $T=13.8 \mathrm{K}.$ Clear fourfold and twofold variations in the field-angle dependence of ${\ensuremath{\kappa}}_{\mathrm{xx}}$ and ${\ensuremath{\kappa}}_{\mathrm{xy}}$ were, respectively, recorded in accordance with the d-wave pairing symmetry of the order parameter. The oscillation amplitude of the transverse thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{xy}}^{0}$ was found to be larger than the longitudinal one, ${\ensuremath{\kappa}}_{\mathrm{xx}}^{0},$ in the range of magnetic field studied here $(0 \mathrm{T}<~B<~9 \mathrm{T}).$ From our data we obtain quantities that are free from nonelectronic contributions, and they allow us a comparison of the experimental results with current models for the quasiparticle transport in the mixed state.

Periodic step density wave on Si(111) vicinal surfaces studied by reflection electron microscopy

A periodic step density wave, which is induced by Au adsorption under step-down current heating, was found on Si(111) vicinal surfaces by reflection electron microscopy. In this step instability extremely straight step bands and (111) terraces are periodically arranged. This step instability is only observed by composition of metal adsorption and dc-heating effects, and the periodic structure vanishes when heating current direction change to the step-up direction. Quantitative analysis of growth of the terrace width during step bunching process shows that the permeability of steps for Si adatoms depends on the heating current direction and steps are permeable for step-up current heating and are impermeable for step-down current heating. The changes of bunching pattern and of the step permeability should be due to change of step structure depending on the heating current direction.

Angle dependence of the transverse thermal conductivity in YBa(2)Cu(3)O(7) single crystals: Doppler shift and Andreev scattering contributions.

We have measured the transverse thermal conductivity kappa(xy) of twinned and untwinned YBa(2)Cu(3)O(7) single crystals as a function of angle theta between the magnetic field applied parallel to the CuO(2) planes and the heat current direction. For both crystals we observed a clear twofold variation in the field-angle dependence of kappa(xy)(theta) = -kappa(0)(xy)(T,B)sin(2 theta). We have found that the oscillation amplitude kappa(0)(xy) depends on temperature and magnetic field. Our results show that kappa(0)(xy) = aBln(1/bB) with the temperature- and sample-dependent parameters a and b. We discuss our results in terms of Andreev scattering of quasiparticles by vortices and a recently proposed theory based on the Doppler shift in the quasiparticle spectrum.

REM Studies of Surface Dynamics on Si Surfaces

Recent developments in techniques of real space observations of surfaces are notable. In them reflection electron microscopy (REM) is a unique technique where in-situ real time observations of wide areas of surfaces are possible. In the present paper recent studies of surface dynamic processes on Si surfaces are reviewed. Observed dynamic processes are adsorption induced successive phase transitions, adsorbate induced facet formations and step rearrangements induced by a reversal of specimen heating current direction(so called current effect) and by a reversal of the sign of the surface strain(strain effects).Figure 1 reproduces a series of REM images and RHEED patterns taken during successive phase transition induced by Au deposition on a clean Si(lll) surface at 780°C[l]. (a) shows a REM image taken before Au deposition. Line images with zigzag in shape are atomic steps on the surface. The surface steps up to the right as indicated by a step mark. The corresponding RHEED pattern in (b) shows that the surface has the 7×7 structure.