arXiv subjects

Pascal Gehring

Publications and source records attributed to Pascal Gehring.

At least 19 recordsLinked to original sources

arXiv2026

Layer- and Field-Dependent Magnetic Order in 2D CrSBr Revealed by Pulsed Nanocalorimetry

Understanding the evolution of magnetic order in the two-dimensional limit remains a central challenge in van der Waals magnets, where thermodynamic measurements are constrained by the femtogram-scale mass of exfoliated flakes. Here, we use microsecond pulse-heating nanocalorimetry to measure the heat capacity and magnetic entropy of CrSBr flakes down to the monolayer limit. The measurements reveal the entropy landscape associated with magnetic ordering, uncovering a reduction of the interlayer transition temperature and an entropy-derived effective magnetic moment approaching the monolayer limit. Thermodynamic anomalies capture a crossover from bulk-like interlayer antiferromagnetism to a regime dominated by intralayer ferromagnetic correlations. A pronounced layer-parity effect further emerges, with odd-layer samples displaying an additional high-temperature contribution associated with uncompensated magnetic layers. Under in-plane magnetic fields applied along the easy axis, antiferromagnetic order is progressively suppressed, allowing extraction of a thickness-dependent critical field reflecting weakened interlayer exchange coupling. Entropy analysis further reveals an extended regime of magnetic fluctuations persisting well above the interlayer ordering transition. Together, these results establish nanocalorimetry as a powerful thermodynamic probe of low-dimensional magnetism, providing direct access to magnetic entropy, exchange interactions, and dimensional crossover in atomically thin van der Waals magnets.

cond-mat.mtrl-sci↗

arXiv2026

Quantum Heat Under the Microscope: A Perspective on Cryogenic Scanning Thermal Microscopy

Exploring thermal transport at cryogenic temperatures presents both significant challenges and valuable insights. By uncovering the thermal counterpart of well-known quantum phenomena, researchers investigated fascinating phenomena ranging from the violation of the Wiedemann-Franz law to the quantisation of phonons. One key frontier remains : no existing method can image local heat transport at the nanoscale under cryogenic conditions. In this Perspective, we review the current state state of the art of local heat transport characterisation techniques and highlight their limitations. As a motivation for the development of cryogenic Scanning Thermal Microscopy, we provide five case studies illustrating how this approach could deepen our understanding of exotic quantum phases and enable the emergence of transformative technologies.

cond-mat.mes-hall↗

arXiv2026

Direct Nanoscale Pyroelectric Characterization of a CuInP${}_2$S${}_6$ van der Waals Nanogenerator

Pyroelectric energy conversion offers a route for harvesting time-dependent thermalfluctuations that are abundant in natural and technological environments. Twodimensional ferroelectrics are particularly attractive for this purpose because reduced dimensionality enables ultrathin, mechanically compliant device architectures. Here, we demonstrate direct nanoscale pyroelectric characterization of an out-of-plane van der Waals nanogenerator based on CuInP2S6 (CIPS) encapsulated between few-layer graphene electrodes. A scanning thermal microscopy (SThM) probe is employed as a localized nanoscale heat source while the electrically generated response is measured in situ through the device electrodes. Harmonic detection isolates the pyroelectric signal from parasitic first-harmonic electromechanical contributions, while finite-element thermal modeling combined with probe calibration enables direct determination of the local pyroelectric coefficient from the measured electrical response. Beyond quantitative characterization, the spatially resolved measurements directly identify electrically inactive regions associated with device defects, revealing local performance-limiting features that remain hidden in conventional spatially averaged pyroelectric measurements. The presented approach establishes a versatile platform for quantitative nanoscale pyroelectric characterization and the optimization of van der Waals pyroelectric devices.

cond-mat.mes-hall↗

arXiv2026

Probing the dynamics and configurations of single molecule junctions via Seebeck coefficient spectroscopy

Single molecule junctions exhibit dynamic structural configurations that strongly influence their electronic and thermoelectric properties. Here, we combine conductance (G) and Seebeck coefficient (S) measurements using the novel AC based scanning tunnelling microscope break junction technique to probe the real-time evolution of oligo(phenylene ethynylene) molecular junctions. We show that most junctions undergo configuration changes that lead to notable changes in S while G remains nearly constant. Density functional theory and quantum transport simulations link these observations to variations in contact geometry and charge transfer at the molecule electrode interface. Our results demonstrate that simultaneous G and S measurements enable direct access to the dynamic reconfiguration of single molecule junctions and offer design insights for thermoelectric molecular devices and new routes for increasing single molecule junction stability.

cond-mat.mes-hall↗

arXiv2025

Characterization of heat transfer in 3D CMOS structures using Sideband Scanning Thermal Wave Microscopy

Efficient thermal management is critical for cryogenic CMOS circuits, where local heating can compromise device performance and qubit coherence. Understanding heat flow at the nanoscale in these multilayer architectures requires localized, high-resolution thermal probing techniques capable of accessing buried structures. Here, we introduce a sideband thermal wave detection scheme for Scanning Thermal Microscopy, S-STWM, to probe deeply buried heater structures within CMOS dies. By extracting the phase of propagating thermal waves, this method provides spatially resolved insight into heat dissipation pathways through complex multilayer structures. Our approach enables quantitative evaluation of thermal management strategies, informs the design of cryo-CMOS circuits, and establishes a foundation for in situ thermal characterization under cryogenic operating conditions.

cond-mat.other↗

arXiv2025

A Single-Molecule Quantum Heat Engine

Particle-exchange heat engines operate without moving parts or time-dependent driving, relying solely on static energy-selective transport. Here, we realize a particle-exchange quantum heat engine based on a single diradical molecule, only a few nanometers in size. We experimentally investigate its operation at low temperatures and demonstrate that both the power output and efficiency are significantly enhanced by Kondo correlations, reaching up to 53 % of the Curzon-Ahlborn limit. These results establish molecular-scale particle-exchange engines as promising candidates for low-temperature applications where extreme miniaturization and energy efficiency are paramount.

cond-mat.mes-hall↗

arXiv2025

Thermal transport mapping in twisted double bilayer graphene

Two-dimensional (2D) materials have attracted significant interest due to their tunable physical properties when stacked into homo- and hetero-structures. Twisting adjacent layers introduces moir\'{e} patterns that strongly influence the material electronic and thermal behavior. In twisted graphene systems, the twist angle critically alters phonon transport, leading to reduced thermal conductivity compared to Bernal-stacked configurations. However, experimental investigations into thermal transport in twisted structures remain limited. Here, we study the local thermal properties of twisted double bilayer graphene (TDBG) using Scanning Thermal Microscopy (SThM). We find an increase in thermal resistance of $0.3 \pm 0.1 \times 10^6 KW^{-1}$ compared to untwisted bilayers, attributed to changes in both intrinsic thermal conductivity and the tip-sample interface. These results, supported by analytical modeling, provide new insight into thermal transport mechanisms in twisted 2D systems and offer a pathway toward thermal engineering in twistronic devices.

cond-mat.mes-hall↗

arXiv2024

Direct measurement of the local electrocaloric effect in 2D ferroelectric In${}_2$Se${}_3$ by Scanning Electrocaloric Thermometry

The electrocaloric effect refers to the temperature change in a material when an electric field is applied or removed. Significant breakthroughs revealed its potential for solid-state cooling technologies in past decades. These devices offer a sustainable alternative to traditional vapor compression refrigeration, with advantages such as compactness, silent operation, and the absence of moving parts or refrigerants. Electrocaloric effects are typically studied using indirect methods using polarization data, and which suffer from inaccuracies related to assumptions about heat capacity. Direct methods, although more precise, require device fabrication and face challenges in studying meso- or nanoscale systems, like 2D materials, and materials with non-uniform polarization textures where high spatial resolution is required. In this study, a novel technique, Scanning Electrocaloric Thermometry, is introduced for characterizing the local electrocaloric effect in nanomaterials. This approach achieves high spatial resolution by locally applying electric fields and by simultaneously measuring the resulting temperature change. By employing AC excitation, the measurement sensitivity is further enhanced and the electrocaloric effect is disentangled from other heating mechanisms such as Joule heating and dielectric losses. The effectiveness of the method is demonstrated by examining electrocaloric and heat dissipation phenomena in two-dimensional In${}_2$Se${}_3$ micrometer-sized flakes.

cond-mat.mes-hall↗

arXiv2024

Violation of the Wiedemann-Franz law and ultra-low thermal conductivity of Ti$_3$C$_2$T$_x$ MXene

The high electrical conductivity and good chemical stability of MXenes offer hopes for their use in many applications, such as wearable electronics, energy storage, or electromagnetic interference shielding. While their optical, electronic and electrochemical properties have been widely studied, the information on thermal properties of MXenes is scarce. In this study, we investigate the heat transport properties of Ti$_3$C$_2$T$_x$ MXene single flakes using scanning thermal microscopy and find exceptionally low anisotropic thermal conductivities within the Ti$_3$C$_2$T$_x$ flakes, leading to an effective thermal conductivity of 0.78$\pm$0.21 W m$^{-1}$ K$^{-1}$. This observation is in stark contrast to the predictions of the Wiedemann-Franz law, as the estimated Lorenz number is only 0.25 of the classical value. Due to the combination of low thermal conductivity and low emissivity of Ti$_3$C$_2$T$_x$, the heat loss from it is two orders of magnitude smaller than that from common metals. Our study explores the heat transport mechanisms of MXenes and highlights a promising approach for developing thermal insulation, two-dimensional thermoelectric, or infrared stealth materials.

cond-mat.mtrl-sci↗

arXiv2024

Versatile polymer method to dry-flip two-dimensional moir\'e hetero structures for nanoscale surface characterization

The recent discovery of magic angle twisted bilayer graphene (MATBG), in which two sheets of monolayer graphene are precisely stacked to a specific angle, has opened up a plethora of new opportunities in the field of topology, superconductivity, and other strongly correlated effects. Most conventional ways of preparing twisted bilayer devices require the use of high process temperatures and solvents and are not well-suited for preparing samples which need to be flipped to be compatible with characterization techniques like STM, ARPES, PFM, SThM etc. Here, we demonstrate a very simple polymer-based method using Polyvinyl Chloride (PVC), which can be used for making flipped twisted bilayer graphene devices. This allowed us to produce flipped twisted samples without the need of any solvents and with high quality as confirmed by Piezoresponse Force Microscopy. We believe that this dry flip technique can be readily extended to twist 2D materials beyond graphene, especially air-sensitive materials which require operation under inert atmosphere, where often solvents cannot be used.

cond-mat.mes-hall↗

arXiv2024

Quantifying the local mechanical properties of twisted double bilayer graphene

Nanomechanical measurements of minimally twisted van der Waals materials remained elusive despite their fundamental importance for device realisation. Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the variation of out-of-plane Young's modulus in minimally twisted double bilayer graphene (TDBG). We reveal a softening of the Young's modulus by 7\% and 17\% along single and double domain walls, respectively. Our experimental results are confirmed by force-field relaxation models. This study highlights the strong tunability of nanomechanical properties in engineered twisted materials, and paves the way for future applications of designer 2D nanomechanical systems.

cond-mat.mes-hall↗

arXiv2024

Record-high Anomalous Ettingshausen effect in a micron-sized magnetic Weyl semimetal on-chip cooler

Solid-state cooling devices offer compact, quiet, reliable and environmentally friendly solutions that currently rely primarily on the thermoelectric (TE) effect. Despite more than two centuries of research, classical thermoelectric coolers suffer from low efficiency which hampers wider application. In this study, the less researched Anomalous Ettingshausen effect (AEE), a transverse thermoelectric phenomenon, is presented as a new approach for on-chip cooling. This effect can be boosted in materials with non-trivial band topologies as demonstrated in the Heusler alloy $\text{Co}_2\text{MnGa}$. Enabled by the high quality of our material, in situ scanning thermal microscopy experiments reveal a record-breaking anomalous Ettingshausen coefficient of $-2.1$~mV in $\mu$m-sized on-chip cooling devices at room temperature. A significant 44\% of the effect is contributed by the intrinsic topological properties, in particular the Berry curvature of $\text{Co}_2\text{MnGa}$, emphasising the unique potential of magnetic Weyl semimetals for high-performance spot cooling in nanostructures.

cond-mat.mes-hall↗

arXiv2024

Impact of spin-entropy on the thermoelectric properties of a 2D magnet

Heat-to-charge conversion efficiency of thermoelectric materials is closely linked to the entropy per charge carrier. Thus, magnetic materials are promising building blocks for highly efficient energy harvesters, as their carrier entropy is boosted by a spin degree of freedom. In this work, we investigate how this spin entropy impacts heat-to-charge conversion in A-type antiferromagnet CrSBr. We perform simultaneous measurements of electrical conductance and thermocurrent while changing magnetic order using temperature and magnetic field as tuning parameters. We find a strong enhancement of the thermoelectric power factor around the N\'eel temperature. We further reveal that the power factor at low temperature can be increased by up to 600% upon applying a magnetic field. Our results demonstrate that the thermoelectric properties of 2D magnets can be optimized by exploiting the sizeable impact of spin entropy and confirm thermoelectric measurements as a sensitive tool to investigate subtle magnetic phase transitions in low-dimensional magnets.

cond-mat.mes-hall↗

arXiv2023

Implementation of SNS thermometers into molecular devices for cryogenic thermoelectric experiments

Thermocurrent flowing through a single-molecule device contains valuable information about the quantum properties of the molecular structure and, in particular, on its electronic and phononic excitation spectra, and entropy. Furthermore, accessing the thermoelectric heat-to-charge conversion efficiency experimentally can help to select suitable molecules for future energy conversion devices, which - predicted by theoretical studies - could reach unprecedented efficiencies. However, one of the major challenges in quantifying thermocurrents in nanoscale devices is to determine the exact temperature bias applied to the junction. In this work, we have incorporated a superconductor-normal metal-superconductor (SNS) Josephson junction thermometer into a single-molecule device. The critical current of the Josephson junction depends accurately on minute changes of the electronic temperature in a wide temperature range from 100 mK to 1.6 K. Thus, we present a device architecture which can enable thermoelectric experiments on single molecules down to millikelvin temperatures with high precision.

cond-mat.mes-hall↗

arXiv2023

Single-material MoS$_{2}$ thermoelectric junction enabled by substrate engineering

To realize a thermoelectric power generator, typically a junction between two materials with different Seebeck coefficient needs to be fabricated. Such difference in Seebeck coefficients can be induced by doping, which renders difficult when working with two-dimensional (2d) materials. Here, we employ substrate effects to form a thermoelectric junction in ultra-thin few-layer MoS2 films. We investigated the junctions with a combination of scanning photocurrent microscopy and scanning thermal microscopy. This allows us to reveal that thermoelectric junctions form across the substrate-engineered parts. We attribute this to a gating effect induced by interfacial charges in combination with alterations in the electron-phonon scattering mechanisms. This work demonstrates that substrate engineering is a promising strategy to develop future compact thin-film thermoelectric power generators.

cond-mat.mtrl-sci↗

arXiv2022

Electronic measurements of entropy in meso- and nanoscale systems

Entropy is one of the most fundamental quantities in physics. For systems with few degrees of freedom, the value of entropy provides a powerful insight into its microscopic dynamics, such as the number, degeneracy and relative energies of electronic states, the value of spin, degree of localisation and entanglement, and the emergence of exotic states such as non-Abelian anyons. As the size of a system decreases, the conventional methods for measuring entropy, based on heat capacity, quickly become infeasible due to the requirement of increasingly accurate measurements of heat. Several methods to directly measure entropy of mesoscopic quantum systems have recently been developed. These methods use electronic measurements of charge, conductance and thermocurrent, rather than heat, and have been successfully applied to a wide range of systems, from quantum dots and molecules, to quantum Hall states and twisted bilayer graphene. In this Review, we provide an overview of electronic direct entropy measurement methods, discuss their theoretical background, compare their ranges of applicability and look into the directions for their future extensions and applications.

cond-mat.mes-hall↗

arXiv2021

Magnetic Field Universality of the Kondo Effect Revealed by Thermocurrent Spectroscopy

Probing the universal low temperature magnetic field scaling of Kondo-correlated quantum dots via electrical conductance has proved to be experimentally challenging. Here, we show how to probe this in nonlinear thermocurrent spectroscopy applied to a molecular quantum dot in the Kondo regime. Our results demonstrate that the bias-dependent thermocurrent is a sensitive probe of universal Kondo physics, directly measures the splitting of the Kondo resonance in a magnetic field, and opens up possibilities for investigating nanosystems far from thermal and electrical equilibrium.

cond-mat.mes-hall↗

arXiv2021

Controlling the entropy of a single-molecule junction

Single molecules are nanoscale thermodynamic systems with few degrees of freedom. Thus, the knowledge of their entropy can reveal the presence of microscopic electron transfer dynamics, that are difficult to observe otherwise. Here, we apply thermocurrent spectroscopy to directly measure the entropy of a single free radical molecule in a magnetic field. Our results allow us to uncover the presence of a singlet to triplet transition in one of the redox states of the molecule, not detected by conventional charge transport measurements. This highlights the power of thermoelectric measurements which can be used to determine the difference in configurational entropy between the redox states of a nanoscale system involved in conductance without any prior assumptions about its structure or microscopic dynamics.

cond-mat.mes-hall↗