arXiv subjects

Herre S. J. van der Zant

Publications and source records attributed to Herre S. J. van der Zant.

At least 73 records · Page 4Linked to original sources

arXiv2020

Anisotropic magnetoresistance in spin-orbit semimetal SrIrO3

SrIrO3, the three-dimensional member of the Ruddlesden-Popper iridates, is a paramagnetic semimetal characterised by a the delicate interplay between spin-orbit coupling and Coulomb repulsion. In this work, we study the anisotropic magnetoresistance (AMR) of SrIrO3 thin films, which is closely linked to spin-orbit coupling and probes correlations between electronic transport, magnetic order and orbital states. We show that the low-temperature negative magnetoresistance is anisotropic with respect to the magnetic field orientation, and its angular dependence reveals the appearance of a fourfold symmetric component above a critical magnetic field. We show that this AMR component is of magnetocrystalline origin, and attribute the observed transition to a field-induced magnetic state in SrIrO3.

cond-mat.str-el↗

arXiv2019

Magnetic and electronic phase transitions probed by nanomechanical resonance

Two-dimensional (2D) materials enable new types of magnetic and electronic phases mediated by their reduced dimensionality like magic-angle induced phase transitions, 2D Ising antiferromagnets and ferromagnetism in 2D atomic layers and heterostructures. However, only a few methods are available to study these phase transitions, which for example is particularly challenging for antiferromagnetic materials. Here, we demonstrate that these phases can be probed by the mechanical motion: the temperature dependent resonance frequency and quality factor of multilayer 2D material membranes show clear anomalies near the phase transition temperature, which are correlated to anomalies in the specific heat of the materials. The observed coupling of mechanical degrees of freedom to magnetic and electronic order is attributed to thermodynamic relations that are not restricted to van der Waals materials. Nanomechanical resonators, therefore, offer the potential to characterize phase transitions in a wide variety of materials, including those that are antiferromagnetic, insulating or so thin that conventional bulk characterization methods become unsuitable.

cond-mat.mes-hall↗

arXiv2019

Phonon Scattering at Kinks in Suspended Graphene

Recent experiments have shown surprisingly large thermal time constants in suspended graphene ranging from 10 to 100 ns in drums with a diameter ranging from 2 to 7 microns. The large time constants and their scaling with diameter points towards a thermal resistance at the edge of the drum. However, an explanation of the microscopic origin of this resistance is lacking. Here, we show how phonon scattering at a kink in the graphene, e.g. formed by sidewall adhesion at the edge of the suspended membrane, can cause a large thermal time constant. This kink strongly limits the fraction of flexural phonons that cross the suspended graphene edge, which causes a thermal interface resistance at its boundary. Our model predicts thermal time constants that are of the same order of magnitude as experimental data, and shows a similar dependence on the circumference. Furthermore, the model predicts the relative in-plane and out-of-plane phonon contributions to graphene's thermal expansion force, in agreement with experiments. We thus show, that in contrast to conventional thermal (Kapitza) resistance which occurs between two different materials, in 2D materials another type of thermal interface resistance can be geometrically induced in a single material.

cond-mat.mes-hall↗

arXiv2019

Nonequilibrium Thermodynamics of Acoustic Phonons in Suspended Graphene

Recent theory has predicted large temperature differences between the in-plane (LA and TA) and out-of-plane (ZA) acoustic phonon baths in locally-heated suspended graphene. To verify these predictions, and their implications for understanding the nonequilibrium thermodynamics of 2D materials, experimental techniques are needed. Here, we present a method to determine the acoustic phonon bath temperatures from the frequency-dependent mechanical response of suspended graphene to a power modulated laser. The mechanical motion reveals two counteracting contributions to the thermal expansion force, that are attributed to fast positive thermal expansion by the in-plane phonons and slower negative thermal expansion by the out-of-plane phonons. The magnitude of the two forces reveals that the in-plane and flexural acoustic phonons are at very different temperatures in the steady-state, with typically observed values of the ratio $ΔT_{\mathrm{LA+TA}}/ΔT_{\mathrm{ZA}}$ between 0.2 and 3.7. These deviations from the generally used local thermal equilibrium assumption ($ΔT_{\mathrm{LA+TA}}=ΔT_{\mathrm{ZA}}$) can affect the experimental analysis of thermal properties of 2D materials.

cond-mat.mes-hall↗

arXiv2019

Efficient heating of single-molecule junctions for thermoelectric studies at cryogenic temperatures

The energy dependent thermoelectric response of a single molecule contains valuable information about its transmission function and its excited states. However, measuring it requires devices that can efficiently heat up one side of the molecule while being able to tune its electrochemical potential over a wide energy range. Furthermore, to increase junction stability devices need to operate at cryogenic temperatures. In this work we report on a new device architecture to study the thermoelectric properties and the conductance of single molecules simultaneously over a wide energy range. We employ a sample heater in direct contact with the metallic electrodes contacting the single molecule which allows us to apply temperature biases up to $ΔT = 60$K with minimal heating of the molecular junction. This makes these devices compatible with base temperatures $T_\mathrm{bath} <2$K and enables studies in the linear ($ΔT \ll T_\mathrm{molecule}$) and non-linear ($ΔT \gg T_\mathrm{molecule}$) thermoelectric transport regimes.

cond-mat.mes-hall↗

arXiv2019

Tunneling spectroscopy of localized states of $\mathrm{WS}_2$ barriers in vertical van der Waals heterostructures

In transition metal dichalcogenides, defects have been found to play an important role, affecting doping, spin-valley relaxation dynamics, and assisting in proximity effects of spin-orbit coupling. Here, we study localized states in $\mathrm{WS}_2$ and how they affect tunneling through van der Waals heterostructures of h-BN/graphene/$\mathrm{WS}_2$/metal. The obtained conductance maps as a function of bias and gate voltage reveal single-electron transistor behavior (Coulomb blockade) with a rich set of transport features including excited states and negative differential resistance regimes. Applying a perpendicular magnetic field, we observe a shift in the energies of the quantum levels and information about the orbital magnetic moment of the localized states is extracted.

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arXiv2019

Ultrathin complex oxide nanomechanical resonators

Complex oxide thin films and heterostructures exhibit a profusion of exotic phenomena, often resulting from the intricate interplay between film and substrate. Recently it has become possible to isolate epitaxially grown single-crystalline layers of these materials, enabling the study of their properties in the absence of interface effects. In this work, we create ultrathin membranes of strongly correlated materials and demonstrate top-down fabrication of nanomechanical resonators made out of \ce{SrTiO3} and \ce{SrRuO3}. Using laser interferometry, we successfully actuate and measure the motion of the nanodrum resonators. By measuring their temperature-dependent mechanical response, we observe signatures of structural phase transitions in \ce{SrTiO3}, which affect the strain and mechanical dissipation in the resonators. This approach can be extended to investigate phase transitions in a wide range of materials. Our study demonstrates the feasibility of integrating ultrathin complex oxide membranes for realizing nanoelectromechanical systems on arbitrary substrates.

physics.app-ph↗

arXiv2019

Weak localization in boron nitride encapsulated bilayer MoS$_2$

We present measurements of weak localization on hexagonal boron nitride encapsulated bilayer MoS$_2$. From the analysis we obtain information regarding the phase-coherence and the spin diffusion of the electrons. We find that the encapsulation with boron nitride provides higher mobilities in the samples, and the phase-coherence shows improvement, while the spin relaxation does not exhibit any significant enhancement compared to non-encapsulated MoS$_2$. The spin relaxation time is in the order of a few picoseconds, indicating a fast intravalley spin-flip rate. Lastly, the spin-flip rate is found to be independent from electron density in the current range, which can be explained through counteracting spin-flip scattering processes based on electron-electron Coulomb scattering and extrinsic Bychkov-Rashba spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2019

Mass measurement of graphene using quartz crystal microbalances

Current wafer-scale fabrication methods for graphene-based electronics and sensors involve the transfer of single-layer graphene by a support polymer. This often leaves some polymer residue on the graphene, which can strongly impact its electronic, thermal, and mechanical resonance properties. To assess the cleanliness of graphene fabrication methods, it is thus of considerable interest to quantify the amount of contamination on top of the graphene. Here, we present a methodology for direct measurement of the mass of the graphene sheet using quartz crystal microbalances (QCM). By monitoring the QCM resonance frequency during removal of graphene in an oxygen plasma, the total mass of the graphene and contamination is determined with sub-graphene-monolayer accuracy. Since the etch-rate of the contamination is higher than that of graphene, quantitative measurements of the mass of contaminants below, on top, and between graphene layers are obtained. We find that polymer-based dry transfer methods can increase the mass of a graphene sheet by a factor of 10. The presented mass measurement method is conceptually straightforward to interpret and can be used for standardized testing of graphene transfer procedures in order to improve the quality of graphene devices in future applications.

cond-mat.mes-hall↗

arXiv2018

High-frequency stochastic switching of graphene resonators near room temperature

Stochastic switching between the two bistable states of a strongly driven mechanical resonator enables detection of weak signals based on probability distributions, in a manner that mimics biological systems. However, conventional silicon resonators at the microscale require a large amount of fluctuation power to achieve a switching rate in the order of a few Hertz. Here, we employ graphene membrane resonators of atomic thickness to achieve a stochastic switching rate of 7.8 kHz, which is 200 times faster than current state-of-the-art. The (effective) temperature of the fluctuations is approximately 400 K, which is 3000 times lower than the state-of-the-art. This shows that these membranes are potentially useful to transduce weak signals in the audible frequency domain. Furthermore, we perform numerical simulations to understand the transition dynamics of the resonator and derive simple analytical expressions to investigate the relevant scaling parameters that allow high-frequency, low-temperature stochastic switching to be achieved in mechanical resonators.

cond-mat.mes-hall↗

arXiv2018

Ground-state spin blockade in a single-molecule junction

It is known that the quantum-mechanical ground state of a nano-scale junction has a significant impact on its electrical transport properties. This becomes particularly important in transistors consisting of a single molecule. Due to strong electron-electron interactions and the possibility to access ground states with high spins, these systems are eligible hosts of a current-blockade phenomenon called ground-state spin blockade. This effect arises from the inability of a charge carrier to account for the spin difference required to enter the junction, as that process would violate the spin selection rules. Here, we present a direct experimental demonstration of ground-state spin blockade in a high-spin single-molecule transistor. The measured transport characteristics of this device exhibit a complete suppression of resonant transport due to a ground-state spin difference of 3/2 between subsequent charge states. Strikingly, the blockade can be reversibly lifted by driving the system through a magnetic ground-state transition in one charge state, using the tunability offered by both magnetic and electric fields.

cond-mat.mes-hall↗

arXiv2018

Mechanically Controlled Quantum Interference in Graphene Break Junctions

The ability to detect and distinguish quantum interference signatures is important for both fundamental research and for the realization of devices including electron resonators, interferometers and interference-based spin filters. Consistent with the principles of subwavelength optics, the wave nature of electrons can give rise to various types of interference effects, such as Fabry-Pérot resonances, Fano resonances and the Aharonov-Bohm effect. Quantum-interference conductance oscillations have indeed been predicted for multiwall carbon nanotube shuttles and telescopes, and arise from atomic-scale displacements between the inner and outer tubes. Previous theoretical work on graphene bilayers indicates that these systems may display similar interference features as a function of the relative position of the two sheets. Experimental verification is, however, still lacking. Graphene nanoconstrictions represent an ideal model system to study quantum transport phenomena due to the electronic coherence and the transverse confinement of the carriers. Here, we demonstrate the fabrication of bowtie-shaped nanoconstrictions with mechanically controlled break junctions (MCBJs) made from a single layer of graphene. We find that their electrical conductance displays pronounced oscillations at room temperature, with amplitudes that modulate over an order of magnitude as a function of sub-nanometer displacements. Surprisingly, the oscillations exhibit a period larger than the graphene lattice constant. Charge-transport calculations show that the periodicity originates from a combination of quantum-interference and lattice-commensuration effects of two graphene layers that slide across each other. Our results provide direct experimental observation of Fabry-Pérot-like interference of electron waves that are partially reflected/transmitted at the edges of the graphene bilayer overlap region.

cond-mat.mes-hall↗

arXiv2018

Investigating laser induced phase engineering in MoS2 transistors

Phase engineering of MoS2 transistors has recently been demonstrated and has led to record low contact resistances. The phase patterning of MoS2 flakes with laser radiation has also been realized via spectroscopic methods, which invites the potential of controlling the metallic and semiconducting phases of MoS2 transistors by simple light exposure. Nevertheless, the fabrication and demonstration of laser patterned MoS2 devices starting from the metallic polymorph has not been demonstrated yet. Here, we study the effects of laser radiation on 1T/1T'-MoS2 transistors with the prospect of driving an in-situ phase transition to the 2H-polymorph through light exposure. We find that although the Raman peaks of 2H-MoS2 become more prominent and the ones from the 1T/1T' phase fade after the laser exposure, the semiconducting properties of the laser patterned devices are not fully restored and the laser treatment ultimately leads to degradation of the transport channel.

physics.app-ph↗

arXiv2018

Massively parallel fabrication of crack-defined gold break junctions featuring sub-3 nm gaps for molecular devices

Break junctions provide tip-shaped contact electrodes that are fundamental components of nano and molecular electronics. However, the fabrication of break junctions remains notoriously time-consuming and difficult to parallelize. Here we demonstrate true parallel fabrication of gold break junctions featuring sub-3 nm gaps on the wafer-scale, by relying on a novel self-breaking mechanism based on controlled crack formation in notched bridge structures. We achieve fabrication densities as high as 7 million junctions per cm$^{2}$, with fabrication yields of around 7% for obtaining crack-defined break junctions with sub-3 nm gaps of fixed gap width that exhibit electron tunneling. We also form molecular junctions using dithiol-terminated oligo(phenylene ethynylene) (OPE3) to demonstrate the feasibility of our approach for electrical probing of molecules down to liquid helium temperatures. Our technology opens a whole new range of experimental opportunities for nano and molecular electronics applications, by enabling very large-scale fabrication of solid-state break junctions.

physics.app-ph↗

arXiv2018

Large Conductance Variations in a Mechanosensitive Single-Molecule Junction

The appealing feature of molecular electronics is the possibility of exploiting functionality built within a single molecule. This functionality can be employed, for example, for sensing or switching purposes. Thus, ideally, the associated conductance changes should be sizable upon application of external stimuli. Here, we show that a molecular spring can be mechanically compressed or elongated to tune its conductance by up to an order of magnitude by controlling the quantum interference between electronic pathways. Oscillations in the conductance occur when the stress built up in the molecule is high enough to allow the anchoring groups to move along the surface in a stick-slip-like fashion. The mechanical control of quantum interference effects and the resulting large change in molecular conductance open the door for applications in, e.g., a minute mechanosensitive sensing device functional at room temperature.

cond-mat.mes-hall↗

arXiv2018

Transient thermal characterization of suspended monolayer MoS$_2$

We measure the thermal time constants of suspended single layer molybdenum disulfide drums by their thermomechanical response to a high-frequency modulated laser. From this measurement the thermal diffusivity of single layer MoS$_2$ is found to be 1.14 $\times$ 10$^{-5}$ m$^2$/s on average. Using a model for the thermal time constants and a model assuming continuum heat transport, we extract thermal conductivities at room temperature between 10 to 40 W/(m$\cdot$K). Significant device-to-device variation in the thermal diffusivity is observed. Based on statistical analysis we conclude that these variations in thermal diffusivity are caused by microscopic defects that have a large impact on phonon scattering, but do not affect the resonance frequency and damping of the membrane's lowest eigenmode. By combining the experimental thermal diffusivity with literature values of the thermal conductivity, a method is presented to determine the specific heat of suspended 2D materials, which is estimated to be 255 $\pm$ 104 J/(kg$\cdot$K) for single layer MoS$_2$.

cond-mat.mes-hall↗

arXiv2018

Robust organic radical molecular junctions using acetylene terminated groups for C-Au bond formation

Organic paramagnetic and electroactive molecules are attracting interest as core components of molecular electronic and spintronic devices. Currently, further progress is hindered by the modest stability and reproducibility of the molecule/electrode contact. We report the synthesis of a persistent organic radical bearing one and two terminal alkyne groups to form Au-C σ bonds. The formation and stability of self-assembled monolayers and the electron transport through single-molecule junctions at room temperature have been studied. The combined analysis of both systems demonstrates that this linker forms a robust covalent bond with gold and a better-defined contact when compared to traditional sulfur-based linkers. Density functional theory and quantum transport calculations support the experimental observation highlighting a reduced variability of conductance values for the C-Au based junction. Our findings advance the quest for robustness and reproducibility of devices based on electroactive molecules.

cond-mat.mes-hall↗

arXiv2018

Redox-Induced Gating of the Exchange Interactions in a Single Organic Diradical

Embedding a magnetic electroactive molecule in a three-terminal junction allows for the fast and local electric field control of magnetic properties desirable in spintronic devices and quantum gates. Here, we provide an example of this control through the reversible and stable charging of a single all-organic neutral diradical molecule. By means of inelastic electron tunnel spectroscopy (IETS) we show that the added electron occupies a molecular orbital distinct from those containing the two radical electrons, forming a spin system with three antiferromagnetically-coupled spins. Changing the redox state of the molecule therefore switches on and off a parallel exchange path between the two radical spins through the added electron. This electrically-controlled gating of the intramolecular magnetic interactions constitutes an essential ingredient of a single-molecule $\sqrt{\text{SWAP}}$ quantum gate.

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