arXiv subjects

Thomas Greber

Publications and source records attributed to Thomas Greber.

At least 19 recordsLinked to original sources

arXiv2024

Benchmarking the integration of hexagonal boron nitride crystals and thin films into graphene-based van der Waals heterostructures

We present a benchmarking protocol that combines the characterization of boron nitride (BN) crystals and films with the evaluation of the electronic properties of graphene on these substrates. Our study includes hBN crystals grown under different conditions and scalable BN films deposited by either chemical or physical vapor deposition (CVD or PVD). We explore the complete process from boron nitride growth, over its optical characterization by time-resolved cathodoluminescence (TRCL), to the optical and electronic characterization of graphene by Raman spectroscopy after encapsulation and Hall bar processing. Within our benchmarking protocol we achieve a homogeneous electronic performance within each Hall bar device through a fast and reproducible processing routine. We find that a free exciton lifetime of 1 ns measured on as-grown hBN crystals by TRCL is sufficient to achieve high graphene room temperature charge carrier mobilities of 80,000 cm$^2$/(Vs) at a carrier density of |n| = 10$^{12}$ cm$^{-2}$, while respective exciton lifetimes around 100 ps yield mobilities up to 30,000 cm$^2$/(Vs). For scalable PVD-grown BN films, we measure carrier mobilities exceeding 10,000 cm$^2$/(Vs) which correlates with a graphene Raman 2D peak linewidth of 22 cm$^{-1}$. Our work highlights the importance of the Raman 2D linewidth of graphene as a critical metric that effectively assesses the interface quality (i.e. surface roughness) to the BN substrate, which directly affects the charge carrier mobility of graphene. Graphene 2D linewidth analysis is suitable for all BN substrates and is particularly advantageous when TRCL or BN Raman spectroscopy cannot be applied to specific BN materials such as amorphous or thin films. This underlines the superior role of spatially-resolved spectroscopy in the evaluation of BN crystals and films for the use of high-mobility graphene devices.

cond-mat.mtrl-sci↗

arXiv2024

Quantum tunneling of the magnetization in systems with anisotropic 4f ion pairs: Rates from low temperature zero field relaxation

Anisotropic open shell 4f ions have magnetic moments that can be read and written as atomic bits. If it comes to qbits where the phase of the wave function has to be written, controlled and read, it is of advantage to rely on more than one atom that carries the quantum information of the system because states with different susceptibilities may be addressed. Such systems are realized for pairs of lanthanides in single molecule magnets, where four pseudospin states are found and mixed in quantum tunneling processes. For the case of endohedral fullerenes like Dy2S@C82 or Tb2ScN@C80 the quantum tunneling of the magnetisation is imprinted in the magnetisation lifetimes at sub-Kelvin temperatures. A Hamiltonian that includes quantum tunneling of the magnetisation predicts the lifting of the zero field ground state degeneracy and non-linear coupling to magnetic fields in such systems.

quant-ph↗

arXiv2022

The Winner Takes It All: Carbon Supersedes Hexagonal Boron Nitride with Graphene on Transition Metals at High Temperatures

The production of high-quality hexagonal boron nitride (h-BN) is essential for the ultimate performance of two-dimensional (2D) materials-based devices, since it is the key 2D encapsulation material. Here, a decisive guideline is reported for fabricating high-quality h-BN on transition metals: It is crucial to exclude carbon from h-BN related process. Otherwise carbon prevails over boron and nitrogen due to its larger binding energy, thereupon forming graphene on metals after high-temperature annealing. We demonstrate the surface reaction-assisted conversion from h-BN to graphene with high-temperature treatments. The pyrolysis temperature Tp is an important quality indicator for h-BN/metals. When the temperature is lower than Tp, the quality of h-BN layer is improved upon annealing. While the annealing temperature is above Tp, in case of carbon-free conditions, the h-BN disintegrates and nitrogen desorbs from the surface more easily than boron, eventually leading to clean metal surfaces. However, once the h-BN layer is exposed to carbon, graphene forms on Pt(111) in the high-temperature regime. This not only provides an indispensable principle (avoid carbon) for fabricating high-quality h-BN materials on transition metals, but also offers a straightforward method for the surface reaction-assisted conversion from h-BN to graphene on Pt(111).

cond-mat.mtrl-sci↗

arXiv2022

Precise measurement of angles between two magnetic moments and their configurational stability in single-molecule magnets

A key parameter for the low-temperature magnetic coupling of in dinuclear lanthanide single-molecule magnets (SMMs) is the barrier $U_{FA}$ resulting from the exchange and dipole interactions between the two $4f$ moments. Here we extend the pseudospin model previously used to describe the ground state of dinuclear endofullerenes to account for variations in the orientation of the single-ion anisotropy axes and apply it to the two SMMs Dy$_2$ScN@C$_{80}$ and Dy$_2$TiC@C$_{80}$. While x-ray magnetic circular dichroism (XMCD) indicates the same $J_z=15/2$ Dy groundstate in both molecules, the Dy-Dy coupling strength and the stability of magnetization is distinct. We demonstrate that both the magnitude of the barrier $U_{FA}$ and the angle between the two $4f$ moments are determined directly from precise temperature-dependent magnetization data to an accuracy better than $1^{\circ}$. The experimentally found angles between the $4f$ moments are in excellent agreement with calculated angles between the quantisation axes of the two Dy ions. Theory indicates a larger deviation of the orientation of the Dy magnetic moments from the Dy bond axes to the central ion in Dy$_2$TiC@C$_{80}$. This may explain the lower stability of the magnetisation in Dy$_2$TiC@C$_{80}$, although it exhibits a $\sim 49\%$ stronger exchange coupling than in Dy$_2$ScN@C$_{80}$.

cond-mat.mtrl-sci↗

arXiv2021

Wafer-scale, epitaxial growth of single layer hexagonal boron nitride on Pt(111)

Single layer hexagonal boron nitride is produced on 2 inch Pt(111)/sapphire wafers. The growth with borazine vapour deposition at process temperatures between 1000 and 1300 K is in-situ investigated by photoelectron yield measurements. The growth kinetics is slower at higher temperatures and follows a tanh$^2$ law which better fits for higher temperatures. The crystal-quality of h-BN/Pt(111) is inferred from scanning low energy electron diffraction (x-y LEED). The data indicate a strong dependence of the epitaxy on the growth temperature. The dominant structure is an aligned coincidence lattice with 10 h-BN on 9 Pt(1$\times$1) unit cells and follows the substrate twinning at the millimeter scale.

cond-mat.mtrl-sci↗

arXiv2020

High-quality hexagonal boron nitride from 2D distillation

The production of high-quality two-dimensional (2D) materials is essential for the ultimate performance of single layers and their hybrids. Hexagonal boron nitride (h-BN) is foreseen to become the key 2D hybrid and packaging material since it is insulating, tight, flat, transparent and chemically inert, though it is difficult to attain in ultimate quality. Here, a new scheme is reported for producing single layer h-BN that shows higher quality and much more uniformity than material from chemical vapor deposition (CVD). We delaminate CVD h-BN from Rh(111) and transfer it to a clean metal surface. The twisting angle between BN and the new substrate yields metastable moir\'e structures. Annealing above 1000 K leads to 2D distillation, i.e., catalyst-assisted BN sublimation from the edges of the transferred layer and subsequent condensation into superior quality h-BN. This provides a new and low-cost way of high-quality 2D material production remote from CVD instrumentation.

cond-mat.mtrl-sci↗

arXiv2020

Gadolinium as a single atom catalyst in a single molecule magnet

Endohedral fullerenes are perfect nanolaboratories for the study of magnetism. The substitution of a diamagnetic scandium atom in Dy2ScN@C80 with gadolinium decreases the stability of a given magnetization and demonstrates Gd to act as a single atom catalyst that accelerates the reaching of thermal equilibrium. X-ray magnetic circular dichroism at the M4,5 edges of Gd and Dy shows that the Gd magnetic moment follows the sum of the external and the dipolar magnetic field of the two Dy ions and compared to Dy2ScN@C80 a lower exchange barrier is found between the ferromagnetic and the antiferromagnetic Dy configuration. The Arrhenius equilibration barrier as obtained from superconducting quantum interference device magnetometry is more than one order of magnitude larger, though a much smaller prefactor imposes faster equilibration in Dy2GdN@C80. This sheds light on the importance of the angular momentum balance in magnetic relaxation.

cond-mat.mtrl-sci↗

arXiv2019

The true corrugation of a h-BN nanomesh layer

Hexagonal boron nitride (h-BN) nanomesh, a two-dimensional insulating monolayer, grown on the (111) surface of rhodium exhibits an intriguing hexagonal corrugation pattern with a lattice constant of 3.2 nm. Despite numerous experimental and theoretical studies no quantitative agreement has been found on some details of the adsorption geometry such as the corrugation amplitude. The issue highlights the differences in chemical and electronic environment in the strongly bound pore regions and the weakly bound wire regions of the corrugated structure. For reliable results it is important to probe the structure with a method that is intrinsically sensitive to the position of the atomic cores rather than the electron density of states. In this work, we determine the corrugation of h-BN nanomesh from angle- and energy-resolved photoelectron diffraction measurements with chemical state resolution. By combining the results from angle and energy scans and comparing them to multiple-scattering simulations true adsorbate-substrate distance can be measured with high precision, avoiding pitfalls of apparent topography observed in scanning probe techniques. Our experimental results give accurate values for the peak to peak corrugation amplitude (0.80 A), the bonding distance to the substrate (2.20 A) and the buckling of the boron and nitrogen atoms in the strongly bound pore regions (0.07 A). The results are important for the development of theoretical methods involving the quantitative description of van der Waals systems like it requires the understanding of the physics of two-dimensional sp2 layers.

cond-mat.mtrl-sci↗

arXiv2019

The sub-Kelvin hysteresis of the dilanthanide single molecule magnet Tb2ScN@C80

Magnetic hysteresis is a direct manifestation of non-equilibrium physics that has to be understood if a system shall be used for information storage and processing. The dilanthanide endofullerene Tb2ScN@C80 is shown to be a single-molecule magnet with a remanence time in the order of 100 s at 400 mK. Three different temperature dependent relaxation barriers are discerned. The lowest 1 K barrier is assigned to intermolecular interaction. The 10 K barrier to intramolecular exchange and dipolar coupling and the 50 K barrier to molecular vibrations as it was observed for Dy2ScN@C80. The four orders of magnitude difference in the prefactor between the Tb and the Dy compound in the decay process across the 10 K barrier is assigned to the electron number in the 4f shells that evidences lack of Kramers protection in Tb3+. The sub-Kelvin hysteresis follows changes in the magnetisation at adiabatic and non-adiabatic level crossings of the four possible Tb2 ground state configurations as is inferred from a zero temperature hysteresis model.

cond-mat.mtrl-sci↗

arXiv2018

An electron acceptor molecule in a nanomesh: F4TCNQ on h-BN/Rh(111)

The adsorption of molecules on surfaces affects the surface dipole and thus changes in the work function may be expected. The effect in change of work function is particularly strong if charge between substrate and adsorbate is involved. Here we report the deposition of a strong electron acceptor molecule, tetrafluorotetracyanoquinodimethane C$_{12}$F$_4$N$_4$ (F$_{4}$TCNQ) on a monolayer of hexagonal boron nitride nanomesh ($h$-BN on Rh(111)). The work function of the F$_{4}$TCNQ/$h$-BN/Rh system increases upon increasing molecular coverage. The magnitude of the effect indicates electron transfer from the substrate to the F$_{4}$TCNQ molecules. Density functional theory calculations confirm the work function shift and predict doubly charged F$_{4}$TCNQ$^{2-}$ in the nanomesh pores, where the $h$-BN is closest to the Rh substrate, and to have the largest binding energy there. The preferred adsorption in the pores is conjectured from a series of ultraviolet photoelectron spectroscopy data, where the $\sigma$ bands in the pores are first attenuated. Scanning tunneling microscopy measurements indicate that F$_{4}$TCNQ molecules on the nanomesh are mobile at room temperature, as "hopping" between neighboring pores is observed.

cond-mat.mtrl-sci↗

arXiv2016

Reading and Writing Single-Atom Magnets

The highest-density magnetic storage media will code data in single-atom bits. To date, the smallest individually addressable bistable magnetic bits on surfaces consist of 5-12 atoms. Long magnetic relaxation times were demonstrated in molecular magnets containing one lanthanide atom, and recently in ensembles of single holmium (Ho) atoms supported on magnesium oxide (MgO). Those experiments indicated the possibility for data storage at the fundamental limit, but it remained unclear how to access the individual magnetic centers. Here we demonstrate the reading and writing of individual Ho atoms on MgO, and show that they independently retain their magnetic information over many hours. We read the Ho states by tunnel magnetoresistance and write with current pulses using a scanning tunneling microscope. The magnetic origin of the long-lived states is confirmed by single-atom electron paramagnetic resonance (EPR) on a nearby Fe sensor atom, which shows that Ho has a large out-of-plane moment of $(10.1 \pm 0.1)$ $\mu_{\rm B}$ on this surface. In order to demonstrate independent reading and writing, we built an atomic scale structure with two Ho bits to which we write the four possible states and which we read out remotely by EPR. The high magnetic stability combined with electrical reading and writing shows that single-atom magnetic memory is possible.

cond-mat.mes-hall↗

arXiv2015

An operational definition of the 100 second blocking temperature T$_{\mathrm{B}100}$ for single molecule magnets

An important figure of merit for the performance of single-molecule magnets (SMMs) is the 100 s blocking temperature T$_{\mathrm{B}100}$. It is the temperature at which the remanence or zero field relaxation time is 100 seconds. If there is more than one relaxation process of the magnetisation, the determination of the relaxation times may, however, become ambiguous. Here we propose an operational definition for the zero-field magnetic relaxation times from which T$_{\mathrm{B}100}$ may be determined. This definition allows for a direct comparison of the performance of different samples independent of the details of the relaxation processes involved in the demagnetization.

cond-mat.mtrl-sci↗

arXiv2014

Surface aligned magnetic moments and hysteresis of an endohedral single-molecule magnet on a metal

The interaction between the endohedral unit in the single-molecule magnet Dy$_2$ScN@C$_{80}$ and a rhodium (111) substrate leads to alignment of the Dy 4$f$ orbitals. The resulting orientation of the Dy$_2$ScN plane parallel to the surface is inferred from comparison of the angular anisotropy of x-ray absorption spectra and multiplet calculations in the corresponding ligand field. The x-ray magnetic circular dichroism (XMCD) is also angle dependent and signals strong magnetocrystalline anisotropy. This directly relates geometric and magnetic structure. Element specific magnetization curves from different coverages exhibit hysteresis at a sample temperature of $\sim4$ K. From the measured hysteresis curves we estimate the zero field remanence life-time during x-ray exposure of a sub-monolayer to be about 30 seconds.

cond-mat.mtrl-sci↗

arXiv2013

Tunneling, Remanence, and Frustration in Dysprosium based Endohedral Single Molecule Magnets

A single molecule magnet (SMM) can maintain its magnetization direction over a long period of time [1,2]. It consists in a low number of atoms that facilitates the understanding and control of the ground state, which is essential in future applications such as high-density information storage or quantum computers [3,4]. Endohedral fullerenes realize robust, nanometer sized, and chemically protected magnetic clusters that are not found as free species in nature. Here we demonstrate how adding one, two, or three dysprosium atoms to the carbon cage results in three distinct magnetic ground states. The significantly different hysteresis curves demonstrate the decisive influence of the number of magnetic moments and their interactions. At zero field the comparison relates tunneling of the magnetization, with remanence, and frustration. The ground state of the tridysprosium species turns out to be one of the simplest realizations of a frustrated, ferromagnetically coupled magnetic system.

cond-mat.mtrl-sci↗

arXiv2013

Adsorption of silicon on Au(110): an ordered two dimensional surface alloy

We report on experimental evidence for the formation of a two dimensional Si/Au(110) surface alloy. In this study, we have used a combination of scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy and ab initio calculations based on density functional theory. A highly ordered and stable Si-Au surface alloy is observed subsequent to growth of a sub-monolayer of silicon on an Au(110) substrate kept above the eutectic temperature.

cond-mat.mtrl-sci↗

arXiv2013

Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2x1)

We report results on the self-assembly of silicon nanoribbons on the (2x1) reconstructed Au(110) surface under ultra-high vacuum conditions. Upon adsorption of 0.2 monolayer (ML) of silicon the (2x1) reconstruction of Au(110) is replaced by an ordered surface alloy. Above this coverage a new superstructure is revealed by low electron energy diffraction (LEED) which becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons all oriented along the [-110] direction as revealed by LEED and scanning tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In addition the silicon atoms show signatures of two chemical environments corresponding to the edge and center of the ribbons.

cond-mat.mtrl-sci↗

arXiv2011

Mechanism of Laser-induced Field Emission

We have measured electron energy distribution curves (EDCs) of the laser-induced field emission from a tungsten tip. Field emission from photo-excited nonequilibrium electron distributions were clearly observed, while no enhanced field emission due to optical electric fields appeared up to values of 1.3 V/nm. Thus, we experimentally confirm the emission mechanism. Simulated transient EDCs show that electron dynamics plays a significant role in the laser-induced field emission. The results should be useful to find optimal parameters for defining the temporal and spectral characteristics of electron pulses for many applications based on pulsed field emission.

cond-mat.mes-hall↗

arXiv2010

A measure for the chirality of triangles

A measure for the description of the chirality of triangles is introduced. The measure $X\delta$ is zero for triangles with at least one mirror axe, i.e. equilateral or isosceles triangles, and positive or negative for scalane, i.e. left or right handed triangles, respectively.

math-ph↗