rami dana | Massachusetts Institute of Technology (MIT) (original) (raw)

Papers by rami dana

Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays ma... more Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays may show emergent photonic or electronic behavior, and once the nuclei coalesce into thin films, the nucleation density influences parameters such as surface roughness, stress, and grain boundary structure. Tailoring substrate properties to control nucleation is therefore a powerful tool for designing functional thin films and nanomaterials. Here, we examine nucleation control for metals deposited on two-dimensional materials in a situation where substrate effects are absent and heterogeneous nucleation sites are minimized. Through quantification of faceted, epitaxial Au island nucleation on graphene, we show that ultralow nucleation densities with nuclei several micrometers apart can be achieved on suspended graphene under conditions where we measure 2−3 orders of magnitude higher nucleation density on the adjacent supported substrate. We estimate diffusion distances using nucleation theory and find a strong sensitivity of nucleation and diffusion to suspended graphene thickness. Finally, we discuss the role of surface roughness as the main factor determining nucleation density on clean free-standing graphene.

ACS nano

Atomic-level defects in van der Waals (vdW) materials are essential building blocks for quantum t... more Atomic-level defects in van der Waals (vdW) materials are essential building blocks for quantum technologies and quantum sensing applications. The layered magnetic semiconductor CrSBr is an outstanding candidate for exploring optically active defects because of a direct gap, in addition to a rich magnetic phase diagram, including a recently hypothesized defect-induced magnetic order at low temperature. Here, we show optically active defects in CrSBr that are probes of the local magnetic environment. We observe a spectrally narrow (1 meV) defect emission in CrSBr that is correlated with both the bulk magnetic order and an additional low-temperature, defectinduced magnetic order. We elucidate the origin of this magnetic order in the context of local and nonlocal exchange coupling effects. Our work establishes vdW magnets like CrSBr as an exceptional platform to optically study defects that are correlated with the magnetic lattice. We anticipate that controlled defect creation allows for tailor-made complex magnetic textures and phases with direct optical access.

ACS Nano, 2023

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic an... more Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga−Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higherdimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr−S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening, with an effective electron mass ratio of m X e /m Y e ∼ 50. This extreme anisotropy experimentally manifests in strong electron−phonon and exciton−phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D-correlated manybody physics in the presence of magnetic order.

We describe the design, construction, and performance of an ultra-low temperature, high-field sca... more We describe the design, construction, and performance of an ultra-low temperature, high-field scanning
tunneling microscope (STM) with two independent tips. The STM is mounted on a dilution
refrigerator and operates at a base temperature of 30 mK with magnetic fields of up to 13.5 T. We
focus on the design of the two-tip STM head, as well as the sample transfer mechanism, which allows
in situ transfer from an ultra high vacuum preparation chamber while the STM is at 1.5 K. Other
design details such as the vibration isolation and rf-filtered wiring are also described. Their effectiveness
is demonstrated via spectral current noise characteristics and the root mean square roughness
of atomic resolution images. The high-field capability is shown by the magnetic field dependence
of the superconducting gap of CuxBi2Se3. Finally, we present images and spectroscopy taken with
superconducting Nb tips with the refrigerator at 35 mK that indicate that the effective temperature of
our tips/sample is approximately 184 mK, corresponding to an energy resolution of 16 μeV.

Israel Journal of Chemistry, 2008

A new design for a dual-tip scanning tunneling microscope (STM) is presented. The design is a var... more A new design for a dual-tip scanning tunneling microscope (STM) is presented. The design is a variation on the mechanically controllable break-junction with two electron beam-induced deposition nano-tips. The new design enables one to scan surfaces simultaneously with two probes having a nano-gap separation. By collecting the lateral current flowing between the tips, the transconductance map can then be compared with the STM images for local characterizations of the electron transport. Since the lateral beam carries the property of the density of states of the surface at momentum space, the dispersion of the electronic structure should give an orientation and position dependence of the local transconductance current. In addition, the reduced terminal separation, on the order of the characteristic mesoscopic length scales, is likely to be sensitive to a variety of typically observed interactions and interference effects.

Usually strain relaxations are predicted on the basis of the macroscopic lattice mismatch between... more Usually strain relaxations are predicted on the basis of the macroscopic lattice mismatch between two materials. In the case of homoepitaxy, according to the classical rule, no difference between deposited material and substrate has to be made and no effects arise due to a misfit between substrate and the film. The principal drawback of this approach is that mesoscopic islands should adopt their intrinsic bond lengths, which can be different from the bond lengths in bulk. And indeed, it was demonstrated that strain relaxations in homoepitaxy are determined by the size dependent mesoscopic mismatch. In mesoscopic islands the relaxation of edge atoms can be the dominating process. These atoms are relaxing in the direction of the center of the island and take other equilibrium positions with shorter bonds than in macroscopic systems. Therefore, the mesoscopic mismatch (finite-size misfit) between islands and the substrate in homoepitaxy exists and can locally affect the growth process.

Abstract – The steady-state morphology of submonolayer Si/Si(111)7×7 islands is characterized by ... more Abstract – The steady-state morphology of submonolayer Si/Si(111)7×7 islands is characterized
by a size-dependent transition from compact through ramified to 1D-like forms. The transition
is described by the linear-chain model (LCM), which explains this shape transition in strained
heteroepitaxial layers, as a mechanism for strain relaxation without dislocations. We found that
above the percolation coverage θc, the entire structure adopts new steady-state morphology and
reduces its typical width by a factor of e, to its optimal-energy value. The LCM predicts this value
as the asymptotic behavior for infinite elongated islands. Our experimental results, which are
supported by energy calculations, confirm the LCM predictions for the first time in homoepitaxy.
These results are explained by a size-dependent mesoscopic mismatch between the islands and the
substrate.

Fractal islands are normally observed when the growth is a result of many random coalescence even... more Fractal islands are normally observed when the growth is a result of many random coalescence events of small
islands or atoms with the growing cluster. In this paper, we show that fractalization can be observed also for growing
islands at a coverage which is close to 0.5 monolayers. This was shown for a Si(1 1 1) surface covered by 0.53 monolayer
of silicon. This fractalization is explained by the simple conservative Ising model, where the diffusion of a single atom is
simulated by a single spin flip. In this model, fractal islands are observed over a finite scaling range where smaller
islands have a dimension of 2 and larger ones are fractal. The fractal dimension and the scaling range are dependent on
the fraction (equivalent to coverage) p of spin up (or down). Both the dimension and range increase as p approaches 0.5.
We show that the growth of the clusters is in agreement with a classical t0:33 law [Phys. Rev. B 34 (1986) 7845].
2002 Elsevier Science B.V. All rights reserved.

We have developed a reproducible technique for the fabrication of sharp superconducting Nb tips f... more We have developed a reproducible technique for the fabrication of sharp superconducting Nb tips for
scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. Sections of Nb wire with
250 lm diameter are dry etched in an SF6 plasma in a Reactive Ion Etcher. The gas pressure, etching
time, and applied power are chosen to control the ratio of isotropic to anisotropic etch rates and
produce the desired tip shape. The resulting tips are atomically sharp, with radii of less than 100 nm,
mechanically stable, and superconducting. They generate good STM images and spectroscopy on
single crystal samples of Au(111), Au(100), and Nb(100), as well as a doped topological insulator
Bi2Se3 at temperatures ranging from 30 mK to 9K.

Drafts by rami dana

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic an... more Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic
and magnetic orders are of fundamental interest. Interaction effects in low-dimensional systems
can lead to fundamental excitations which are completely different from the quasi-particles one
would expect in a higher-dimensional counterpart, such as Tomonaga-Luttinger liquids and topological
orders and defects. However, clean 1D electronic systems are difficult to realize experimentally,
particularly magnetically ordered systems. Here, we show that the van der Waals layered magnetic
semiconductor CrSBr behaves like a quasi-1D electronic material embedded in a magnetically ordered
environment. The strong 1D electronic character is due to the unique combination of weak
interlayer hybridization and anisotropy in effective mass and dielectric screening. The band structure
and quasi-particle excitations are dominated by the Cr-S chains and a shallow 1D quantum
confinement normal to these chains, manifesting in an anisotropic band with an effective electron
mass ratio of me
X/me
Y 50. Strong quasi-particle interactions and 1D electronic character are
indicated by Fano resonances from a van Hove singularity of similar strength as in metallic carbon
nanotubes. The spectrally narrow excitons (1meV) inherit the 1D character and show pronounced
exciton-phonon coupling effects. Overall, CrSBr appears to be an experimentally clean candidate
for the study of 1D correlated many-body physics in the presence of magnetic order.

Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays ma... more Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays may show emergent photonic or electronic behavior, and once the nuclei coalesce into thin films, the nucleation density influences parameters such as surface roughness, stress, and grain boundary structure. Tailoring substrate properties to control nucleation is therefore a powerful tool for designing functional thin films and nanomaterials. Here, we examine nucleation control for metals deposited on two-dimensional (2D) materials in a situation where substrate effects are absent and heterogeneous nucleation sites are minimized. Through quantification of faceted, epitaxial Au island nucleation on graphene, we show that ultra-low nucleation densities with nuclei several micrometers apart can be achieved on suspended graphene under conditions where we measure the nucleation density to be 2-3 orders of magnitude higher on the adjacent supported substrate. We estimate diffusion distances using nucleation theory and find a strong sensitivity of nucleation and diffusion to suspended graphene thickness. We suggest that nucleation site density control via substrate tuning may act as a platform for applications where non-lithographic patterning of metals on graphene is beneficial.

Research Background General: I have a complete experience in the design, purchase, assembly, test... more Research Background General: I have a complete experience in the design, purchase, assembly, testing and operation of experimental systems, versatile lab experience from UHV to cryogenics to fabrication to measurement techniques and data analysis. My interest lies in the broad field of condense matter, specifically, a record in surface science and characterization. M.sc: My M.sc thesis describes the fractal nature of islands in submonolayer near coverage of  = 0.5. The islands where formed by ripening and coalescence after starting from a random distribution of the atoms and heating the substrate [1]. In my work I compare between scanning tunneling microscope (STM) experimental results of silicon island on si(111) 7x7 and Monte Carlo simulations on a new Conservative Ising Model with Kawasaki dynamics[2] and curvature effect. See left image-bottom. PhD: My PhD thesis has two parts; STM realization of finite-size misfit in homoepitaxy [3] and a dual-tip STM (DTSTM) for the characterization of mesoscopic transport on surfaces [4]. The finite-size misfit was used to explain the shape transition of homoepitaxial silicon islands above the percolation threshold as predicted by the Linear Chain Model (LCM) [5]. See left image-top. The motivation to characterize surface transport on the mesoscale was in the heart of a new approach for a dual-tip STM. The design was based on the mechanically controllable break-junction (MCBJ) with two electron-beam induced deposition nano-tips. See center image. PostDoc projects: a) Single donors near Silicon surfaces as candidates for quantum qubits. b) Search for Majorana Fermions in Topological Insulators (TI)-Superconductors (SC) heterostructures. One interesting result is the realization of p(E) [6] theory in SIS junction between SC Nb tip [7] and proximity induced SC on the surface of the TI Bi 2 Se 3 [8]. See right image-top. Current work-For the last two years I was working with a dilution refrigerator equipped with a single/dual tip mK-STM [9] and 13.5 T magnetic fields. I was looking for Majorana Fermions in condense matter physics by using normal Tungsten or SC Nb tips on the TI Bi 2 Se 3 or the SC TI Cu 0.2 Bi 2 Se 3. My recent results are evidence for the effect of vortexes on the sub-gap spectrum of Cu 0.2 Bi 2 Se 3. Here I found the appearance of a zero-bias conductance peak at the vortex core (and edge) when the SC gap is closed [10]. See right image-bottom. Left: Top – model [5] & Bottom-transition from compact (green) to ramified (purple) to linear chains (pink) both in experiment and simulation. Center: Top-DTSTM model to measure trans-conductance current [11], center-new design for a MCBJ-DTSTM with virtual pivot & Bottom-two induced deposition tips on both sides of the DTSTM. Right: Top-P(E) applied to proximity induced SIS junction, center-STM image of Cu0.2Bi2Se3 & Bottom – ZBCP at the center (left), center and edge (right) of a vortex when the SC gap is closed. core edge

ACS nano

ACS Nano, 2023

Israel Journal of Chemistry, 2008

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic an... more Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic
and magnetic orders are of fundamental interest. Interaction effects in low-dimensional systems
can lead to fundamental excitations which are completely different from the quasi-particles one
would expect in a higher-dimensional counterpart, such as Tomonaga-Luttinger liquids and topological
orders and defects. However, clean 1D electronic systems are difficult to realize experimentally,
particularly magnetically ordered systems. Here, we show that the van der Waals layered magnetic
semiconductor CrSBr behaves like a quasi-1D electronic material embedded in a magnetically ordered
environment. The strong 1D electronic character is due to the unique combination of weak
interlayer hybridization and anisotropy in effective mass and dielectric screening. The band structure
and quasi-particle excitations are dominated by the Cr-S chains and a shallow 1D quantum
confinement normal to these chains, manifesting in an anisotropic band with an effective electron
mass ratio of me
X/me
Y 50. Strong quasi-particle interactions and 1D electronic character are
indicated by Fano resonances from a van Hove singularity of similar strength as in metallic carbon
nanotubes. The spectrally narrow excitons (1meV) inherit the 1D character and show pronounced
exciton-phonon coupling effects. Overall, CrSBr appears to be an experimentally clean candidate
for the study of 1D correlated many-body physics in the presence of magnetic order.

Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays ma... more Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays may show emergent photonic or electronic behavior, and once the nuclei coalesce into thin films, the nucleation density influences parameters such as surface roughness, stress, and grain boundary structure. Tailoring substrate properties to control nucleation is therefore a powerful tool for designing functional thin films and nanomaterials. Here, we examine nucleation control for metals deposited on two-dimensional (2D) materials in a situation where substrate effects are absent and heterogeneous nucleation sites are minimized. Through quantification of faceted, epitaxial Au island nucleation on graphene, we show that ultra-low nucleation densities with nuclei several micrometers apart can be achieved on suspended graphene under conditions where we measure the nucleation density to be 2-3 orders of magnitude higher on the adjacent supported substrate. We estimate diffusion distances using nucleation theory and find a strong sensitivity of nucleation and diffusion to suspended graphene thickness. We suggest that nucleation site density control via substrate tuning may act as a platform for applications where non-lithographic patterning of metals on graphene is beneficial.