Bulk conducting states of intrinsically doped Bi$_2$Se$_3$. (arXiv:2306.00827v1 [cond-mat.mtrl-sci])

2023-06-02T02:29:50+00:00June 2nd, 2023|Categories: Publications|Tags: |

With a large band gap and a single Dirac cone responsible for the topological surface states, Bi2Se3 is widely regarded as a prototypical 3D topological insulator. Further applications of the bulk material has, however, been hindered by inherent structural defects that donate electrons and make the bulk conductive. Consequently, controlling these defects is of great interest for future technological applications, and while past literature has focused on adding external doping elements to the mixture, a complete study on undoped Bi2Se3 was still lacking. In this work, we use the self-flux method to obtain high-quality Bi2Se3 single-crystals in the entire concentration range available on the phase-diagram for the technique. By combining basic structural characterization with measurements of the resistivity, Hall effect and Shubnikov-de Haas (SdH) quantum oscillations, the effects of these impurities on the bulk transport are investigated in samples with electron densities ranging from 10^17 cm^-3 to 10^19 cm^-3, from Se-rich to Bi-rich mixtures, respectively, evidencing the transition into a degenerate semiconductor regime. We find that electron-donor impurities, likely Se vacancies, unavoidably shift the Fermi level up to 200 meV inside the conduction band. Other impurities, like interstitial Bi and Se, are shown to play a significant role as scattering centres, specially at low temperatures and in the decoherence of the SdH oscillations. Previous open questions on Bi2Se3, such as the upturn in resistivity below 30 K, the different scattering times in transport and quantum oscillations, and the presence of additional low mobility bands, are addressed. The results outlined here

Published in: "arXiv Material Science".

Emergent One-Dimensional Helical Channel in Higher-Order Topological Insulators with Step Edges. (arXiv:2206.15206v3 [cond-mat.mes-hall] UPDATED)

2023-06-02T02:29:47+00:00June 2nd, 2023|Categories: Publications|Tags: |

We study theoretically the electronic structure of three-dimensional (3D) higher-order topological insulators in the presence of step edges. We numerically find that a 1D conducting state with a helical spin structure, which also has a linear dispersion near the zero energy, emerges at a step edge and on the opposite surface of the step edge. We also find that the 1D helical conducting state on the opposite surface of a step edge emerges when the electron hopping in the direction perpendicular to the step is weak. In other words, the existence of the 1D helical conducting state on the opposite surface of a step edge can be understood by considering an addition of different-sized independent blocks of 3D higher-order topological insulators. On the other hand, when the electron hopping in the direction perpendicular to the step is strong, the location of the emergent 1D helical conducting state moves from the opposite surface of a step edge to the dip ($270^{circ}$ edge) just below the step edge. In this case, the existence at the dip below the step edge can be understood by assigning each surface with a sign ($+$ or $-$) of the mass of the surface Dirac fermions. Namely, the 1D helical conducting states at the step edge and at the dip below the step edge emerge where the mass changes its sign, as well as the ordinary 1D topological hinge states. Thus, in the both strong and weak coupling regimes, the emergent 1D helical conducting states have a

Published in: "arXiv Material Science".

Imaging Moir’e Excited States with Photocurrent Tunneling Microscopy. (arXiv:2306.00859v1 [cond-mat.mes-hall])

2023-06-02T02:29:45+00:00June 2nd, 2023|Categories: Publications|Tags: , |

Moir’e superlattices provide a highly tunable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators1-17 to moir’e excitons7-10,18. Scanning tunneling microscopy has played a key role in probing microscopic behaviors of the moir’e correlated ground states at the atomic scale1,11-15,19. Atomic-resolution imaging of quantum excited state in moir’e heterostructures, however, has been an outstanding experimental challenge. Here we develop a novel photocurrent tunneling microscopy by combining laser excitation and scanning tunneling spectroscopy (laser-STM) to directly visualize the electron and hole distribution within the photoexcited moir’e exciton in a twisted bilayer WS2 (t-WS2). We observe that the tunneling photocurrent alternates between positive and negative polarities at different locations within a single moir’e unit cell. This alternating photocurrent originates from the exotic in-plane charge-transfer (ICT) moir’e exciton in the t-WS2 that emerges from the competition between the electron-hole Coulomb interaction and the moir’e potential landscape. Our photocurrent maps are in excellent agreement with our GW-BSE calculations for excitonic states in t-WS2. The photocurrent tunneling microscopy creates new opportunities for exploring photoexcited non-equilibrium moir’e phenomena at the atomic scale.

Published in: "arXiv Material Science".

Electronic structure of few-layer black phosphorus from $mu$-ARPES. (arXiv:2306.00749v1 [cond-mat.mes-hall])

2023-06-02T02:29:43+00:00June 2nd, 2023|Categories: Publications|Tags: |

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based micro-focus angle resolved photoemission ($mu$-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters which provide an accurate description of the electronic structure for all thicknesses.

Published in: "arXiv Material Science".

Kinetic Friction of Structurally Superlubric 2D Material Interfaces. (arXiv:2306.00205v1 [cond-mat.mtrl-sci])

2023-06-02T02:29:38+00:00June 2nd, 2023|Categories: Publications|Tags: |

The ultra-low kinetic friction F_k of 2D structurally superlubric interfaces, connected with the fast motion of the incommensurate moir’e pattern, is often invoked for its linear increase with velocity v_0 and area A, but never seriously addressed and calculated so far. Here we do that, exemplifying with a twisted graphene layer sliding on top of bulk graphite — a demonstration case that could easily be generalized to other systems. Neglecting quantum effects and assuming a classical Langevin dynamics, we derive friction expressions valid in two temperature regimes. At low temperatures the nonzero sliding friction velocity derivative dF_k/dv_0 is shown by Adelman-Doll-Kantorovich type approximations to be equivalent to that of a bilayer whose substrate is affected by an analytically derived effective damping parameter, replacing the semi-infinite substrate. At high temperatures, friction grows proportional to temperature as analytically required by fluctuation-dissipation. The theory is validated by non-equilibrium molecular dynamics simulations with different contact areas, velocities, twist angles and temperatures. Using 6^{circ}-twisted graphene on Bernal graphite as a prototype we find a shear stress of measurable magnitude, from 25 kPa at low temperature to 260 kPa at room temperature, yet only at high sliding velocities such as 100 m/s. However, it will linearly drop many orders of magnitude below measurable values at common experimental velocities such as 1 {mu}m/s, a factor 10^{-8} lower. The low but not ultra-low “engineering superlubric” friction measured in existing experiments should therefore be attributed to defects and/or edges, whose contribution surpasses by far the negligible moir’e contribution.

Published in: "arXiv Material Science".

Diffusive and Ballistic Transport in Ultra-thin InSb Nanowire Devices Using a Few-layer-Graphene-AlOx Gate. (arXiv:2306.00117v1 [cond-mat.mes-hall])

2023-06-02T02:29:34+00:00June 2nd, 2023|Categories: Publications|Tags: |

Quantum devices based on InSb nanowires (NWs) are a prime candidate system for realizing and exploring topologically-protected quantum states and for electrically-controlled spin-based qubits. The influence of disorder on achieving reliable topological regimes has been studied theoretically, highlighting the importance of optimizing both growth and nanofabrication. In this work we investigate both aspects. We developed InSb nanowires with ultra-thin diameters, as well as a new gating approach, involving few-layer graphene (FLG) and Atomic Layer Deposition (ALD)-grown AlOx. Low-temperature electronic transport measurements of these devices reveal conductance plateaus and Fabry-P’erot interference, evidencing phase-coherent transport in the regime of few quantum modes. The approaches developed in this work could help mitigate the role of material and fabrication-induced disorder in semiconductor-based quantum devices.

Published in: "arXiv Material Science".

Twisto-electrochemical activity volcanoes in Trilayer Graphene. (arXiv:2306.00028v1 [cond-mat.mes-hall])

2023-06-02T02:29:32+00:00June 2nd, 2023|Categories: Publications|Tags: |

In this work, we develop a twist-dependent electrochemical activity map, combining a tight-binding electronic structure model with modified Marcus-Hush-Chidsey kinetics in trilayer graphene. We identify a counterintuitive rate enhancement region spanning the magic angle curve and incommensurate twists of the system geometry. We find a broad activity peak with a ruthenium hexamine redox couple in regions corresponding to both magic angles and incommensurate angles, a result qualitatively distinct from the twisted bilayer case. Flat bands and incommensurability offer new avenues for reaction rate enhancements in electrochemical transformations.

Published in: "arXiv Material Science".

Sliding and Pinning in Structurally Lubric 2D Material Interfaces. (arXiv:2305.19740v1 [cond-mat.mtrl-sci])

2023-06-01T02:29:32+00:00June 1st, 2023|Categories: Publications|Tags: |

A plethora of two-dimensional (2D) materials entered the physics and engineering scene in the last two decades. Their robust, membrane-like sheet permit — mostly require — deposition, giving rise to solid-solid dry interfaces whose bodily mobility, pinning, and general tribological properties under shear stress are currently being understood and controlled, experimentally and theoretically. In this Colloquium we use simulation case studies of twisted graphene system as a prototype workhorse tool to demonstrate and discuss the general picture of 2D material interface sliding. First, we highlight the crucial mechanical difference, often overlooked, between small and large incommensurabilities, corresponding e.g., to small and large twist angles in graphene interfaces. In both cases, focusing on flat, structurally lubric, “superlubric” geometries, we elucidate and review the generally separate scaling with area of static friction in pinned states and of kinetic friction during sliding, tangled as they are with the effects of velocity, temperature, load, and defects. Including the role of island boundaries and of elasticity, and corroborating when possible the existing case-by-case results in literature beyond graphene, the overall picture proposed is meant for general 2D material interfaces, that are of importance for the physics and technology of existing and future bilayer and multilayer systems.

Published in: "arXiv Material Science".

Atomically smooth films of CsSb: a chemically robust visible light photocathode. (arXiv:2305.19553v1 [physics.acc-ph])

2023-06-01T02:29:29+00:00June 1st, 2023|Categories: Publications|Tags: |

Alkali antimonide semiconductor photocathodes provide a promising platform for the generation of high brightness electron beams, which are necessary for the development of cutting-edge probes including x-ray free electron lasers and ultrafast electron diffraction. However, to harness the intrinsic brightness limits in these compounds, extrinsic degrading factors, including surface roughness and contamination, must be overcome. By exploring the growth of CsxSb thin films monitored by in situ electron diffraction, the conditions to reproducibly synthesize atomically smooth films of CsSb on 3C-SiC (100) and graphene coated TiO2 (110) substrates are identified, and detailed structural, morphological, and electronic characterization is presented. These films combine high quantum efficiency in the visible (up to 1.2% at 400 nm), an easily accessible photoemission threshold of 550 nm, low surface roughness (down to 600 pm on a 1 um scale), and a robustness against oxidation up to 15 times greater then Cs3Sb. These properties suggest that CsSb has the potential to operate as an alternative to Cs$_3$Sb in electron source applications where the demands of the vacuum environment might otherwise preclude the use of traditional alkali antimonides.

Published in: "arXiv Material Science".

Fabrication of thin diamond membranes by Ne$^+$ implantation. (arXiv:2305.19133v1 [cond-mat.mtrl-sci])

2023-05-31T02:30:18+00:00May 31st, 2023|Categories: Publications|Tags: |

Color centers in diamond are one of the most promising tools for quantum information science. Of particular interest is the use of single-crystal diamond membranes with nanoscale-thickness as hosts for color centers. Indeed, such structures guarantee a better integration with a variety of other quantum materials or devices, which can aid the development of diamond-based quantum technologies, from nanophotonics to quantum sensing. A common approach for membrane production is what is known as “smart-cut”, a process where membranes are exfoliated from a diamond substrate after the creation of a thin sub-surface amorphous carbon layer by He$^+$ implantation. Due to the high ion fluence required, this process can be time-consuming. In this work, we demonstrated the production of thin diamond membranes by neon implantation of diamond substrates. With the target of obtaining membranes of $sim$ 200 nm thickness and finding the critical damage threshold, we implanted different diamonds with 300 keV Ne$^+$ ions at different fluences. We characterized the structural properties of the implanted diamonds and the resulting membranes through SEM, Raman spectroscopy, and photoluminescence spectroscopy. We also found that a SRIM model based on a two-layer diamond/sp$^2$-carbon target better describes ion implantation, allowing us to estimate the diamond critical damage threshold for Ne$^+$ implantation. Compared to He$^+$ smart-cut, the use of a heavier ion like Ne$^+$ results in a ten-fold decrease in the ion fluence required to obtain diamond membranes and allows to obtain shallower smart-cuts, i.e. thinner membranes, at the same ion energy.

Published in: "arXiv Material Science".

Healing of a Topological Scar: Coordination Defects in a Honeycomb Lattice. (arXiv:2305.12246v2 [cond-mat.mtrl-sci] UPDATED)

2023-05-31T02:30:16+00:00May 31st, 2023|Categories: Publications|

A crystal structure with a point defect typically returns to its ideal local structure upon moving a few bond lengths away from the defect; topological defects such as dislocations or disclinations also heal rapidly in this regard. Here we describe a simple point defect — a two-fold atom incorporated at the growth edge of a 2D hexagonal honeycomb material — whose healing may require a defect complex with 50 or more atoms. $textit{Topologically}$ the two-fold atom disappears into a single ‘long bond’ between its neighbors, thereby inducing a pentagonal disclination. However, $textit{chemically}$ this disclination occupies as much physical space as a six-fold ring. This incompatibility of chemistry and topology can cause a ”ringing” of the Gaussian curvature that creates an expansive healing region and may even spawn a semi-infinite grain boundary propagating outwards from the topological scar.

Published in: "arXiv Material Science".

Search for magnetoelectric monopole response in Cr$_2$O$_3$ powder. (arXiv:2305.19098v1 [cond-mat.mtrl-sci])

2023-05-31T02:30:14+00:00May 31st, 2023|Categories: Publications|

Powder samples have been suggested as a pathway to fabricate isotropic magnetoelectric (ME) materials which effectively only have a pseudoscalar or monopole ME response. We demonstrate that random distribution of ME grains alone does not warrant isotropic ME response because the activation of a non-vanishing ME response requires a ME field cooling protocol which tends to induce preferred axes. We investigate the evolution of ME susceptibility in powder chromia samples for various ME field cooling protocols both theoretically and experimentally. In particular, we work out the theoretical expressions for ME susceptibility for powder Chromia in the framework of statistical mechanics where Boltzmann factors weigh the orientation of the N’eel vector relative to the local orientation of the c-axis of a grain. Previous approximations oversimplified the thermodynamic nature of the annealing process giving rise to misleading conclusions on the role of the magnitude of the applied product of electric and magnetic fields on the ME response. In accordance with our refined theory, a strong dependence of the functional form of $alpha$ vs. $T$ of Chromia powders on the ME field cooling protocol is observed. It shows that Chromia powder is not generically an isotropic ME effective medium but provides a pathway to realize the elusive isotropic ME response.

Published in: "arXiv Material Science".

Kinetics of Graphene Growth on Liquid Copper by Chemical Vapor Deposition. (arXiv:2305.18331v1 [cond-mat.mtrl-sci])

2023-05-31T02:30:12+00:00May 31st, 2023|Categories: Publications|Tags: , |

We report a combined experimental and computational study of the kinetics of graphene growth during chemical vapor deposition on a liquid copper catalyst. The use of liquid metal catalysts offers bright perspectives for controllable large-scale, high-quality synthesis technologies of two-dimensional materials. We carried out a series of growth experiments varying CH4-to-H2 pressure ratios and deposition temperature. By monitoring the graphene flake morphology in real time during growth using in situ optical microscopy in radiation mode, we explored the morphology and kinetics of the growth within a wide range of experimental conditions. Following an analysis of the flakes’ growth rates, we conclude that the growth mode was attachment-limited. The attachment and detachment activation energies of carbon species are derived as 1.9 +- 0.3 eV and 2.0 +- 0.1 eV, respectively. We also conducted free-energy calculations by a moment tensor potential trained to density functional theory data. Our simulations propose that carbon dimers are most likely the active carbon species during growth, with attachment and detachment barriers of 1.71 +- 0.15 eV and 2.09 +- 0.02 eV, respectively, being in good agreement with the experimental results.

Published in: "arXiv Material Science".

Perpendicular in-plane negative magnetoresistance in ZrTe5. (arXiv:2305.19009v1 [cond-mat.mes-hall])

2023-05-31T02:30:10+00:00May 31st, 2023|Categories: Publications|

The unique band structure in topological materials frequently results in unusual magneto-transport phenomena, one of which is in-plane longitudinal negative magnetoresistance (NMR) with the magnetic field aligned parallel to the electrical current direction. This NMR is widely considered as a hallmark of chiral anomaly in topological materials. Here we report the observation of in-plane NMR in the topological material ZrTe5 when the in-plane magnetic field is both parallel and perpendicular to the current direction, revealing an unusual case of quantum transport beyond the chiral anomaly. We find that a general theoretical model, which considers the combined effect of Berry curvature and orbital moment, can quantitatively explain this in-plane NMR. Our results provide new insights into the understanding of in-plane NMR in topological materials.

Published in: "arXiv Material Science".

Ion irradiation-induced sinking of Ag nanocubes into substrates. (arXiv:2305.18968v1 [cond-mat.mtrl-sci])

2023-05-31T02:30:08+00:00May 31st, 2023|Categories: Publications|

Ion irradiation can cause burrowing of nanoparticles in substrates, strongly depending on the material properties and irradiation parameters. In this study, we demonstrate that the sinking process can be accomplished with ion irradiation of cube-shaped Ag nanoparticles on top of silicon; how ion channeling affects the sinking rate; and underline the importance of the amorphous state of the substrate upon ion irradiation. Based on our experimental findings, the sinking process is described as being driven by capillary forces enabled by ion-induced plastic flow of the substrate.

Published in: "arXiv Material Science".

High temperature decomposition and age hardening of single-phase wurtzite Ti$_{1-x}$Al$_{x}$N thin films grown by cathodic arc deposition. (arXiv:2305.18950v1 [cond-mat.mtrl-sci])

2023-05-31T02:30:06+00:00May 31st, 2023|Categories: Publications|

We investigated the high temperature decomposition behavior of wurtzite phase Ti$_{1-x}$Al$_{x}$N films using experimental methods and first-principles calculations. Single phase metastable wurtzite Ti$_{1-x}$Al$_{x}$N (x = 0.65, 0.75, 085 and 0.95) solid solution films were grown by cathodic arc deposition using low duty cycle pulsed substrate-bias voltage. First-principles calculated elastic constants of the wurtzite Ti$_{1-x}$Al$_{x}$N phase show a strong dependence on alloy composition. The predicted phase diagram shows a miscibility gap with an unstable region. High resolution scanning transmission electron microscopy and chemical mapping demonstrate decomposition of the films after high temperature annealing (950$^{circ}$C), which resulted in nanoscale chemical compositional modulations containing Ti-rich and Al-rich regions with coherent or semi coherent interfaces. This spinodal decomposition of the wurtzite film causes age hardening of 1-2 GPa.

Published in: "arXiv Material Science".

Terahertz emission from transient currents and coherent phonons in layered MoSe$_2$ and WSe$_2$. (arXiv:2305.18805v1 [cond-mat.mtrl-sci])

2023-05-31T02:30:04+00:00May 31st, 2023|Categories: Publications|Tags: , |

Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe2 and 2H-WSe2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, analysis of the THz emission waveforms were performed, separating the two contributions to the total waveform. Results of the fitting show several vibrational modes in the range of 5.87 to 32.75 cm-1 for the samples, attributed to infrared-active interlayer shear and breathing modes. This study demonstrates that THz emission spectroscopy provides a means of observing these low frequency vibrational modes in layered materials.

Published in: "arXiv Material Science".

A plate-type condenser platform with engineered wettability for space applications. (arXiv:2305.19070v1 [physics.flu-dyn])

2023-05-31T02:30:01+00:00May 31st, 2023|Categories: Publications|Tags: |

Vapor condensation is extensively used in applications that demand the exchange of a substantial amount of heat energy or the vapor-liquid phase conversion. In conventional condensers, the condensate removal from a subcooled surface is caused by gravity force. This restricts the use of such condensers in space applications or in horizontal orientations. The current study demonstrates proof-of-concept of a novel plate-type condenser platform for passively removing condensate from a horizontally oriented surface to the surrounded wicking reservoir without gravity. The condensing surface is engineered with patterned wettabilities, which enables the continuous migration of condensate from the inner region of the condenser surface to the side edges via surface energy gradient. The surrounding wicking reservoir facilitates the continuous absorption of condensate from the side edges. The condensation dynamics on different substrates with patterned wettabilities are investigated, and their condensation heat transfer performance is compared. The continuous migration of condensate drops from a superhydrophobic to a superhydrophilic area can rejuvenate the nucleation sites in the superhydrophobic area, resulting in increased heat transport. We can use the condenser design with engineered wettability mentioned above for temperature and humidity management applications in space.

Published in: "arXiv Material Science".

Strain Relaxation in Core-Shell Pt-Co Catalyst Nanoparticles. (arXiv:2305.18686v1 [cond-mat.mtrl-sci])

2023-05-31T02:29:59+00:00May 31st, 2023|Categories: Publications|Tags: , |

Surface strain plays a key role in enhancing the activity of Pt-alloy nanoparticle oxygen reduction catalysts. However, the details of strain effects in real fuel cell catalysts are not well-understood, in part due to a lack of strain characterization techniques that are suitable for complex supported nanoparticle catalysts. This work investigates these effects using strain mapping with nanobeam electron diffraction and a continuum elastic model of strain in simple core-shell particles. We find that surface strain is relaxed both by lattice defects at the core-shell interface and by relaxation across particle shells caused by Poisson expansion in the spherical geometry. The continuum elastic model finds that in the absence of lattice dislocations, geometric relaxation results in a surface strain that scales with the average composition of the particle, regardless of the shell thickness. We investigate the impact of these strain effects on catalytic activity for a series of Pt-Co catalysts treated to vary their shell thickness and core-shell lattice mismatch. For catalysts with the thinnest shells, the activity is consistent with an Arrhenius dependence on the surface strain expected for coherent strain in dislocation-free particles, while catalysts with thicker shells showed greater activity losses indicating strain relaxation caused by dislocations as well.

Published in: "arXiv Material Science".

Local step-flow dynamics in thin film growth with desorption. (arXiv:2305.18661v1 [cond-mat.mtrl-sci])

2023-05-31T02:29:57+00:00May 31st, 2023|Categories: Publications|

Desorption of deposited species plays a role in determining the evolution of surface morphology during crystal growth when the desorption time constant is short compared to the time to diffuse to a defect site, step edge or kink. However, experiments to directly test the predictions of these effects are lacking. Novel techniques such as emph{in-situ} coherent X-ray scattering can provide significant new information. Herein we present X-ray Photon Correlation Spectroscopy (XPCS) measurements during diindenoperylene (DIP) vapor deposition on thermally oxidized silicon surfaces. DIP forms a nearly complete two-dimensional first layer over the range of temperatures studied (40 – 120 $^{circ}$C), followed by mounded growth during subsequent deposition. Local step flow within mounds was observed, and we find that there was a terrace-length-dependent behavior of the step edge dynamics. This led to unstable growth with rapid roughening ($beta>0.5$) and deviation from a symmetric error-function-like height profile. At high temperatures, the grooves between the mounds tend to close up leading to nearly flat polycrystalline films. Numerical analysis based on a 1 + 1 dimensional model suggests that terrace-length dependent desorption of deposited ad-molecules is an essential cause of the step dynamics, and it influences the morphology evolution.

Published in: "arXiv Material Science".

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