Showing 1–50 of 90 results for author: Tran, T T

Co2SeO3Cl2: Studies of Emerging Magnetoelectric Coupling in a Polar, Buckled Honeycomb Material

Authors: Faith O. Adeyemi, Xudong Huai, Mohamed Kandil, Pradip Karki, Wencan Jin, Thao T. Tran

Abstract: The development of magnetoelectric materials requires chemical design strategies that integrate structural polarity with magnetic lattices capable of supporting competing spin interactions. Here, we demonstrate such an approach in the polar, buckled honeycomb magnet Co2SeO3Cl2. Magnetization and heat-capacity measurements reveal strong magnetic anisotropy and four successive magnetic transitions a… ▽ More The development of magnetoelectric materials requires chemical design strategies that integrate structural polarity with magnetic lattices capable of supporting competing spin interactions. Here, we demonstrate such an approach in the polar, buckled honeycomb magnet Co2SeO3Cl2. Magnetization and heat-capacity measurements reveal strong magnetic anisotropy and four successive magnetic transitions at 25.4, 16.8, 11, and 3 K. The recovered magnetic entropy through the ordering regime is only around half of the expected 2Rln(2), indicating persistent spin fluctuations. Second-harmonic generation measurements show three pronounced intensity anomalies at 11, 17, and 26 K that coincide with magnetic transitions while revealing that the crystallographic symmetry is preserved. Together, these results demonstrate that polar, buckled honeycomb magnets offer an unconventional phase space for coupling magnetic and electric dipoles in magnetoelectric materials. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Heat Capacity-A Powerful Tool for Studying Exotic States of Matter

Authors: K. Ramesh Kumar, Xudong Huai, Michał J. Winiarski, Allen O. Scheie, Thao T. Tran

Abstract: Heat capacity measurements are a powerful tool that researchers rely on when studying the relationship between microscopic degrees of freedom and macroscopic behavior in condensed matter. This uniqueness stems from heat capacity capturing contributions from lattice, electronic, and magnetic components, as well as energy-level populations, enabling an effective approach to studying phase transition… ▽ More Heat capacity measurements are a powerful tool that researchers rely on when studying the relationship between microscopic degrees of freedom and macroscopic behavior in condensed matter. This uniqueness stems from heat capacity capturing contributions from lattice, electronic, and magnetic components, as well as energy-level populations, enabling an effective approach to studying phase transitions and excitations across different classes of materials. However, analyzing heat capacity data presents a common, appreciable challenge for new researchers. Although comprehensive theoretical aspects of heat capacity are presented in several elegant textbooks, practical application remains a daunting task. To overcome this challenge, this tutorial guides researchers in collecting, analyzing, and interpreting heat capacity data in contemporary quantum materials. We outline the connections between thermodynamics, heat capacity, and entropy, as well as measurement methodology and data analysis for representative examples, including phonon dynamics, spin waves, superconductors, magnetic skyrmions, proximate quantum spin liquids, and heavy-fermion materials. Our goal is to provide a concise, accessible guide that enables new researchers to utilize heat capacity as a quantitative lens for understanding exotic states of matter. △ Less

Submitted 14 April, 2026; v1 submitted 13 March, 2026; originally announced March 2026.

Coexisting Paramagnetic Spins and Long-Range Magnetic Order in Ba$_4$(Ru$_{0.92}$Ir$_{0.08}$)$_3$O$_{10}$

Authors: Farhan Islam, Jiasen Guo, Wei Tian, Bing Li, Xudong Huai, Thao T. Tran, Gang Cao, Zachary Morgan, Feng Ye

Abstract: We investigate the effect of dilute Ir substitution on the magnetism of the trimer-based ruthenate Ba$_4$Ru$_3$O$_{10}$ using neutron diffraction, magnetic susceptibility measurements, atomistic simulations, and first-principles calculations. Neutron diffraction shows that Ir doping preserves the zigzag antiferromagnetic structure and the ordered-moment magnitude of the parent compound, in which t… ▽ More We investigate the effect of dilute Ir substitution on the magnetism of the trimer-based ruthenate Ba$_4$Ru$_3$O$_{10}$ using neutron diffraction, magnetic susceptibility measurements, atomistic simulations, and first-principles calculations. Neutron diffraction shows that Ir doping preserves the zigzag antiferromagnetic structure and the ordered-moment magnitude of the parent compound, in which the moments reside exclusively on the two outer Ru(2) sites of each $\rm Ru_3O_{12}$ trimer, while the central Ru(1) site remains nonmagnetic. The Néel temperature is reduced from $\approx\!105$ K to 84.0(1) K upon 8% Ir substitution, while magnetic susceptibility reveals a pronounced low-temperature Curie-like upturn, indicating the coexistence of paramagnetic spins with long-range antiferromagnetic order. Density-functional calculations shows that Ir preferentially occupies the central Ru(1) site, where its extended $5d$ orbitals disrupt the Ru-Ru molecular-orbital network and intra/inter-trimer exchange pathways. Atomistic simulations incorporating this paramagnetic dilution reproduce the suppressed ordering temperature and the coexistence of ordered and paramagnetic components. △ Less

Submitted 5 March, 2026; originally announced March 2026.

Possible Proximity to Ferromagnetism in the V$_2$Ga$_5$ Superconductor

Authors: Szymon Królak, Xudong Huai, Wiktoria Jarosz, Filip Košuth, Pavol Szabó, Michał J. Winiarski, Sudip Malick, Thao T. Tran, Tomasz Klimczuk

Abstract: Superconductivity and ferromagnetism are generally competing ground states in $d$-electron systems, making their interplay of fundamental interest. We report a comprehensive study of high-quality single- and polycrystalline V$_2$Ga$_5$, a bulk type-II superconductor ($T_c = 3.54 \ K$) with a quasi-one-dimensional crystal structure, supplemented with density functional theory (DFT) calculations, su… ▽ More Superconductivity and ferromagnetism are generally competing ground states in $d$-electron systems, making their interplay of fundamental interest. We report a comprehensive study of high-quality single- and polycrystalline V$_2$Ga$_5$, a bulk type-II superconductor ($T_c = 3.54 \ K$) with a quasi-one-dimensional crystal structure, supplemented with density functional theory (DFT) calculations, suggesting possible proximity to ferromagnetic order. Below $T \approx 10 \ K$, magnetic susceptibility shows ZFC/FC splitting, along with saturation and hysteresis in $M(H)$. Moreover, electrical transport measurements reveal a magnetic-field-dependent resistivity upturn, while specific heat is enhanced in magnetic fields. DFT calculations show that the Fermi level in V$_2$Ga$_5$ is located at a peak in the density of states, with a small magnetic moment per unit cell comparable to the experimental value. Together, these results indicate the possibility that ferromagnetic correlations develop below $T \approx 10 \ K$, well above $T_c$, with long-range ferromagnetic order suppressed by the superconducting transition. △ Less

Submitted 10 February, 2026; originally announced February 2026.

Comments: 13 pages, 8 figures

Chiral Polar Eu2(SeO3)2(SO4)(H2O)2: A Pathway Toward Narrow Optical Linewidths and Microsecond Lifetimes for Quantum Memory Candidates

Authors: Uchenna Chinaegbomkpa, Ebube Oyeka, Xudong Huai, Ramesh Kumar, Mingli Liang, Jakoah Brgoch, Hugo Sanabria, Thao T. Tran

Abstract: Stoichiometric materials of Eu(III) offer a promising platform for quantum memories attributable to their unique capability to display a distinctive, nondegenerate J = 0 transition, which enables precise mapping of optical quantum states into their hyperfine structure for reliable storage and retrieval on demand. However, placing Eu(III) into chiral polar structures, which are necessary for achiev… ▽ More Stoichiometric materials of Eu(III) offer a promising platform for quantum memories attributable to their unique capability to display a distinctive, nondegenerate J = 0 transition, which enables precise mapping of optical quantum states into their hyperfine structure for reliable storage and retrieval on demand. However, placing Eu(III) into chiral polar structures, which are necessary for achieving narrow spectral linewidths and long optical lifetimes, is a daunting task. Here, we discover Eu2(SeO3)2(SO4)(H2O)2, a rare Eu(III) material that exhibits chiral polar symmetries encompassing both local and global structures. This unique structure is shaped by an appropriate combination of asymmetric ligands. The chirality fosters dipole-dipole interactions and J-mixing, as characterized by second-harmonic generation, photoluminescence, and magnetic susceptibility. The broken inversion symmetry is supported by the phase-matching behavior of second-harmonic generation. The J = 0 transition is observed at 578 nm with a narrow linewidth at 78 K and a microsecond-scale optical lifetime. The analysis of magnetic susceptibility data using Van Vleck theory results in an effective magnetic moment of 3.33 μB/Eu3+ and J-mixing. Heat capacity data reveal underlying phonon dynamics in the material. This study demonstrates a pathway toward realizing new stoichiometric Eu3+ compounds with potential for optically addressable quantum memory applications. △ Less

Submitted 9 February, 2026; originally announced February 2026.

Probing formation and epitaxy of ultrathin Titanium Silicide using low and medium energy ion scattering

Authors: Philipp M. Wolf, Eduardo Pitthan, Zhen Zhang, Tuan T. Tran, Radek Holeňák, Daniel Primetzhofer

Abstract: Titanium silicide is a key contact material in advanced three-dimensional semiconductor device architectures. Here, we examine the formation of ultrathin Ti-silicide on Si(100) using a combination of non-destructive in-situ and ex-situ ion scattering techniques capable of resolving composition and structure at the nanoscale. In-situ Time-of-Flight Low-Energy Ion Scattering (ToF-LEIS) indicates int… ▽ More Titanium silicide is a key contact material in advanced three-dimensional semiconductor device architectures. Here, we examine the formation of ultrathin Ti-silicide on Si(100) using a combination of non-destructive in-situ and ex-situ ion scattering techniques capable of resolving composition and structure at the nanoscale. In-situ Time-of-Flight Low-Energy Ion Scattering (ToF-LEIS) indicates intermixing after annealing at 350 °C, with further compositional changes after annealing at 500 °C, including the emergence of a Si terminating layer at the surface. Consecutive ex-situ Time-of-Flight Medium-Energy Ion Scattering (ToF-MEIS) reveals a Ti-rich polycrystalline surface layer and a Si-rich interface layer exhibiting strong ordering along the Si [100] axis. High-Resolution Transmission Electron Microscopy (HR-TEM) images confirm these findings, revealing a $\approx$1.5 nm thick epitaxial silicide layer at the interface. The presence of an epitaxial interface is particularly promising for minimizing contact resistivity in ultrathin contact layers, where interfacial order can dominate electronic performance. In addition, both ToF-MEIS and HR-TEM unveil significant variations in the thickness of the silicide layer, with a substantial interface roughness but no translation of this roughness to the surface. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 19 pages, 5 figures

Laser-Induced Heating in Diamonds: Influence of Substrate Thermal Conductivity and Interfacial Polymer Layers

Authors: Md Shakhawath Hossain, Jiatong Xu, Thi Ngoc Anh Mai, Nhat Minh Nguyen, Trung Vuong Doan, Chaohao Chen, Qian Peter Su, Yongliang Chen, Evgeny Ekimov, Toan Dinh, Xiaoxue Xu, Toan Trong Tran

Abstract: Diamonds hosting color centers possess intrinsically high thermal conductivity; therefore, laser-induced heating has often received little attention. However, when placed on substrates with low thermal conductivity, localized heating of diamonds under laser excitation can become significant, and the presence of an interfacial polymer layer between substrate and diamond further amplifies this effec… ▽ More Diamonds hosting color centers possess intrinsically high thermal conductivity; therefore, laser-induced heating has often received little attention. However, when placed on substrates with low thermal conductivity, localized heating of diamonds under laser excitation can become significant, and the presence of an interfacial polymer layer between substrate and diamond further amplifies this effect. Yet, the relationship between substrate thermal conductivity, polymer thickness, and laser heating remains to be established. Here, a systematic investigation is presented on laser-induced heating of silicon-vacancy diamond on substrates with varying thermal conductivity and interfacial polymer thickness. Results reveal that even at a low excitation power of 737~$μ$W/$μ$m$^2$, thin amorphous holey carbon -- the lowest-conductivity substrate ($\sim$0.2~W~m$^{-1}$~K$^{-1}$) studied -- exhibits substantial heating, while glass ($\sim$1.4~W~m$^{-1}$~K$^{-1}$) and polydimethylsiloxane (PDMS, $\sim$0.35~W~m$^{-1}$~K$^{-1}$) show noticeable heating only above 2.95~mW/$μ$m$^2$. For polymer interlayers, a thickness of just 2.2~$μ$m induces significant heating at 2.95~mW/$μ$m$^2$ and above, highlighting strong influence of both substrate and polymer thickness on local heating response. Experimental findings are further validated using COMSOL Multiphysics simulations with a steady-state 3D heat transfer model. These results provide practical guidance for substrate selection and sample preparation, enabling optimization of conditions for optical thermometry and quantum sensing applications. △ Less

Submitted 16 October, 2025; originally announced October 2025.

Kinetics of the photochromic effect in oxygen-containing rare-earth hydrides

Authors: Dmitrii Moldarev, Tuan T. Tran, Max Wolff, Daniel Primetzhofer

Abstract: The kinetics of the photochromic reaction of oxygen-containing rare-earth hydrides is commonly described by an exponential function assuming a single-step process. In this paper, we elaborate on the origin of the photochromic effect in oxygen-containing rare-earth metal hydrides, considering the kinetics of the reaction as a two-step process. We show that the fit to the experimental data is improv… ▽ More The kinetics of the photochromic reaction of oxygen-containing rare-earth hydrides is commonly described by an exponential function assuming a single-step process. In this paper, we elaborate on the origin of the photochromic effect in oxygen-containing rare-earth metal hydrides, considering the kinetics of the reaction as a two-step process. We show that the fit to the experimental data is improved drastically when two processes that cause the photodarkening are assumed: a fast reaction rate-limited - for example, electronic or local - process and a slow, e.g. diffusion-limited process. △ Less

Submitted 30 September, 2025; originally announced September 2025.

Comments: 5 pages, 4 figures

Machine Learning Based Optical Thermometry Using Photoluminescence and Raman Spectra of Diamonds Containing SiV Centers

Authors: Md Shakhawath Hossain, Dylan G. Stone, G. Landry, Xiaoxue Xu, Carlo Bradac, Toan Trong Tran

Abstract: Micro- and nanothermometry enable precise temperature monitoring and control at the micro- and nanoscale, and have become essential diagnostic tools in applications ranging from high-power microelectronics to biosensing and nanomedicine. Most existing techniques rely on secondary micro- and nanothermometers that require individual calibration of each sensor, ideally both off- and in-situ, before u… ▽ More Micro- and nanothermometry enable precise temperature monitoring and control at the micro- and nanoscale, and have become essential diagnostic tools in applications ranging from high-power microelectronics to biosensing and nanomedicine. Most existing techniques rely on secondary micro- and nanothermometers that require individual calibration of each sensor, ideally both off- and in-situ, before use. We present an alternative approach that overcomes this limitation by employing fluorescent diamonds containing silicon-vacancy centers, where the thermo-sensitive physical quantities are the centers' photoluminescence and the diamond host's Raman signals. The photoluminescence and Raman data are analyzed using two multi-feature regression algorithms that leverage a minimal number of calibration diamonds and temperature set points to predict the temperature of previously unseen diamonds. Using this approach, the models achieve accuracies as low as 0.7 K, resolutions down to 0.6 K Hz$^{-1/2}$, and sensitivity as high as 0.04 K$^{-1}$. These correspond to improvements of roughly 70 percent (over threefold) in accuracy, 50 percent (twofold) in resolution, and 567 percent (sevenfold) in sensitivity compared with traditional single-feature models. Our approach is particularly suited to applications where pre-deployment calibration of every thermosensor is impractical, and it is generalizable to any thermometry platform with two or more simultaneously measurable temperature-dependent observables. △ Less

Submitted 24 October, 2025; v1 submitted 26 September, 2025; originally announced September 2025.

NiSi$_2$ as seed layer for epitaxial growth of NiAl and Cr on Si(001)

Authors: Mohamed Ben Chroud, Thu-Huong Thi Tran, Johan Swerts, Kristiaan Temst, Robert Carpenter

Abstract: In general, metal layers cannot be grown epitaxially on Si due to the tendency of metals to react and form a silicide. Even in case the metal layer has a matching lattice symmetry and atomic distance, the Si/metal interface is disturbed by the silicide thus preventing epitaxial growth. One exception is NiAl which is known to grow epitaxially when deposited on Si(001). During the growth, NiAl react… ▽ More In general, metal layers cannot be grown epitaxially on Si due to the tendency of metals to react and form a silicide. Even in case the metal layer has a matching lattice symmetry and atomic distance, the Si/metal interface is disturbed by the silicide thus preventing epitaxial growth. One exception is NiAl which is known to grow epitaxially when deposited on Si(001). During the growth, NiAl reacts with Si to form NiSi$_2$. In this work, epitaxial NiAl is grown and significant silicidation is observed in accordance with previous reports. However, the role that this silicide plays as a template for the epitaxial growth of NiAl has not been clear to this date. We hypothesize that NiSi$_2$ acts as a necessary seed layer between the Si substrate and the NiAl layer. Additionally, NiSi$_2$ can be used as a seed layer for the epitaxial growth of other metals besides NiAl. This was tested by growing NiSi$_2$ seperately and replacing the NiAl layer with Cr. Growing Cr directly on Si(001) produced a polycrystalline layer. When NiSi$_2$ was used as a seed layer, the Cr layer was found to be a single crystal with Si(001)//Cr(001) and Si(100)//Cr(100). NiSi was also tried as seed layer for Cr and was found to produce a polycrystalline Cr layer. Using NiSi$_2$ as a seed layer could enable the growth of various epitaxial materials for industrial semiconductor applications. △ Less

Submitted 11 September, 2025; originally announced September 2025.

Treasure Map Toward Skyrmion Evolution in Ambient Conditions: A Perspective from Electronic Instabilities and the Density of Energy

Authors: Xudong Huai, Thao T. Tran

Abstract: Magnetic skyrmions with topologically protected properties are anticipated to shape the future of electronics. Understanding how skyrmions may evolve in ambient conditions presents a key challenge in the pursuit of technologically significant materials. In this perspective, we focus on electronic instabilities and the density of energy of established skyrmion hosts, where a pathway to a skyrmion p… ▽ More Magnetic skyrmions with topologically protected properties are anticipated to shape the future of electronics. Understanding how skyrmions may evolve in ambient conditions presents a key challenge in the pursuit of technologically significant materials. In this perspective, we focus on electronic instabilities and the density of energy of established skyrmion hosts, where a pathway to a skyrmion phase transition is readily available, to identify signposts for the emergence of skyrmions. We value the impressive research efforts in the field that have built the foundation for many more enticing breakthroughs to come. We share a framework that connects the electronic origins of skyrmion formation to the temperature and field requirements, allowing predictions of candidate materials that may host skyrmions at ambient conditions (the treasure). △ Less

Submitted 10 September, 2025; originally announced September 2025.

Field-Tailoring Quantum Materials via Magneto-Synthesis: Metastable Metallic and Magnetically Suppressed Phases in a Trimer Iridate

Authors: Tristan R. Cao, Hengdi Zhao, Xudong Huai, Arabella Quane, Thao T. Tran, Feng Ye, Gang Cao

Abstract: We demonstrate that applying modest magnetic fields during high-temperature crystal growth can profoundly alter the structure and ground state of a spin-orbit-coupled, antiferromagnetic trimer lattice. Using BaIrO3 as a model system, whose ground state is intricately dictated by the trimer lattice, we show that magneto-synthesis, a field-assisted synthesis approach, stabilizes a structurally compr… ▽ More We demonstrate that applying modest magnetic fields during high-temperature crystal growth can profoundly alter the structure and ground state of a spin-orbit-coupled, antiferromagnetic trimer lattice. Using BaIrO3 as a model system, whose ground state is intricately dictated by the trimer lattice, we show that magneto-synthesis, a field-assisted synthesis approach, stabilizes a structurally compressed, metastable metallic and magnetically suppressed phases inaccessible via conventional methods. These effects include a 0.85% reduction in unit cell, 4-order-of-magnitude decrease in resistivity, a 10-fold enhancement of the Sommerfeld coefficient, and the collapse of long-range magnetic order -- all intrinsic and bulk in origin. First-principles calculations confirm that the field-stabilized structure lies substantially above the ground state in energy, highlighting its metastable character. These large, coherent and correlated changes across multiple bulk properties, unlike those caused by dilute impurities, defects or off-stoichiometry, point to an intrinsic field-induced mechanism. The findings establish magneto-synthesis as a powerful new pathway for accessing non-equilibrium quantum phases in strongly correlated materials. △ Less

Submitted 5 November, 2025; v1 submitted 10 August, 2025; originally announced August 2025.

Comments: 4 figures

Journal ref: npj Quantum Materials (2026)

Antibonding and Electronic Instabilities in GdRu2X2 (X = Si, Ge, Sn): A New Pathway Toward Developing Centrosymmetric Skyrmion Materials

Authors: Dasuni N. Rathnaweera, Xudong Huai, K. Ramesh Kumar, Sumanta Tewari, Michał J. Winiarski, Richard Dronskowski, Thao T. Tran

Abstract: Chemical bonding is key to unlocking the potential of magnetic materials for future information technology. Magnetic skyrmions are topologically protected nano-sized spin textures that can enable high-density low-power spin-based electronics. Despite increasing interest in the discovery of new skyrmion hosts and their characterization, the electronic origins of the skyrmion formation remain unknow… ▽ More Chemical bonding is key to unlocking the potential of magnetic materials for future information technology. Magnetic skyrmions are topologically protected nano-sized spin textures that can enable high-density low-power spin-based electronics. Despite increasing interest in the discovery of new skyrmion hosts and their characterization, the electronic origins of the skyrmion formation remain unknown. Here, we study GdRu2X2 (X = Si, Ge, Sn) as a model system to study the connection among chemical bonding, electronic instability, and the critical temperature and magnetic field at which skyrmions evolve. The nature of the electronic structure of GdRu2X2 is characterized by chemical bonding, Fermi surface analysis, and density of energy function. As X-p orbitals become more extended from Si-3p to Ge-4p and Sn-5p, improved interactions between the Gd spins and the [Ru2X2] conduction layer and increased destabilizing energy contributions are obtained. GdRu2Si2 possesses a Fermi surface nesting (FSN) vector [Q = (q, 0, 0)], whereas GdRu2Ge2 displays two inequivalent FSN vectors [Q = (q, 0, 0); QA = (q, q, 0)] and GdRu2Sn2 features multiple Q vectors. In addition, competing ferromagnetic and antiferromagnetic exchange interactions in the Gd plane become more pronounced as a function of X. These results reveal some correlation among the electronic instability, the competing interaction strength, and the temperature and magnetic field conditions at which the skyrmions emerge. This work demonstrates how chemical bonding and electronic structure enable a new framework for understanding and developing skyrmions under desired conditions that would otherwise be impossible. △ Less

Submitted 24 July, 2025; originally announced July 2025.

Inductive-Effect-Driven Tunability of Magnetism and Lumines-cence in Triangular Layers ANd(SO4)2 (A = Rb, Cs)

Authors: Xudong Huai, Ebube E. Oyeka, Uchenna Chinaegbomkpa, Michal J. Winiarski, Hugo Sanabria, Thao T. Tran

Abstract: Tuning the energy landscape of manybody electronic states in extended solids through the inductive effect-a concept widely used in organic chemistry-offers a new, effective strategy for materials development. Here, we demonstrate this approach using the ANd(SO4)2 (A = Rb, Cs) model system, which possesses different A-site electronegativity and displays a distorted triangular lattice of Nd3+ (4I9/2… ▽ More Tuning the energy landscape of manybody electronic states in extended solids through the inductive effect-a concept widely used in organic chemistry-offers a new, effective strategy for materials development. Here, we demonstrate this approach using the ANd(SO4)2 (A = Rb, Cs) model system, which possesses different A-site electronegativity and displays a distorted triangular lattice of Nd3+ (4I9/2 ground term). Magnetization data indicate appreciable antiferromagnetic interactions without long-range ordering down to 1.8 K while highlighting the tunable population of the electronic states. Temperature-dependent and time-resolved photoluminescence measurements reveal that emissions and nonradiative processes can be modified by the inductive effect at the atomic level. Heat capacity data confirm no magnetic ordering and add insight into the role of phonons in emission lifetime. Density functional theory calculations prove enhanced covalency in the Cs compound compared to the Rb counterpart while acknowledging the adjustable magnetic intralayer and inter-layer exchange pathways. These results demonstrate a viable framework for utilizing the inductive effect as an important knob for simultaneously dialing in magnetic, optical, and electronic properties in quantum materials. △ Less

Submitted 2 June, 2025; originally announced June 2025.

Mobility of single vacancies and adatoms in graphene at room temperature

Authors: Tuan T. Tran, Per O. Å. Persson, Ngan Pham, Radek Holenak, Daniel Primetzhofer

Abstract: We investigate the mobility of structural defects, adatoms, and defect-adatom combinations in self-supporting graphene subjected to keV ion irradiation. In the first scenario, homogeneous irradiation using 20 keV Ar$^+$ ions at a dose of $3 \times 10^{14}$ ions/cm$^2$ induces tensile strain of up to 0.8\%. This strain diminishes with increasing defect density at the dose of $5 \times 10^{14}$ ions… ▽ More We investigate the mobility of structural defects, adatoms, and defect-adatom combinations in self-supporting graphene subjected to keV ion irradiation. In the first scenario, homogeneous irradiation using 20 keV Ar$^+$ ions at a dose of $3 \times 10^{14}$ ions/cm$^2$ induces tensile strain of up to 0.8\%. This strain diminishes with increasing defect density at the dose of $5 \times 10^{14}$ ions/cm$^2$, indicating a strain-relaxation mechanism. Contrary to the expected localized behavior, vacancies exhibit long-range interactions, contributing to global strain effects across the lattice. In the second scenario, by employing a nanopore mask, we spatially confined defect generation to periodically aligned circular regions surrounded by non-irradiated material, enabling direct observation of vacancy and adatom dynamics. Selected area electron diffraction (SAED) reveals significant structural damage in areas adjacent to the irradiated regions, suggesting that single vacancies migrate over distances on the order of 100 nm from irradiated to non-irradiated zones even at room temperature. The build-up of lattice strain observed in this study may play a key role in lowering the migration barrier of single vacancies, thereby facilitating their diffusion into pristine lattice regions. Furthermore, the findings highlight the role of pre-existing surface contaminants in preserving lattice integrity through a self-healing mechanism, where adatom-induced lattice reconstruction mitigates defect-induced structural degradation. △ Less

Submitted 26 March, 2025; originally announced March 2025.

Jeff = 1/2 Diamond Magnet CaCo2TeO6: A Pathway toward New Spin Physics and Quantum Functions

Authors: Xudong Huai, Luke Pritchard Cairns, Bridget Delles, Michal J. Winiarski, Maurice Sorolla II, Xinshu Zhang, Youzhe Chen, Stuart Calder, Tatenda Kanyowa, Anshul Kogar, Huibo Cao, Danielle Yahne, Robert Birgeneau, James Analytis, Thao T. Tran

Abstract: Diamond lattice magnets, formed by a framework of corner-sharing tetrahedra of magnetic cations, offer unique opportunities to realize novel states of matter for potential utility in information technology. However, research has mostly focused on AB2X4 spinels with Td magnetic ions. This hinders the atomically enabled tunability of competing interactions at different energy scales and the ability… ▽ More Diamond lattice magnets, formed by a framework of corner-sharing tetrahedra of magnetic cations, offer unique opportunities to realize novel states of matter for potential utility in information technology. However, research has mostly focused on AB2X4 spinels with Td magnetic ions. This hinders the atomically enabled tunability of competing interactions at different energy scales and the ability to harness many-body electronic states in quantum materials, making the discovery of quantum fluctuations and spin dynamics less accessible. We discover a new material CaCo2TeO6 featuring a diamond lattice of two distinct Oh-Co2+ sites. This material displays strong quantum fluctuations, increased competing magnetic exchange interactions, and field-induced tunability of magnetic structures. The results demonstrate how simple, fundamental refinements in ligand fields can profoundly influence the phase space of quantum matter. △ Less

Submitted 26 July, 2025; v1 submitted 22 March, 2025; originally announced March 2025.

Atomically Modulating Competing Exchange Interactions in Centrosymmetric Skyrmion Hosts GdRu2X2 (X = Si, Ge)

Authors: Dasuni N. Rathnaweera, Xudong Huai, K. Ramesh Kumar, Michal J. Winiarski, Tomasz Klimczuk, Allana G. Iwanicki, Satya Kushwaha, Martin Mourigal, Tyrel M. McQueen, Thao T. Tran

Abstract: Magnetic skyrmions are topologically protected spin states enabling high-density, low-power spin electronics. Despite growing efforts to find new skyrmion host systems, the microscopic mechanisms leading to skyrmion phase transitions at specific temperatures and magnetic fields remain elusive. Here, we systematically study the isostructural centrosymmetric magnets- GdRu2X2 (X = Si and Ge), and the… ▽ More Magnetic skyrmions are topologically protected spin states enabling high-density, low-power spin electronics. Despite growing efforts to find new skyrmion host systems, the microscopic mechanisms leading to skyrmion phase transitions at specific temperatures and magnetic fields remain elusive. Here, we systematically study the isostructural centrosymmetric magnets- GdRu2X2 (X = Si and Ge), and the role of X-p orbitals in modifying magnetic exchange interactions. GdRu2Ge2 single crystals, synthesized by arc melting, exhibit two high-entropy pockets associated with skyrmion phases at 0.9 T < H < 1.2 T and 1.3 T < H < 1.7 T, 2 K < T < 30 K-more accessible condition at lower fields and higher temperatures than that in the Si counterpart. Entropy estimations from heat capacity measurements align with magnetization data, and transport studies confirm a topological Hall effect, highlighting the system's nontrivial spin textures and Berry curvature. Compared to GdRu2Si2, electronic structure and exchange interaction evaluations reveal the more extended Ge-4p orbitals enhance competing exchange interactions in GdRu2Ge2, thereby manifesting the rich skyrmion behavior. This work demonstrates how modifying exchange interactions at the atomic level enables the tunability of topologically nontrivial electronic states while advancing our understanding of skyrmion formation mechanisms for future spintronics. △ Less

Submitted 12 August, 2025; v1 submitted 28 February, 2025; originally announced February 2025.

Quantum Emitters in Hexagonal Boron Nitride: Principles, Engineering and Applications

Authors: Thi Ngoc Anh Mai, Md Shakhawath Hossain, Nhat Minh Nguyen, Yongliang Chen, Chaohao Chen, Xiaoxue Xu, Quang Thang Trinh, Toan Dinh, Toan Trong Tran

Abstract: Solid-state quantum emitters, molecular-sized complexes releasing a single photon at a time, have garnered much attention owing to their use as a key building block in various quantum technologies. Among these, quantum emitters in hexagonal boron nitride (hBN) have emerged as front runners with superior attributes compared to other competing platforms. These attributes are attainable thanks to the… ▽ More Solid-state quantum emitters, molecular-sized complexes releasing a single photon at a time, have garnered much attention owing to their use as a key building block in various quantum technologies. Among these, quantum emitters in hexagonal boron nitride (hBN) have emerged as front runners with superior attributes compared to other competing platforms. These attributes are attainable thanks to the robust, two-dimensional lattice of the material formed by the extremely strong B-N bonds. This review discusses the fundamental properties of quantum emitters in hBN and highlights recent progress in the field. The focus is on the fabrication and engineering of these quantum emitters facilitated by state-of-the-art equipment. Strategies to integrate the quantum emitters with dielectric and plasmonic cavities to enhance their optical properties are summarized. The latest developments in new classes of spin-active defects, their predicted structural configurations, and the proposed suitable quantum applications are examined. Despite the current challenges, quantum emitters in hBN have steadily become a promising platform for applications in quantum information science. △ Less

Submitted 22 January, 2025; originally announced January 2025.

Energy and momentum relaxation through the Curie temperature in an itinerant ferromagnet

Authors: Rishi Bhandia, Tim Priessnitz, Jiahao Liang, Ksenia S. Rabinovich, Ralph Romero III, Kota Katsumi, Thi Thu Huong Tran, Georg Christiani, Gennady Logvenov, Bernhard Keimer, N. P. Armitage

Abstract: In this work, we combine conventional linear response time-domain THz spectroscopy with non-linear THz-pump THz-probe techniques to study metallic strained thin films of $\mathrm{Ca}_2\mathrm{RuO}_4$, which undergo a transition into a ferromagnetic state at 10 K. Such measurements allowing us to independently measure momentum and energy relaxation rates. We find that while the momentum relaxation… ▽ More In this work, we combine conventional linear response time-domain THz spectroscopy with non-linear THz-pump THz-probe techniques to study metallic strained thin films of $\mathrm{Ca}_2\mathrm{RuO}_4$, which undergo a transition into a ferromagnetic state at 10 K. Such measurements allowing us to independently measure momentum and energy relaxation rates. We find that while the momentum relaxation rate decreases significantly at the ferromagnetic transition, the energy relaxation rate remains unaffected by the emergence of magnetic order. This shows that the dominant changes to scattering across the transition correspond to scatterings that relax momentum without relaxing energy. It is consistent with a scenario where energy is not carried off by coupling to collective magnetic degrees of freedom. Instead, the principal channel for energy relaxation remains the conventional one e.g. coupling to acoustic phonons. This observation validates the approximation used in the conventional understanding of resistive anomalies of ferromagnets across the Curie temperature, which due to critical slowing down, spin fluctuations can be treated as effectively static and scattering off of them elastic. This scenario can likely be extended to resistive anomalies at other phase transitions to charge- and spin-density wave states in kagome metals or pnictide system △ Less

Submitted 11 December, 2024; originally announced December 2024.

Comments: 6 pages, 4 figures

La$_2$O$_3$Mn$_2$Se$_2$: a correlated insulating layered d-wave altermagnet

Authors: Chao-Chun Wei, Xiaoyin Li, Sabrina Hatt, Xudong Huai, Jue Liu, Birender Singh, Kyung-Mo Kim, Rafael M. Fernandes, Paul Cardon, Liuyan Zhao, Thao T. Tran, Benjamin M. Frandsen, Kenneth S. Burch, Feng Liu, Huiwen Ji

Abstract: Altermagnets represent a new class of magnetic phases without net magnetization that are invariant under a combination of rotation and time reversal. Unlike conventional collinear antiferromagnets (AFM), altermagnets could lead to new correlated states and important material properties deriving from their non-relativistic spin-split band structure. Indeed, they are the magnetic analogue of unconve… ▽ More Altermagnets represent a new class of magnetic phases without net magnetization that are invariant under a combination of rotation and time reversal. Unlike conventional collinear antiferromagnets (AFM), altermagnets could lead to new correlated states and important material properties deriving from their non-relativistic spin-split band structure. Indeed, they are the magnetic analogue of unconventional superconductors and can yield spin polarized electrical currents in the absence of external magnetic fields, making them promising candidates for next-generation spintronics. Here, we report altermagnetism in the correlated insulator, magnetically-ordered tetragonal oxychalcogenide, La$_2$O$_3$Mn$_2$Se$_2$. Symmetry analysis reveals a $\mathit{d}_{x^2 - y^2}$-wave type spin momentum locking, which is supported by density functional theory (DFT) calculations. Magnetic measurements confirm the AFM transition below $\sim$166 K while neutron pair distribution function analysis reveals a 2D short-range magnetic order that persists above the Néel temperature. Single crystals are grown and characterized using X-ray diffraction, optical and electron microscopy, and microRaman spectroscopy to confirm the crystal structure, stoichiometry, and uniformity. △ Less

Submitted 18 October, 2024; originally announced October 2024.

Magnetic metamaterials by ion-implantation

Authors: Christina Vantaraki, Petter Ström, Tuan T. Tran, Matías P. Grassi, Giovanni Fevola, Michael Foerster, Jerzy T. Sadowski, Daniel Primetzhofer, Vassilios Kapaklis

Abstract: We present a method for the additive fabrication of planar magnetic nanoarrays with minimal surface roughness. Synthesis is accomplished by combining electron-beam lithography, used to generate nanometric patterned masks, with ion implantation in thin films. By implanting $^{56}$Fe$^{+}$ ions, we are able to introduce magnetic functionality in a controlled manner into continuous Pd thin films, ach… ▽ More We present a method for the additive fabrication of planar magnetic nanoarrays with minimal surface roughness. Synthesis is accomplished by combining electron-beam lithography, used to generate nanometric patterned masks, with ion implantation in thin films. By implanting $^{56}$Fe$^{+}$ ions, we are able to introduce magnetic functionality in a controlled manner into continuous Pd thin films, achieving 3D spatial resolution down to a few tens of nanometers. Our results demonstrate the application of this technique in fabricating square artificial spin ice lattices, which exhibit well-defined magnetization textures and interactions among the patterned magnetic elements. △ Less

Submitted 23 October, 2024; v1 submitted 16 September, 2024; originally announced September 2024.

Comments: 5 pages, 4 figures, Supplemental Material

Journal ref: Appl. Phys. Lett. 125, 202403 (2024)

Noncentrosymmetric Triangular Magnet CaMnTeO$_6$: Strong Quantum Fluctuations and Role of s0 vs. s2 Electronic States in Competing Exchange Interactions

Authors: Xudong Huai, Emmanuel Acheampong, Erich Delles, Michał J. Winiarski, Maurice Sorolla II, Lila Nassar, Mingli Liang, Caleb Ramette, Huiwen Ji, Allen Scheie, Stuart Calder, Martin Mourigal, Thao T. Tran

Abstract: Noncentrosymmetric triangular magnets offer a unique platform for realizing strong quantum fluctuations. However, designing these quantum materials remains an open challenge attributable to a knowledge gap in the tunability of competing exchange interactions at the atomic level. Here, we create a new noncentrosymmetric triangular S = 3/2 magnet CaMnTeO$_6$ based on careful chemical and physical co… ▽ More Noncentrosymmetric triangular magnets offer a unique platform for realizing strong quantum fluctuations. However, designing these quantum materials remains an open challenge attributable to a knowledge gap in the tunability of competing exchange interactions at the atomic level. Here, we create a new noncentrosymmetric triangular S = 3/2 magnet CaMnTeO$_6$ based on careful chemical and physical considerations. The model material displays competing magnetic interactions and features nonlinear optical responses with the capability of generating coherent photons. The incommensurate magnetic ground state of CaMnTeO$_6$ with an unusually large spin rotation angle of 127 deg.(1) indicates that the anisotropic interlayer exchange is strong and competing with the isotropic interlayer Heisenberg interaction. The moment of 1.39(1) $μ$B, extracted from low-temperature heat capacity and neutron diffraction measurements, is only 46% of the expected value of the static moment 3 $μ$B. This reduction indicates the presence of strong quantum fluctuations in the half-integer spin S = 3/2 CaMnTeO$_6$ magnet, which is rare. By comparing the spin-polarized band structure, chemical bonding, and physical properties of AMnTeO$_6$ (A = Ca, Sr, Pb), we demonstrate how quantum-chemical interpretation can illuminate insights into the fundamentals of magnetic exchange interactions, providing a powerful tool for modulating spin dynamics with atomically precise control. △ Less

Submitted 12 March, 2024; originally announced March 2024.

Fiber-based Ratiometric Optical Thermometry with Silicon-Vacancy in Microdiamonds

Authors: Md Shakhawath Hossain, Miguel Bacaoco, Thi Ngoc Anh Mai, Guillaume Ponchon, Chaohao Chen, Lei Ding, Yongliang Chen, Evgeny Ekimov, Helen Xu, Alexander S. Solntsev, Toan Trong Tran

Abstract: Fiber optic all-optical thermometry is a promising technology to track temperature at a micro-scale while designing efficient and reliable microelectronic devices and components. In this work, we demonstrate a novel real-time ratiometric fiber optic thermometry technique based on silicon-vacancy (SiV) diamond that shows the highest temperature resolution (22.91 KHz^(-1/2) Wcm^(-2)) and spatial res… ▽ More Fiber optic all-optical thermometry is a promising technology to track temperature at a micro-scale while designing efficient and reliable microelectronic devices and components. In this work, we demonstrate a novel real-time ratiometric fiber optic thermometry technique based on silicon-vacancy (SiV) diamond that shows the highest temperature resolution (22.91 KHz^(-1/2) Wcm^(-2)) and spatial resolution (~7.5 um) among all-optical fiber-based thermosensors reported to date. Instead of analyzing the spectral features of temperature-dependent SiV signal, coming from SiV micro-diamond fixed on the fiber tip, an alternative parallel detection method based on filtering optics and photon counters is proposed to read out the sample temperature in real-time. The signal collection efficiency of the fiber is also investigated numerically with semi-analytic ray-optical analysis and then compared with our experimental study. We finally demonstrate the performance of the thermosensor by monitoring the temperature at distinct locations in a lab-built graphite-based microheater device. Our work introduces a reconfigurable method for temperature monitoring in microelectronic, microfluidic devices, or biological environments and unlocks a new direction for fiber-based all-optical thermometry research. △ Less

Submitted 29 November, 2023; originally announced November 2023.

Cryogenic Thermal Shock Effects on Optical Properties of Quantum Emitters in Hexagonal Boron Nitride

Authors: Thi Ngoc Anh Mai, Sajid Ali, Md Shakhawath Hossain, Chaohao Chen, Lei Ding, Yongliang Chen, Alexander S. Solntsev, Hongwei Mou, Xiaoxue Xu, Nikhil Medhekar, Toan Trong Tran

Abstract: Solid-state quantum emitters are vital building blocks for quantum information science and quantum technology. Among various types of solid-state emitters discovered to date, color centers in hexagonal boron nitride have garnered tremendous traction in recent years thanks to their environmental robustness, high brightness and room-temperature operation. Most recently, these quantum emitters have b… ▽ More Solid-state quantum emitters are vital building blocks for quantum information science and quantum technology. Among various types of solid-state emitters discovered to date, color centers in hexagonal boron nitride have garnered tremendous traction in recent years thanks to their environmental robustness, high brightness and room-temperature operation. Most recently, these quantum emitters have been employed for satellite-based quantum key distribution. One of the most important requirements to qualify these emitters for space-based applications is their optical stability against cryogenic thermal shock. Such understanding has, however, remained elusive to date. Here, we report on the effects caused by such thermal shock which induces random, irreversible changes in the spectral characteristics of the quantum emitters. By employing a combination of structural characterizations and density functional calculations, we attribute the observed changes to lattice strains caused by the cryogenic temperature shock. Our study shed light on the stability of the quantum emitters under extreme conditions, similar to those countered in outer space. △ Less

Submitted 28 November, 2023; originally announced November 2023.

Ultralow-power cryogenic thermometry based on optical-transition broadening of a two-level system in diamond

Authors: Yongliang Chen, Simon White, Evgeny A. Ekimov, Carlo Bradac, Milos Toth, Igor Aharonovich, Toan Trong Tran

Abstract: Cryogenic temperatures are the prerequisite for many advanced scientific applications and technologies. The accurate determination of temperature in this range and at the submicrometer scale is, however, nontrivial. This is due to the fact that temperature reading in cryogenic conditions can be inaccurate due to optically induced heating. Here, we present an ultralow power, optical thermometry tec… ▽ More Cryogenic temperatures are the prerequisite for many advanced scientific applications and technologies. The accurate determination of temperature in this range and at the submicrometer scale is, however, nontrivial. This is due to the fact that temperature reading in cryogenic conditions can be inaccurate due to optically induced heating. Here, we present an ultralow power, optical thermometry technique that operates at cryogenic temperatures. The technique exploits the temperature dependent linewidth broadening measured by resonant photoluminescence of a two level system, a germanium vacancy color center in a nanodiamond host. The proposed technique achieves a relative sensitivity of 20% 1/K, at 5 K. This is higher than any other all optical nanothermometry method. Additionally, it achieves such sensitivities while employing excitation powers of just a few tens of nanowatts, several orders of magnitude lower than other traditional optical thermometry protocols. To showcase the performance of the method, we demonstrate its ability to accurately read out local differences in temperatures at various target locations of a custom-made microcircuit. Our work is a definite step towards the advancement of nanoscale optical thermometry at cryogenic temperatures. △ Less

Submitted 3 November, 2022; originally announced November 2022.

Ln2(SeO3)2(SO4)(H2O)2 (Ln = Sm, Dy, Yb): A Mixed-Ligand Pathway to New Lathanide (III) Multifunctional Materials Featuring Nonlinear Optical and Magnetic Anisotropy Properties

Authors: Ebube Oyeka, Michal J. Winiarski, Hanka Świątek, Wyatt Balliew, Colin D. McMillen, Mingli Liang, Maurice Sorolla II, Thao T. Tran

Abstract: Bottom-up assembly of optically nonlinear and magnetically anisotropic lanthanide materials involving precisely placed spin carriers and optimized metal-ligand coordination offers a potential route to developing electronic architectures for coherent radiation generation and spin-based technologies, but the chemical design historically has been extremely hard to achieve. To address this, we develop… ▽ More Bottom-up assembly of optically nonlinear and magnetically anisotropic lanthanide materials involving precisely placed spin carriers and optimized metal-ligand coordination offers a potential route to developing electronic architectures for coherent radiation generation and spin-based technologies, but the chemical design historically has been extremely hard to achieve. To address this, we developed a worthwhile avenue for creating new noncentrosymmetric chiral Ln3+ materials Ln2(SeO3)2(SO4)(H2O)2 (Ln = Sm, Dy, Yb) by mixed-ligand design. The materials exhibit phase-matching nonlinear optical responses, elucidating the feasibility of the heteroanionic strategy. Ln2(SeO3)2(SO4)(H2O)2 displays paramagnetic property with strong magnetic anisotropy facilitated by large spin-orbit coupling. This study demonstrates a new chemical pathway for creating previously unknown polar chiral magnets with multiple functionalities. △ Less

Submitted 5 October, 2022; originally announced October 2022.

Real-time ratiometric optical nanoscale thermometry

Authors: Yongliang Chen, Chi Li, Tieshan Yang, Evgeny A. Ekimov, Carlo Bradac, Milos Toth, Igor Aharonovich, Toan Trong Tran

Abstract: All optical nanothermometry has become a powerful, noninvasive tool for measuring nanoscale temperatures in applications ranging from medicine to nanooptics and solid-state nanodevices. The key features of any candidate nanothermometer are sensitivity and resolution. Here, we demonstrate a real time, diamond based nanothermometry technique with sensitivity and resolution much larger than those of… ▽ More All optical nanothermometry has become a powerful, noninvasive tool for measuring nanoscale temperatures in applications ranging from medicine to nanooptics and solid-state nanodevices. The key features of any candidate nanothermometer are sensitivity and resolution. Here, we demonstrate a real time, diamond based nanothermometry technique with sensitivity and resolution much larger than those of any existing all optical method. The distinct performance of our approach stems from two factors. First, temperature sensors nanodiamonds cohosting two Group IV colour centers engineered to emit spectrally separated Stokes and AntiStokes fluorescence signals under excitation by a single laser source. Second, a parallel detection scheme based on filtering optics and high sensitivity photon counters for fast readout. We demonstrate the performance of our method by monitoring temporal changes in the local temperature of a microcircuit and a MoTe2 field effect transistor. Our work lays the foundation for time resolved temperature monitoring and mapping of micro, nanoscale devices such as microfluidic channels, nanophotonic circuits, and nanoelectronic devices, as well as complex biological environments such as tissues and cells. △ Less

Submitted 3 December, 2021; originally announced December 2021.

Spin and Orbital Effects on Asymmetric Exchange Interaction in Polar Magnets: M(IO3)2 (M = Cu, Mn)

Authors: Ebube E. Oyeka, Michal J. Winiarski, Maurice Sorolla II, Keith M. Taddei, Allen Scheie, Thao T. Tran

Abstract: We study how spin and orbital effects influence the capability of promoting Dzyaloshinskii-Moriya (DM) interaction by studying the two magnetic polar materials, Cu(IO3)2 (S = 1/2 with orbital contribution) and Mn(IO3)2 (S = 5/2 with quenched orbital magnetism) and connecting their electronic and magnetic properties with their structures. The chemically controlled low-temperature synthesis of these… ▽ More We study how spin and orbital effects influence the capability of promoting Dzyaloshinskii-Moriya (DM) interaction by studying the two magnetic polar materials, Cu(IO3)2 (S = 1/2 with orbital contribution) and Mn(IO3)2 (S = 5/2 with quenched orbital magnetism) and connecting their electronic and magnetic properties with their structures. The chemically controlled low-temperature synthesis of these complexes resulted in pure polycrystalline samples, providing a viable pathway to prepare bulk forms of transition-metal io-dates. Rietveld refinements of the powder synchrotron X-ray diffraction data reveal that these materials exhibit different crystal structures but crystallize in the same polar and chiral P21 space group, giving rise to an electric polarization along the b-axis direction. The presence and absence of an evident phase transition to a possible topologically distinct state observed in Cu(IO3)2 and Mn(IO3)2, respectively, implies the important role of spin-orbit coupling. Neutron diffraction experiments reveal helpful insights into the magnetic ground state of these materials. While the long-wavelength incommensurability of Cu(IO3)2 is in harmony with orbital effects and anisotropic magnetic exchange, the commensurate stripe AFM ground state of Mn(IO3)2 is attributed to quenched orbital angular momentum and isotropic magnetic coupling. The work demonstrates connections between combined spin and orbital effects, magnetic coupling dimensionality and DM exchange, providing a worthwhile approach for tuning asymmetric interaction which promotes evolution of topologically distinct spin phases. △ Less

Submitted 25 September, 2021; v1 submitted 9 August, 2021; originally announced August 2021.

Potential Skyrmion Host Fe(IO3)3: Connecting Stereo-active Lone-Pair Electron Effects to the Dzyaloshinskii-Moriya Interaction

Authors: Ebube E. Oyeka, Michal J. Winiarski, Artur Blachowski, Keith M. Taddei, Allen Scheie, Thao T. Tran

Abstract: Magnetic skyrmions, which are topologically distinct magnetic spin textures, are gaining increased attention for their unique physical properties and potential applications in spintronic devices. Here we present a design strategy for skyrmion host candidates based on combinations of magnetic spin, asymmetric building units having stereo-active lone-pair electrons, and polar lattice symmetry. To de… ▽ More Magnetic skyrmions, which are topologically distinct magnetic spin textures, are gaining increased attention for their unique physical properties and potential applications in spintronic devices. Here we present a design strategy for skyrmion host candidates based on combinations of magnetic spin, asymmetric building units having stereo-active lone-pair electrons, and polar lattice symmetry. To demonstrate the viability of the proposed rational design principles, we successfully synthesized Fe(IO3)3 polycrystalline sample and single crystals by using a new simplified low-temperature pathway, which is experimentally feasible for extending materials growth of transition metal iodates. Single crystal X-ray and powder synchrotron X-ray diffraction measurements demonstrated that Fe(IO3)3 crystallizes in the polar chiral hexagonal lattice with space group P63. The combined structural features of the macroscopic electric polarization along the c axis stemming from the coalignment of the stereo-active lone-pairs of the IO3 trigonal pyramid and the magnetic Fe3+cation residing on the three-fold rotation axis were selected to promote asymmetric exchange coupling. We find evidence of a predicted skyrmion phase at 14 K to 16 K and 2.5 T to 3.2 T driven by Dzyaloshinskii Moriya (DM) interaction, a conclusion supported by the appreciable DM exchange and the zero-field spiral antiferromagnetic ground state of Fe(IO3)3 deduced from neutron diffraction experiments. The associated magnetic modulation wavelength of the putative skyrmions is expected to be short, approximately 18 nm, comparable to the period of the DM-driven incommensurate order. This work links stereo-active lone-pair electron effects to enhanced DM interaction, demonstrating a new approach for chemical guidelines in the search for skyrmionic states of matter. △ Less

Submitted 4 August, 2021; originally announced August 2021.

Journal ref: Chemistry of Materials (2021)

Topological Green function of interacting systems

Authors: Minh-Tien Tran, Duong-Bo Nguyen, Hong-Son Nguyen, Thanh-Mai Thi Tran

Abstract: We construct a Green function, which can identify the topological nature of interacting systems. It is equivalent to the single-particle Green function of effective non-interacting particles, the Bloch Hamiltonian of which is given by the inverse of the full Green function of the original interacting particles at zero frequency. The topological nature of the interacting insulators is originated fr… ▽ More We construct a Green function, which can identify the topological nature of interacting systems. It is equivalent to the single-particle Green function of effective non-interacting particles, the Bloch Hamiltonian of which is given by the inverse of the full Green function of the original interacting particles at zero frequency. The topological nature of the interacting insulators is originated from the coincidence of the poles and the zeros of the diagonal elements of the constructed Green function. The cross of the zeros in the momentum space closely relates to the topological nature of insulators. As a demonstration, using the zero's cross, we identify the topological phases of magnetic insulators, where both the ionic potential and the spin exchange between conduction electrons and magnetic moments are present together with the spin-orbital coupling. The topological phase identification is consistent with the topological invariant of the magnetic insulators. We also found an antiferromagnetic state with topologically breaking of the spin symmetry, where electrons with one spin orientation are in topological insulating state, while electrons with the opposite spin orientation are in topologically trivial one. △ Less

Submitted 26 April, 2021; v1 submitted 6 April, 2021; originally announced April 2021.

Comments: updated reference and corrected figures

Twisting of 2D kagomé sheets in layered intermetallics

Authors: Mekhola Sinha, Hector K. Vivanco, Cheng Wan, Maxime A. Siegler, Veronica J. Stewart, Elizabeth A. Pogue, Lucas A. Pressley, Tanya Berry, Ziqian Wang, Isaac Johnson, Mingwei Chen, Thao T. Tran, W. Adam Phelan, Tyrel M. McQueen

Abstract: Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal and superconducting properties. Here we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ~ 4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measure-ments, enabled by the preparati… ▽ More Chemical bonding in 2D layered materials and van der Waals solids is central to understanding and harnessing their unique electronic, magnetic, optical, thermal and superconducting properties. Here we report the discovery of spontaneous, bidirectional, bilayer twisting (twist angle ~ 4.5°) in the metallic kagomé MgCo6Ge6 at T = 100(2) K via X-ray diffraction measure-ments, enabled by the preparation of single crystals by the Laser Bridgman method. Despite the appearance of static twisting on cooling from T ~ 300 K to 100 K, no evidence for a phase transition was found in physical properties measurements. Combined with the presence of an Einstein phonon mode contribution in the specific heat, this implies that the twisting exists at all temperatures but is thermally fluctuating at room temperature. Crystal Orbital Hamilton Population analysis demonstrates that the cooperative twisting between layers stabilizes the Co-kagomé network when coupled to strongly bonded and rigid (Ge2) dimers that connect adjacent layers. Further modelling of the displacive disorder in the crystal structure shows the presence of second, Mg-deficient, stacking sequence. This alternative stacking sequence also exhibits inter-layer twisting, but with a different pattern, consistent with the change in electron count due to removal of Mg. Magnetization, resistivity, and low-temperature specific heat measurements are all consistent with a Pauli paramagnetic, strongly correlated metal. Our results provide crucial insight into how chemical concepts lead to interesting electronic structures and behaviors in layered materials. △ Less

Submitted 6 August, 2021; v1 submitted 6 February, 2021; originally announced February 2021.

Bottom-Up Synthesis of Hexagonal Boron Nitride Nanoparticles with Intensity-Stabilized Quantum Emitters

Authors: Yongliang Chen, Xiaoxue Xu, Chi Li, Avi Bendavid, Mika T. Westerhausen, Carlo Bradac, Milos Toth, Igor Aharonovich, Toan Trong Tran

Abstract: Fluorescent nanoparticles are widely utilized in a large range of nanoscale imaging and sensing applications. While ultra-small nanoparticles (size <10 nm) are highly desirable, at this size range their photostability can be compromised due to effects such as intensity fluctuation and spectral diffusion caused by interaction with surface states. In this letter, we demonstrate a facile, bottom-up t… ▽ More Fluorescent nanoparticles are widely utilized in a large range of nanoscale imaging and sensing applications. While ultra-small nanoparticles (size <10 nm) are highly desirable, at this size range their photostability can be compromised due to effects such as intensity fluctuation and spectral diffusion caused by interaction with surface states. In this letter, we demonstrate a facile, bottom-up technique for the fabrication of sub-10-nm hBN nanoparticles hosting photostable bright emitters via a catalyst-free hydrothermal reaction between boric acid and melamine. We also implement a simple stabilization protocol that significantly reduces intensity fluctuation by ~85% and narrows the emission linewidth by ~14% by employing a common sol-gel silica coating process. Our study advances a promising strategy for the scalable, bottom-up synthesis of high-quality quantum emitters in hBN nanoparticles. △ Less

Submitted 27 January, 2021; originally announced January 2021.

Photochromic response of encapsulated oxygen-containing yttrium hydride thin films

Authors: Marcos V. Moro, Sigurbjörn M. Aðalsteinsson, Tuan. T. Tran, Dmitrii Moldarev, Ayan Samanta, Max Wolff, Daniel Primetzhofer

Abstract: Photochromic oxygen$-$containing yttrium$-$hydride thin films are synthesized by argon$-$magnetron sputtering on microscope slides. Some of them are encapsulated with a thin, transparent and non$-$photochromic diffusion-barrier layer of either Al2O3 or Si3N4. Ion beam-based methods prove that these protective diffusion barriers are stable and free from pinholes, with thicknesses of only a few tens… ▽ More Photochromic oxygen$-$containing yttrium$-$hydride thin films are synthesized by argon$-$magnetron sputtering on microscope slides. Some of them are encapsulated with a thin, transparent and non$-$photochromic diffusion-barrier layer of either Al2O3 or Si3N4. Ion beam-based methods prove that these protective diffusion barriers are stable and free from pinholes, with thicknesses of only a few tens of nanometers. Optical spectrophotometry reveals that the photochromic response and relaxation time for both $-$ protected and unprotected $-$ samples are almost identical. Ageing effects in the unprotected films lead to degradation of the photochromic performance (self$-$delamination) while the photochromic response for the encapsulated films is stable. Our results show that the environment does not play a decisive role for the photochromic process and encapsulation of oxygen containing rare-earth hydride films with transparent and non-organic thin diffusion barrier layers provides long-time stability of the films, mandatory for applications as photochromic coatings on e.g., smart windows. △ Less

Submitted 30 December, 2020; originally announced December 2020.

Quasi-one-dimensional exchange interactions and short-range magnetic correlations in CuTeO4

Authors: Zubia Hasan, Eli Zoghlin, Michal J. Winiarski, Kathryn E. Arpino, Thomas Halloran, Thao T. Tran, Tyrel M. McQueen

Abstract: CuTeO4 has been proposed as a crystallographically distinct, yet electronic structure analog, of the superconducting cuprates. Here, we present detailed characterization of the of the physical properties of CuTeO4 to address this proposal. Fitting of magnetic susceptibility data indicates unexpected quasi-one-dimensional, antiferromagnetic correlations at high temperature, with a nearest neighbour… ▽ More CuTeO4 has been proposed as a crystallographically distinct, yet electronic structure analog, of the superconducting cuprates. Here, we present detailed characterization of the of the physical properties of CuTeO4 to address this proposal. Fitting of magnetic susceptibility data indicates unexpected quasi-one-dimensional, antiferromagnetic correlations at high temperature, with a nearest neighbour Heisenberg exchange of J1=165(4) K. Low temperature heat capacity measurements reveal a sizable T-linear contribution of $γ= 9.58(8) mJ mol^{-1} K^{-2}$, qualitatively consistent with expectations for a S=1/2, uniform, Heisenberg spin chain. Below T ~ 40 K, the susceptibility shows an upturn inconsistent with quasi-one-dimensional behaviour. While heat capacity measurements show no signs of magnetic order down to low temperature, the upturn in the magnetic susceptibility coincides with the emergence of a diffuse peak (centered at |Q| ~ 0.7 Angstrom) in the neutron diffraction data, indicative of persistent, short-range, antiferromagnetic order with a correlation length of $ξ$ = 10.1(9) Angstrom at T = 10 K. The onset of nonlinearity and hysteresis in the isothermal magnetization curves suggest the presence of a small ferromagnetic component. This persistent, short-range order is understood in the context of structural modeling of the x-ray and neutron diffraction data that show the presence of a significant density of stacking faults. No evidence for substantive dopability is observed and CuTeO4 appears qualitatively, to have a larger band gap than predicted by density functional theory. We ascribe this finding to the inductive withdrawal effect from high-valence Te and suggest that superconductivity in the copper tellurates is more likely to be found in compounds with a decreased reductive withdrawal effect from Te. △ Less

Submitted 29 May, 2024; v1 submitted 11 December, 2020; originally announced December 2020.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. B 109, 195168, 2024

Orbital-hybridization-created optical excitations in Li2GeO3

Authors: Vo Khuong Dien, Hai Duong Pham, Ngoc Thanh Thuy Tran, Nguyen Thi Han, Thi My Duyen Huynh, Thi Dieu Hien Nguyen, Ming Fa-Lin

Abstract: Li2GeO3, a ternary electrolyte compound of Li+-based battery, presents the unusual essential properties. The main features are thoroughly explored from the first-principles calculations. The concise pictures, the critical orbital hybridizations in Li-O and Ge-O bonds, are clearly examined through the optimal Moire superlattice, the atom-dominated electronic energy spectrum, the spatial charge dens… ▽ More Li2GeO3, a ternary electrolyte compound of Li+-based battery, presents the unusual essential properties. The main features are thoroughly explored from the first-principles calculations. The concise pictures, the critical orbital hybridizations in Li-O and Ge-O bonds, are clearly examined through the optimal Moire superlattice, the atom-dominated electronic energy spectrum, the spatial charge densities, the atom- and orbital-decomposed van Hove singularities, and the strong optical responses. The unusual optical transitions cover the red-shift optical gap, 16 frequency-dependent absorption structures and the most prominent plasmon mode in terms of the dielectric functions, energy loss functions, reflectance spectra, and absorption coefficients. Optical excitations, depending on the directions of electric polarization, are strongly affected by the excitonic effects. The close combinations of electronic and optical properties can identify a significant orbital hybridization for each available excitation channel. The developed theoretical framework will be very useful in fully understanding the diverse phenomena of cathode/electrolyte/anode materials in ion-based batteries. △ Less

Submitted 7 October, 2020; v1 submitted 4 September, 2020; originally announced September 2020.

Unprecedented Severe Atomic Redistribution in Germanium Induced by MeV Self-Irradiation

Authors: Tuan T. Tran, Daniel Primetzhofer

Abstract: We present a pronounced unprecedented surface modification of a crystalline Ge layer under heavy ion irradiation with a Ge ion beam at high energy of 2.5 MeV. Under the irradiation conditions, the Ge layer did not become porous as observed for other projectiles and lower energies but develops into an uneven ripple morphology in which the roughness monotonically increases with the irradiation doses… ▽ More We present a pronounced unprecedented surface modification of a crystalline Ge layer under heavy ion irradiation with a Ge ion beam at high energy of 2.5 MeV. Under the irradiation conditions, the Ge layer did not become porous as observed for other projectiles and lower energies but develops into an uneven ripple morphology in which the roughness monotonically increases with the irradiation doses. We show that this phenomenon is caused neither by surface erosion effect nor by a non-uniform volumetric expansion. Rather, atomic redistribution in the bulk of the material is the only drive for the ripple surface. Furthermore, the deformation of the Ge layer likely occurs to largest extend after irradiation, as indicated by the very flat interface around the end-of-range region. The observed morphology modification is discussed based on irradiation-induced plastic flow, coupled with a larger contribution of the electronic component in the ion-solid interactions. △ Less

Submitted 19 August, 2020; originally announced August 2020.

Comments: 12 pages, 6 figures

Magnetic competition in topological kagome magnets

Authors: Minh-Tien Tran, Duong-Bo Nguyen, Hong-Son Nguyen, Thanh-Mai Thi Tran

Abstract: Magnetic competition in topological kagome magnets is studied by incorporating the spin-orbit coupling, the anisotropic Hund coupling and spin exchange into the kagome lattice. Using the Bogoliubov variational principle we find the stable phases at zero and finite temperatures. At zero temperature and in the strong Ising-Hund coupling regime, a magnetic tunability from the out-of-plane ferromagnet… ▽ More Magnetic competition in topological kagome magnets is studied by incorporating the spin-orbit coupling, the anisotropic Hund coupling and spin exchange into the kagome lattice. Using the Bogoliubov variational principle we find the stable phases at zero and finite temperatures. At zero temperature and in the strong Ising-Hund coupling regime, a magnetic tunability from the out-of-plane ferromagnetism (FM) to the in-plane antiferromagnetism (AFM) is achieved by a universal property of the critical in-plane Hund coupling. At two-thirds filling the phase transition from the out-of-plane FM to the in-plane AFM is accompanied by a topological transition from quantum anomalous Hall (QAH) to quantum anomalous spin Hall (QASH) effect. Nearby half filling a large anomalous Hall conductance is observed at the magnetic phase transition. At finite temperature the out-of-plane FM is stable until a crossing temperature, above which the in-plane AFM is stable, but the out-of-plane FM magnetization is still finite. This suggests a coexistence of these magnetic phases in a finite temperature range. △ Less

Submitted 19 August, 2020; originally announced August 2020.

Impact of magnetic dopants on magnetic and topological phases in magnetic topological insulators

Authors: Thanh-Mai Thi Tran, Duc-Anh Le, Tuan-Minh Pham, Kim-Thanh Thi Nguyen, Minh-Tien Tran

Abstract: A topological insulator doped with random magnetic impurities is studied. The system is modelled by the Kane-Mele model with a random spin exchange between conduction electrons and magnetic dopants. The dynamical mean field theory for disordered systems is used to investigate the electron dynamics. The magnetic long-range order and the topological invariant are calculated within the mean field the… ▽ More A topological insulator doped with random magnetic impurities is studied. The system is modelled by the Kane-Mele model with a random spin exchange between conduction electrons and magnetic dopants. The dynamical mean field theory for disordered systems is used to investigate the electron dynamics. The magnetic long-range order and the topological invariant are calculated within the mean field theory. They reveal a rich phase diagram, where different magnetic long-range orders such as antiferromagnetic or ferromagnetic one can exist in the metallic or insulating phases, depending on electron and magnetic impurity fillings. It is found that insulator only occurs at electron half filling, quarter filling and when electron filling is equal to magnetic impurity filling. However, non-trivial topology is observed only in half-filling antiferromagnetic insulator and quarter-filling ferromagnetic insulator. At electron half filling, the spin Hall conductance is quantized and it is robust against magnetic doping, while at electron quarter filling, magnetic dopants drive the ferromagnetic topological insulator to ferromagnetic metal. The quantum anomalous Hall effect is observed only at electron quarter filling and dense magnetic doping. △ Less

Submitted 29 July, 2020; originally announced July 2020.

Journal ref: Phys. Rev. B 102, 205124 (2020)

In-situ Nanoscale Characterization of Composition and Structure during Formation of Ultrathin Nickel Silicide

Authors: Tuan T. Tran, Christian Lavoie, Zhen Zhang, Daniel Primetzhofer

Abstract: We characterize composition and structure of ultrathin nickel silicide during formation from 3 nm Ni films on Si(100) using in-situ high resolution ion scattering and high resolution transmission electron microscopy. We show the transition to occur in discrete steps, in which an intermediate phase is observed within a narrow range of temperature from 230 oC to 290 oC. The film composition of this… ▽ More We characterize composition and structure of ultrathin nickel silicide during formation from 3 nm Ni films on Si(100) using in-situ high resolution ion scattering and high resolution transmission electron microscopy. We show the transition to occur in discrete steps, in which an intermediate phase is observed within a narrow range of temperature from 230 oC to 290 oC. The film composition of this intermediate phase is found to be 50% Ni:50% Si, without evidence for long-range structure, indicating the film to be a homogeneous monosilicide NiSi phase. The final phase is resemblant of the cubic disilicide NiSi2, but with slightly off-stoichiometric composition of 38% Ni and 62% Si. Along the [100] axis, the lattices of the film and the substrate are found in perfect alignment. Due to the epitaxial growth of the silicide, a contraction of the c lattice constant of the film by 0.7-1% is detected. △ Less

Submitted 22 June, 2020; originally announced June 2020.

Comments: 17 pages, 6 figures

Essential electronic properties on stage-1 Li/Li+-graphite-intercalation compounds for different concentrations

Authors: Wei-Bang Li, Shih-Yang Lin, Ngoc Thanh Thuy Tran, Kuang-I Lin, Ming-Fa Lin

Abstract: We use first-principles calculation within the density functional theory (DFT) to explore the electronic properties on stage-1 Li- and Li+-graphite-intercalation compounds (GIC) for different concentrations, LiCx/Li+Cx with x= 6,12,18,24,32 and 36. The essential properties, e.g. geometric structures, band structures and spatial charge distributions are determined by the hybridization of orbitals,… ▽ More We use first-principles calculation within the density functional theory (DFT) to explore the electronic properties on stage-1 Li- and Li+-graphite-intercalation compounds (GIC) for different concentrations, LiCx/Li+Cx with x= 6,12,18,24,32 and 36. The essential properties, e.g. geometric structures, band structures and spatial charge distributions are determined by the hybridization of orbitals, the main focus of our works. The band structures/density of states/spatial charge distribution display that the Li-GIC possesses blue shift of fermi energy and just like metals, but the Li+-GIC still preserves as original graphite or so-call semimetal possessing the same densities of free electrons and holes. According to these properties, we find that there exists weak but significant van der Waals interactions between interlayer of graphite, and 2s-2pz hybridization between Li and C. There scarcely exists strong interactions between Li+-C. The dominant interaction between the Li and C is 2s-2pz orbital-orbital couple; the orbital-orbital couple is not significant in Li+ and C case but the dipole-diploe couple. △ Less

Submitted 23 June, 2020; v1 submitted 22 June, 2020; originally announced June 2020.

Comments: 12 pages, 7 figures, 2 tables

The Chemistry of Quantum Spin Liquids

Authors: J. R. Chamorro, T. T. Tran, T. M. McQueen

Abstract: Quantum spin liquids are an exciting playground for exotic physical phenomena and emergent many-body quantum states. The realization and discovery of quantum spin liquid candidate materials and associated phenomena lie at the intersection of solid-state chemistry, condensed matter physics and materials science and engineering. In this review, we provide the current status of the crystal chemistry,… ▽ More Quantum spin liquids are an exciting playground for exotic physical phenomena and emergent many-body quantum states. The realization and discovery of quantum spin liquid candidate materials and associated phenomena lie at the intersection of solid-state chemistry, condensed matter physics and materials science and engineering. In this review, we provide the current status of the crystal chemistry, synthetic techniques, physical properties, and research methods in the field of quantum spin liquids. We also highlight a number of specific quantum spin liquid candidate materials and their structure-property relationships, elucidating their fascinating behavior and connecting it to the intricacies of their structures. Furthermore, we share our thoughts on defects and their inevitable presence in materials, of which quantum spin liquids are no exception, which can complicate the interpretation of characterization of these materials, and urge the community to extend their attention to materials preparation and data analysis, cognizant of the impact of defects. This review was written with the intention of providing guidance on improving the materials design and growth of quantum spin liquids, and painting a picture of the beauty of the underlying chemistry of this exciting class of materials. △ Less

Submitted 18 June, 2020; originally announced June 2020.

Comments: Submitted for publication in a special issue on quantum materials in Chemical Reviews. Open for comment, please provide suggestions for additional references and improvements

Induced Ferromagnetism in bilayer Hexagonal Boron Nitride (h-BN) on vacancy defects at B and N sites

Authors: B. Chettri, P. K. Patra, Tuan V. Vu, Lalrinkima, Abu Yaya, Kingsley O. Obodo, Ngoc Thanh Thuy Tran, A. Laref, D. P. Rai

Abstract: We investigated the electronic and optical properties of bilayer AB stacked Boron and Nitrogen vacancies in hexagonal Boron Nitride (h-BN) using density functional theory (DFT). The density of states (DOS) and electronic band structure showed that Boron vacancy in bilayer h-BN results in a magnetic and conducting ground state. The band gap energy ranges from 4.56 eV for the pristine BN bilayer to… ▽ More We investigated the electronic and optical properties of bilayer AB stacked Boron and Nitrogen vacancies in hexagonal Boron Nitride (h-BN) using density functional theory (DFT). The density of states (DOS) and electronic band structure showed that Boron vacancy in bilayer h-BN results in a magnetic and conducting ground state. The band gap energy ranges from 4.56 eV for the pristine BN bilayer to 0.12 eV for a single Nitrogen vacancy in the bilayer. Considering the presence of 1,3,4-Boron vacancy, half metallic character is observed. However, the 2-boron vacancy configuration resulted in metallic character. The bilayers with 1,2,3,4- Nitrogen vacancy has a band gap of 0.39, 0.33, 0.28 and 0.12eV respectively, which is significantly less than the pristine band gap. Also B and N vacancy induces ferromagnetism in the h-BN bilayer. The maximum total magnetic moment for the Boron vacant system is 6.583uB in case of 4-Boron vacancy configuration. In case of Nitrogen vacancy system it is 3.926uB for 4-Nitrogen vacancy configuration. The optical response of the system is presented in terms of the absorption coefficient, refractive index and dielectric constant for pristine as well as the defective configurations. Negative value of dielectric constant for Boron vacant system in the energy range 0.9-1.4 eV and for Nitrogen vacant system in the energy range 0.5-0.8 eV opens an opportunity for it to be utilized for negative index optical materials. The current study shows that B and N vacancies in bilayer h-BN could have potential applications in nano-structure based electronics, optoelectronics and spintronic devices. △ Less

Submitted 21 March, 2020; originally announced March 2020.

Optical Thermometry with Quantum Emitters in Hexagonal Boron Nitride

Authors: Yongliang Chen, Thinh Ngoc Tran, Ngoc My Hanh Duong, Chi Li, Milos Toth, Carlo Bradac, Igor Aharonovich, Alexander Solntsev, Toan Trong Tran

Abstract: Nanoscale optical thermometry is a promising non-contact route for measuring local temperature with both high sensitivity and spatial resolution. In this work, we present a deterministic optical thermometry technique based on quantum emitters in nanoscale hexagonal boron-nitride. We show that these nanothermometers exhibit better performance than that of homologous, all-optical nanothermometers bo… ▽ More Nanoscale optical thermometry is a promising non-contact route for measuring local temperature with both high sensitivity and spatial resolution. In this work, we present a deterministic optical thermometry technique based on quantum emitters in nanoscale hexagonal boron-nitride. We show that these nanothermometers exhibit better performance than that of homologous, all-optical nanothermometers both in sensitivity and range of working temperature. We demonstrate their effectiveness as nanothermometers by monitoring the local temperature at specific locations in a variety of custom-built micro-circuits. This work opens new avenues for nanoscale temperature measurements and heat flow studies in miniaturized, integrated devices. △ Less

Submitted 8 March, 2020; originally announced March 2020.

Fundamental Properties of Metal-Adsorbed Silicene: A DFT Study

Authors: Ngoc Thanh Thuy Tran, Godfrey Gumbs, Duy Khanh Nguyen, Ming-Fa Lin

Abstract: Sodium, magnesium and aluminum adatoms, which, respectively, possess one, two and three valence electrons in terms of 3s, $3s^2$, and ($3s^2$, 3p) orbitals, are very suitable for helping us understand the adsorption-induced diverse phenomena. In this study, the revealing properties of metal (Na/Mg/Al)-adsorbed graphene systems are investigated by mean of the first-principles method. The single- an… ▽ More Sodium, magnesium and aluminum adatoms, which, respectively, possess one, two and three valence electrons in terms of 3s, $3s^2$, and ($3s^2$, 3p) orbitals, are very suitable for helping us understand the adsorption-induced diverse phenomena. In this study, the revealing properties of metal (Na/Mg/Al)-adsorbed graphene systems are investigated by mean of the first-principles method. The single- and double-side chemisorption cases, the various adatom concentrations, the hollow/top/valley/bridge sites, and the buckled structures are taken into account. The hollow and valley adsorptions, which, respectively, correspond to the Na/Mg and Al cases, create the extremely non-uniform environments within the Moire superlattices. This lead to diverse orbital hybridizations in Na/Mg/Al-Si bonds, as indicated from the Na/Mg/Al-dominated bands, the spatial charge density distributions and the orbital-projected density of states (DOS). Among three kinds of metal-adatom adsorptions, the Al-adsorption configurations present the strongest chemical modifications. The ferromagnetic configurations are shown to only survive in the specific Mg- and Al-adsorptions, but not the Na-cases. The theoretical predictions could be validated by experimental measurements and the up-to-date potential applications are included. Furthermore, the important similarities and differences with the graphene-related systems are also discussed. △ Less

Submitted 5 March, 2020; originally announced March 2020.

First-principles studies of electronic properties in Lithium metasilicate (Li2SiO3)

Authors: Nguyen Thi Han, Vo Khuong Dien, Ngoc Thanh Thuy Tran, Duy Khanh Nguyen, Wu-Pei Su, Ming-Fa Lin

Abstract: Lithium metasilicate (Li2SiO3) has attracted considerable interest as a promising electrolyte material for potential use in lithium batteries. However, its electronic properties are still not thoroughly understood. In this work, density functional theory calculations were adopted, our calculations find out that Li2SiO3 exhibits unique lattice symmetry (orthorhombic crystal), valence and conduction… ▽ More Lithium metasilicate (Li2SiO3) has attracted considerable interest as a promising electrolyte material for potential use in lithium batteries. However, its electronic properties are still not thoroughly understood. In this work, density functional theory calculations were adopted, our calculations find out that Li2SiO3 exhibits unique lattice symmetry (orthorhombic crystal), valence and conduction bands, charge density distribution, and van Hove singularities. Delicate analyses, the critical multi-orbital hybridizations in Li-O and Si-O bonds 2s- (2s, 2px, 2py, 2pz) and (3s, 3px, 3py, 3pz)- (2s, 2px, 2py, 2pz), respectively was identified. In particular, this system shows a huge indirect-gap of 5.077 eV. Therefore, there exist many strong covalent bonds, with obvious anisotropy and non-uniformity. On the other hand, the spin-dependent magnetic configurations are thoroughly absent. The theoretical framework could be generalized to explore the essential properties of cathode and anode materials of oxide compounds. △ Less

Submitted 20 January, 2020; originally announced January 2020.

Comments: 18 pages, 7 figures

Diverse fundamental properties in stage-n graphite alkali-intercalation compounds: anode materials of Li+-based batteries

Authors: Wei-bang Li, Ngoc Thanh Thuy Tran, Shih-yang Lin, Ming-Fa Lin

Abstract: The diversified essential properties of the stage-n graphite alkali-intercalation compounds are thoroughly explored by the first-principles calculations. According to their main features, the lithium and non-lithium materials might be quite different from each other in stacking configurations, the intercalated alkali-atom concentrations, the free conduction electron densities, and the atom-dominat… ▽ More The diversified essential properties of the stage-n graphite alkali-intercalation compounds are thoroughly explored by the first-principles calculations. According to their main features, the lithium and non-lithium materials might be quite different from each other in stacking configurations, the intercalated alkali-atom concentrations, the free conduction electron densities, and the atom-dominated & (carbon, alkali)-co-dominated energy bands. The close relations between the alkali-doped metallic behaviors and the geometric symmetries will be clarified through the interlayer atomic interactions, in which the significant alkali-carbon chemical bondings are fully examined from the atom- and orbital-decomposed van Hove singularities. The blue shift of the Fermi level, the n-type doping, is clearly identified from the low-energy features of the density of states. This study is able to provide the partial information about anode of Li+-based battery. There are certain important differences between AC$_6$/AC$_8$ and Li$_8$Si$_4$O$_{12}$. △ Less

Submitted 22 June, 2020; v1 submitted 2 December, 2019; originally announced January 2020.

Overcharging of zinc ion in the structure of zinc finger protein is needed for DNA binding stability

Authors: Ly Hai Nguyen, Tuyen Thanh Tran, Lien Ngoc Thi Truong, Hanh Hong Mai, Toan T. Nguyen

Abstract: The zinc finger structure where a Zn2+ ion binds to 4 cysteine or histidine amino acids in a tetrahedral structure is very common motif of nucleic acid binding proteins. The corresponding interaction model is present in 3% of the genes of human genome. As a result, zinc finger has been shown to be extremely useful in various therapeutic and research capacities, as well as in biotechnology. In stab… ▽ More The zinc finger structure where a Zn2+ ion binds to 4 cysteine or histidine amino acids in a tetrahedral structure is very common motif of nucleic acid binding proteins. The corresponding interaction model is present in 3% of the genes of human genome. As a result, zinc finger has been shown to be extremely useful in various therapeutic and research capacities, as well as in biotechnology. In stable configuration, the cysteine amino acids are deprotonated and become negatively charged. This means the Zn2+ ion is overscreened by 4 cysteine charges (overcharged). It is question of whether this overcharged configuration is also stable when such negatively charged zinc finger binds to negatively charged DNA molecule. Using all atom molecular dynamics simulation up to microsecond range of an androgen receptor protein dimer, we investigate how the deprotonated state of cysteine influences its structure, dynamics, and function in binding o DNA molecules. Our results show that the deprotonated state of cysteine residues are essential for mechanical stabilization of the functional, folded conformation. Not only this state stabilizes the protein structure, it also stabilizes the protein-DNA binding complex. The differences in structural and energetic properties of the two (sequence-identical) monomers are also investigated showing the strong influence of DNA on the structure of zinc fingers upon complexation. Our result has potential impact on better molecular understanding of one of the most common classes of zinc fingers △ Less

Submitted 21 February, 2020; v1 submitted 23 November, 2019; originally announced November 2019.

Comments: 33 pages, 9 figures, improved presentation

Strain Engineering of Quantum Emitters in Hexagonal Boron Nitride

Authors: Noah Mendelson, Marcus Doherty, Milos Toth, Igor Aharonovich, Toan Trong Tran

Abstract: Quantum emitters in hexagonal boron nitride (hBN) are promising building blocks for the realization of integrated quantum photonic systems. However, their spectral inhomogeneity currently limits their potential applications. Here, we apply tensile strain to quantum emitters embedded in few-layer hBN films and realize both red and blue spectral shifts with tuning magnitudes up to 65 meV, a record f… ▽ More Quantum emitters in hexagonal boron nitride (hBN) are promising building blocks for the realization of integrated quantum photonic systems. However, their spectral inhomogeneity currently limits their potential applications. Here, we apply tensile strain to quantum emitters embedded in few-layer hBN films and realize both red and blue spectral shifts with tuning magnitudes up to 65 meV, a record for any two-dimensional quantum source. We demonstrate reversible tuning of the emission and related photophysical properties. We also observe rotation of the optical dipole in response to strain, suggesting the presence of a second excited state. We derive a theoretical model to describe strain-based tuning in hBN, and the rotation of the optical dipole. Our work demonstrates the immense potential for strain tuning of quantum emitters in layered materials to enable their employment in scalable quantum photonic networks. △ Less

Submitted 10 January, 2020; v1 submitted 18 November, 2019; originally announced November 2019.

In-situ characterization of ultrathin nickel silicides using 3D medium-energy ion scattering

Authors: Tuan Thien Tran, Lukas Jablonka, Christian Lavoie, Zhen Zhang, Daniel Primetzhofer

Abstract: We demonstrate a novel approach for non-destructive in-situ characterization of phase transitions of ultrathin nickel silicide films using 3D medium-energy ion scattering. The technique provides simultaneously composition and real-space crystallography of silicide films during the annealing process using a single sample. We show, for 10 nm Ni films on Si, that their composition follows the normal… ▽ More We demonstrate a novel approach for non-destructive in-situ characterization of phase transitions of ultrathin nickel silicide films using 3D medium-energy ion scattering. The technique provides simultaneously composition and real-space crystallography of silicide films during the annealing process using a single sample. We show, for 10 nm Ni films on Si, that their composition follows the normal transition sequence, such as Ni-Ni2Si-NiSi. For samples with initial Ni thickness of 3 nm, depth-resolved crystallography using a position-sensitive detector, shows that the Ni film transform from an as-deposited disordered layer to epitaxial silicide layers at a relatively low temperature of ~290 °C. △ Less

Submitted 21 October, 2019; originally announced October 2019.

Comments: 10 pages, 5 figures

Non-localized states and high hole mobility in amorphous germanium

Authors: Tuan T. Tran, Jennifer Wong-Leung, Lachlan A. Smillie, Anders Hallen, Maria G. Grimaldi, Jim S. Williams

Abstract: Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to be by the hopping mechanism through localized states. The carrier mobility of these materials is usually very low, in the order of ~10^-3 - 10^-… ▽ More Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to be by the hopping mechanism through localized states. The carrier mobility of these materials is usually very low, in the order of ~10^-3 - 10^-2 cm^2/(Vs) at room temperature. In this study, we present the Hall effect characterization of a-Ge prepared by self-ion implantation of Ge ions. The a-Ge prepared by this method is highly homogenous and has a mass density within 98.5% of the crystalline Ge. The material exhibits an exceptionally high electrical conductivity and carrier mobility (~100 cm^2/(Vs)) for an amorphous semiconductor. The temperature-dependent resistivity of the material is very-well defined with two distinctive regions, extrinsic and intrinsic conductivity, as in crystalline Ge. These results are direct evidence for a largely-preserved band structure and non-localized states of the valence band in a-Ge, as proposed by Tauc et al. from optical characterization alone. This finding is not only significant for the understanding of electrical conductivity in covalent disordered semiconductors, but the exceptionally high mobility we have observed in amorphous Ge opens up device applications not previously considered for amorphous semiconductors. △ Less

Submitted 22 August, 2019; originally announced August 2019.

Comments: 10 pages, 3 figures