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Observation of correlation-driven topological transport and robust ferromagnetism in 2D CrS$_2$
Authors:
Sk Md Obaidulla,
M. Nur Hasan,
Dayal Das,
Rafiqul Alam,
Antonio Supina,
Muhammad Awais Aslam,
Sherif Kamal,
Iva Šarić Jankovic,
Aleksandar Matkovic,
Christian Teichert,
Atindra Nath Pal,
Heike C. Herper,
Marko Kralj
Abstract:
The realization of correlated layered magnets hosting robust ferromagnetism with emergent topological transport remains a key challenge in quantum materials. Here we report the first catalyst-free chemical vapour deposition growth of layered 1T-CrS$_2$, establishing a highly stable vdWs ferromagnet with an out-of-plane easy-axis anisotropy and a Curie temperature above room temperature. Transport…
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The realization of correlated layered magnets hosting robust ferromagnetism with emergent topological transport remains a key challenge in quantum materials. Here we report the first catalyst-free chemical vapour deposition growth of layered 1T-CrS$_2$, establishing a highly stable vdWs ferromagnet with an out-of-plane easy-axis anisotropy and a Curie temperature above room temperature. Transport measurements reveal a semimetal--insulator crossover near 80 K and pronounced negative magnetoresistance up to 350 K. A topological Hall effect emerges below 30 K, a rare signature of correlated transport in layered transition-metal dichalcogenide ferromagnets. First-principles calculations show that spin--orbit coupling gaps Dirac-like crossings, while electronic correlations reconstruct the Fermi surface by suppressing electron pockets and reducing the carrier density, enhancing momentum-dependent out-of-plane spin polarization. Magnetic measurements, supported by Heisenberg exchange calculations, reveal strong nearest-neighbour ferromagnetic exchange that stabilizes long-range ferromagnetism. Our results establish 1T-CrS$_2$ as a rare correlated 3$d$ layered ferromagnet in which electronic correlations and spin--orbit coupling cooperatively drive emergent topological transport.
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Submitted 26 July, 2026;
originally announced July 2026.
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Quantifying the contribution of material and junction resistances in nano-networks
Authors:
Cian Gabbett,
Adam G. Kelly,
Emmet Coleman,
Luke Doolan,
Tian Carey,
Kevin Synnatschke,
Shixin Liu,
Anthony Dawson,
Domhnall OSuilleabhain,
Jose Munuera,
Eoin Caffrey,
John B. Boland,
Zdenek Sofer,
Goutam Ghosh,
Sachin Kinge,
Laurens D. A. Siebbeles,
Neelam Yadav,
Jagdish K. Vij,
Muhammad Awais Aslam,
Aleksandar Matkovic,
Jonathan N. Coleman
Abstract:
Networks of nanowires and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the inter-particle junction resistance, a property that is challenging to minimise because it is difficult to measure. Here, we develop a simple model for conduction in networks of 1D or 2D nanomaterials, which allows us to extract j…
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Networks of nanowires and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the inter-particle junction resistance, a property that is challenging to minimise because it is difficult to measure. Here, we develop a simple model for conduction in networks of 1D or 2D nanomaterials, which allows us to extract junction and nanoparticle resistances from particle-size-dependent D.C. resistivity data of conducting and semiconducting materials. We find junction resistances in porous networks to scale with nanoparticle resistivity and vary from 5 Ohm for silver nanosheets to 25 GOhm for WS2 nanosheets. Moreover, our model allows junction and nanoparticle resistances to be extracted from A.C. impedance spectra of semiconducting networks. Impedance data links the high mobility (~7 cm2/Vs) of aligned networks of electrochemically exfoliated MoS2 nanosheets to low junction resistances of ~670 kOhm. Temperature-dependent impedance measurements allow us to quantitatively differentiate intra-nanosheet phonon-limited band-like transport from inter-nanosheet hopping for the first time.
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Submitted 28 November, 2023;
originally announced November 2023.
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Optoelectronic and Transport Properties of Vacancy Ordered Double Perovskite Halides: A First-principles Study
Authors:
Supriti Ghorui,
Jiban Kangsabanik,
M. Aslam,
Aftab Alam
Abstract:
In the search for stable lead (Pb) free perovskites, Vacancy ordered double perovskite (VODP), A$_2$BX$_6$ has emerged as a promising class of materials for solar harvesting owing to their nontoxicity, better stability, and unique optoelectronic properties. Here, we present the stability and the key physical attributes of few selected compounds in a systematic manner using state-of-the-art first-p…
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In the search for stable lead (Pb) free perovskites, Vacancy ordered double perovskite (VODP), A$_2$BX$_6$ has emerged as a promising class of materials for solar harvesting owing to their nontoxicity, better stability, and unique optoelectronic properties. Here, we present the stability and the key physical attributes of few selected compounds in a systematic manner using state-of-the-art first-principle calculations. A careful structural and stability analysis via simulating convex hull and compositional phase diagrams for different structural prototypes discloses 14 stable and 1 metastable compounds in this class. The electronic structure calculations using hybrid functional reveals six compounds to acquire band gap in the ideal visible region. These six compounds, namely Cs$_2$SnI$_6$, Cs$_2$PdI$_6$, Cs$_2$TeI$_6$, Cs$_2$TiI$_6$, Cs$_2$PtI$_6$, and Cs$_2$PdBr$_6$, show high optical absorption ($\approx$ 10$^{5}$ cm $^{-1}$) giving rise to high spectroscopic limited maximum efficiency, SLME (15-23\%) in the thin-film thickness range. Close inspection of transport properties reveals polar optical phonon scattering to be the dominant mechanism limiting the overall mobility. Further analysis of the polaron excitations discloses the possibility of large polaron formation at low to moderate defect concentrations. At high defect concentrations, ionized impurity scattering takes over. This suggests that, a simulation based guided control of defect concentrations during synthesis can yield a desired candidate for promissing device application. Additionally, few selected compounds show moderate to high electron mobility values ($\sim$13-63 cm$^2$V$^{-1}$ s$^{-1}$) at room temperature. Overall, the present study paves an important path to help design VODP as Pb-free potential candidates for future optoelectronic applications.
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Submitted 12 September, 2023;
originally announced September 2023.
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Water Induced Ferroelectric Switching: The Crucial Role of Collective Dynamics
Authors:
Muhammad Awais Aslam,
Igor Stankovic,
Gennadiy Murastov,
Amy Carl,
Zehao Song,
Kenji Watanabe,
Takashi Taniguchi,
Alois Lugstein,
Christian Teichert,
Roman Gorbachev,
Raul David Rodriguez,
Aleksandar Matkovic
Abstract:
The interaction mechanisms of water with nanoscale geometries remain poorly understood. This study focuses on behaviour of water clusters under varying external electric fields with a particular focus on molecular ferroelectric devices. We employ a two-fold approach, combining experiments with large-scale molecular dynamics simulations on graphene nanoribbon field effect transistors. We show that…
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The interaction mechanisms of water with nanoscale geometries remain poorly understood. This study focuses on behaviour of water clusters under varying external electric fields with a particular focus on molecular ferroelectric devices. We employ a two-fold approach, combining experiments with large-scale molecular dynamics simulations on graphene nanoribbon field effect transistors. We show that bilayer graphene nanoribbons provide stable anchoring of water clusters on the oxygenated edges, resulting in a ferroelectric effect. A molecular dynamics model is then used to investigate water cluster behaviour under varying external electric fields. Finally, we show that these nanoribbons exhibit significant and persistent remanent fields that can be employed in ferroelectric heterostructures and neuromorphic circuits.
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Submitted 19 April, 2023;
originally announced April 2023.
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Probing magnetic ordering in air stable iron-rich van der Waals minerals
Authors:
Muhammad Zubair Khan,
Oleg E. Peil,
Apoorva Sharma,
Oleksandr Selyshchev,
Sergio Valencia,
Florian Kronast,
Maik Zimmermann,
Muhammad Awais Aslam,
Johann G. Raith,
Christian Teichert,
Dietrich R. T. Zahn,
Georgeta Salvan,
Aleksandar Matković,
Chair of Physics,
Department Physics,
Mechanics,
Electrical engineering,
Montanuniversität Leoben,
8700,
Leoben,
Austria.,
Materials Center Leoben Forschung GmbH,
8700,
Leoben,
Austria.
, et al. (24 additional authors not shown)
Abstract:
In the rapidly expanding field of two-dimensional materials, magnetic monolayers show great promise for the future applications in nanoelectronics, data storage, and sensing. The research in intrinsically magnetic two-dimensional materials mainly focuses on synthetic iodide and telluride based compounds, which inherently suffer from the lack of ambient stability. So far, naturally occurring layere…
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In the rapidly expanding field of two-dimensional materials, magnetic monolayers show great promise for the future applications in nanoelectronics, data storage, and sensing. The research in intrinsically magnetic two-dimensional materials mainly focuses on synthetic iodide and telluride based compounds, which inherently suffer from the lack of ambient stability. So far, naturally occurring layered magnetic materials have been vastly overlooked. These minerals offer a unique opportunity to explore air-stable complex layered systems with high concentration of local moment bearing ions. We demonstrate magnetic ordering in iron-rich two-dimensional phyllosilicates, focusing on mineral species of minnesotaite, annite, and biotite. These are naturally occurring van der Waals magnetic materials which integrate local moment baring ions of iron via magnesium/aluminium substitution in their octahedral sites. Due to self-inherent capping by silicate/aluminate tetrahedral groups, ultra-thin layers are air-stable. Chemical characterization, quantitative elemental analysis, and iron oxidation states were determined via Raman spectroscopy, wavelength disperse X-ray spectroscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Superconducting quantum interference device magnetometry measurements were performed to examine the magnetic ordering. These layered materials exhibit paramagnetic or superparamagnetic characteristics at room temperature. At low temperature ferrimagnetic or antiferromagnetic ordering occurs, with the critical ordering temperature of 38.7 K for minnesotaite, 36.1 K for annite, and 4.9 K for biotite. In-field magnetic force microscopy on iron bearing phyllosilicates confirmed the paramagnetic response at room temperature, present down to monolayers.
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Submitted 13 April, 2023;
originally announced April 2023.
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Exciton tuning in monolayer WSe$_2$ via substrate induced electron doping
Authors:
Yang Pan,
Mahfujur Rahaman,
Lu He,
Ilya Milekhin,
Gopinath Manoharan,
Muhammad Awais Aslam,
Thomas Blaudeck,
Andreas Willert,
Aleksandar Matković,
Teresa I. Madeira,
Dietrich R. T. Zahn
Abstract:
We report on large exciton tuning in WSe$_2$ monolayers via substrate induced non-degenerate doping. We observe a redshift of $\sim$62 meV for the $A$ exciton together with a 1-2 orders of magnitude photoluminescence (PL) quenching when the monolayer WSe$_2$ is brought in contact with highly oriented pyrolytic graphite (HOPG) compared to the dielectric substrates such as hBN and SiO$_2$. As the ev…
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We report on large exciton tuning in WSe$_2$ monolayers via substrate induced non-degenerate doping. We observe a redshift of $\sim$62 meV for the $A$ exciton together with a 1-2 orders of magnitude photoluminescence (PL) quenching when the monolayer WSe$_2$ is brought in contact with highly oriented pyrolytic graphite (HOPG) compared to the dielectric substrates such as hBN and SiO$_2$. As the evidence of doping from HOPG to WSe$_2$, a drastic increase of the trion emission intensity was observed. Using a systematic PL and Kelvin probe force microscopy (KPFM) investigation on WSe$_2$/HOPG, WSe$_2$/hBN, and WSe$_2$/graphene, we conclude that this unique excitonic behavior is induced by electron doping from the substrate. Our results propose a simple yet efficient way for exciton tuning in monolayer WSe$_2$, which plays a central role in the fundamental understanding and further device development.
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Submitted 20 July, 2022; v1 submitted 6 July, 2022;
originally announced July 2022.
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Single Crystalline 2D Material Nanoribbon Networks for Nanoelectronics
Authors:
Muhammad Awais Aslam,
Tuan Hoang Tran,
Antonio Supina,
Olivier Siri,
Vincent Meunier,
Kenji Watanabe,
Takashi Taniguchi,
Marko Kralj,
Christian Teichert,
Evgeniya Sheremet,
Raul D. Rodriguez,
Aleksandar Matković
Abstract:
The last decade has seen a flurry of studies related to graphene nanoribbons owing to their potential applications in the quantum realm. However, little experimental work has been reported towards nanoribbons of other 2D materials due to the absence of synthesis routes. Here, we propose a universal approach to synthesize high-quality networks of nanoribbons from arbitrary 2D materials while mainta…
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The last decade has seen a flurry of studies related to graphene nanoribbons owing to their potential applications in the quantum realm. However, little experimental work has been reported towards nanoribbons of other 2D materials due to the absence of synthesis routes. Here, we propose a universal approach to synthesize high-quality networks of nanoribbons from arbitrary 2D materials while maintaining high crystallinity, sufficient yield, narrow size distribution, and straight-forward device integrability. The wide applicability of this technique is demonstrated by fabricating MoS2, WS2, WSe2, and graphene nanoribbon field effect transistors that inherently do not suffer from interconnection resistances. By relying on self-assembled and self-aligned organic nanostructures as masks, we demonstrate the possibility of controlling the predominant crystallographic direction of the nanoribbon's edges. Electrical characterization shows record mobilities and very high ON currents for various TMDCs despite extreme width scaling. Lastly, we explore decoration of nanoribbon edges with plasmonic particles paving the way towards the development of nanoribbon-based plasmonic sensing and opto-electronic devices.
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Submitted 16 May, 2022;
originally announced May 2022.
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Optoelectronic Properties and Defect Physics of Lead-free Photovoltaic Absorbers Cs$_2$Au$^{I}$Au$^{III}$X$_6$ (X=I, Br)
Authors:
Jiban Kangsabanik,
Supriti Ghorui,
M. Aslam,
Aftab Alam
Abstract:
Stability and toxicity issues with the hybrid lead iodide perovskite MAPbI$_3$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites Cs$_2$Au$_2$X$_6$ (X = I, Br, Cl). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding ene…
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Stability and toxicity issues with the hybrid lead iodide perovskite MAPbI$_3$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites Cs$_2$Au$_2$X$_6$ (X = I, Br, Cl). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding energy. Our study shows a favorable electronic structure, resulting in a high optical-transition strength, and thus a sharp rise in the absorption spectrum near the band gap. This, in turn, yields a very high short-circuit current density and hence higher simulated efficiency compared with MAPbI$_3$. However, careful investigation of defect physics reveals the possibility of deep-level defects (such as V$_X$ , V$_{Cs}$, X$_{Au}$, X$_{Cs}$, Au$_i$, and Au$_X$ , X = I, Br), depending on the growth conditions. These can act as carrier traps and become detrimental to photovoltaic performance. The present study should help in taking necessary precautions in synthesizing these compounds in a controlled chemical environment, which should minimize performance limiting defects and pave the way for future studies on this class of materials.
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Submitted 6 January, 2020; v1 submitted 23 May, 2019;
originally announced May 2019.
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Tip-induced Superconductivity Coexisting with Preserved Topological Properties in Line-nodal Semimetal ZrSiS
Authors:
Leena Aggarwal,
Chandan K. Singh,
Mohammad Aslam,
Ratnadwip Singha,
Arnab Pariari,
Sirshendu Gayen,
Mukul Kabir,
Prabhat Mandal,
Goutam Sheet
Abstract:
ZrSiS was recently shown to be a new material with topologically non-trivial band structure which exhibits multiple Dirac nodes and a robust linear band dispersion up to an unusually high energy of 2\,eV. Such a robust linear dispersion makes the topological properties of ZrSiS insensitive to perturbations like carrier doping or lattice distortion. Here we show that a novel superconducting phase w…
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ZrSiS was recently shown to be a new material with topologically non-trivial band structure which exhibits multiple Dirac nodes and a robust linear band dispersion up to an unusually high energy of 2\,eV. Such a robust linear dispersion makes the topological properties of ZrSiS insensitive to perturbations like carrier doping or lattice distortion. Here we show that a novel superconducting phase with a remarkably high $T_c$ of 7.5\,K can be induced in single crystals of ZrSiS by a non-superconducting metallic tip of Ag. From first-principles calculations we show that the observed superconducting phase might originate from dramatic enhancement of density of states due to the presence of a metallic tip on ZrSiS. Our calculations also show that the emerging tip-induced superconducting phase co-exists with the well preserved topological properties of ZrSiS.
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Submitted 22 February, 2018;
originally announced February 2018.
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Superconductivity of Cobalt in Thin Films
Authors:
Nasrin Banu,
M. Aslam,
Arpita Paul,
Sanjib Banik,
S. Das,
S. Datta,
A. Roy,
I. Das,
G. Sheet,
U. V. Waghmare,
B. N. Dev
Abstract:
Due to competing long range ferromagnetic order, the transition metals Fe, Co and Ni are not superconductors at ambient pressure. While superconductivity was observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to be discovered in Co and Ni under any experimental conditions. Here, we report emergence of superconductivity in the recently discovered high-density nonmagnetic f…
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Due to competing long range ferromagnetic order, the transition metals Fe, Co and Ni are not superconductors at ambient pressure. While superconductivity was observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to be discovered in Co and Ni under any experimental conditions. Here, we report emergence of superconductivity in the recently discovered high-density nonmagnetic face centered cubic phase in Co thin films below a transition temperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact spectroscopy and resistance, and four-probe measurements of resistance at ambient pressure. We confirm the non-magnetic nature of the dense fcc phase of Co within first-principles density functional theory, and show that its superconductivity below 5 K originates from anomalous softening of zone-boundary phonons and their enhanced coupling with electrons upon biaxial strain.
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Submitted 17 October, 2017;
originally announced October 2017.
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Giant Enhancement of Superconductivity in Zr Point Contacts
Authors:
Mohammad Aslam,
Chandan K. Singh,
Shekhar Das,
Ritesh Kumar,
Soumya Datta,
Soumyadip Halder,
Sirshendu Gayen,
Mukul Kabir,
Goutam Sheet
Abstract:
For certain complex superconducting systems, the superconducting properties get enhanced under mesoscopic point contacts made of elemental non-superconducting metals. However, understanding of the mechanism through which such contact induced local enhancement of superconductivity happens has been limited due to the complex nature of such compounds. In this paper we present giant enhancement of sup…
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For certain complex superconducting systems, the superconducting properties get enhanced under mesoscopic point contacts made of elemental non-superconducting metals. However, understanding of the mechanism through which such contact induced local enhancement of superconductivity happens has been limited due to the complex nature of such compounds. In this paper we present giant enhancement of superconducting transition temperature (T$_c$) and superconducting energy gap ($Δ$) in a simple elemental superconductor Zr. While bulk Zr shows a critical temperature around 0.6\,K, superconductivity survives at Ag/Zr and Pt/Zr point contacts up to 3\,K with a corresponding five-fold enhancement of $Δ$. From first principles calculations we show that the enhancement in superconducting properties can be attributed to a modification in the electron-phonon coupling accompanied by an enhancement of the density of states which involves the appearance of a new electron band at the Ag/Zr interfaces.
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Submitted 14 September, 2017;
originally announced September 2017.
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Unexpected superconductivity at nanoscale junctions made on the topological crystalline insulator Pb$_{0.6}$Sn$_{0.4}$Te
Authors:
Shekhar Das,
Leena Aggarwal,
Subhajit Roychowdhury,
Mohammad Aslam,
Sirshendu Gayen,
Kanishka Biswas,
Goutam Sheet
Abstract:
Discovery of exotic phases of matter from the topologically non-trivial systems not only makes the research on topological materials more interesting but also enriches our understanding of the fascinating physics of such materials. Pb$_{0.6}$Sn$_{0.4}$Te was recently shown to be a topological crystalline insulator. Here we show that by forming a mesoscopic point-contact using a normal non-supercon…
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Discovery of exotic phases of matter from the topologically non-trivial systems not only makes the research on topological materials more interesting but also enriches our understanding of the fascinating physics of such materials. Pb$_{0.6}$Sn$_{0.4}$Te was recently shown to be a topological crystalline insulator. Here we show that by forming a mesoscopic point-contact using a normal non-superconducting elemental metal on the surface of Pb$_{0.6}$Sn$_{0.4}$Te a novel superconducting phase is created locally in a confined region under the point-contact. This happens while the bulk of the sample remains to be non-superconducting and the superconducting phase emerges as a nano-droplet under the point-contact. The superconducting phase shows a high transition temperature $T_c$ that varies for different point-contacts and falls in a range between 3.7 K and 6.5 K. Therefore, this Letter presents the discovery of a new superconducting phase on the surface of a topological crystalline insulator and the discovery is expected to shed light on the mechanism of induced superconductivity in topologically non-trivial systems in general.
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Submitted 6 July, 2016;
originally announced July 2016.
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Anisotropic superconductivity in La(O,F)BiSeS crystals revealed by field-angle dependent Andreev reflection spectroscopy
Authors:
Mohammad Aslam,
Sirshendu Gayen,
Avtar Singh,
Masashi Tanaka,
Takuma Yamaki,
Yoshihiko Takano,
Goutam Sheet
Abstract:
From field-angle dependent Andreev reflection spectroscopy on single crystals of La(O,F)BiSeS, which belongs to the recently discovered BiCh 2 (Ch = S, Se) based layered superconductors, we found that the superconductivity in La(O,F)BiSeS is highly anisotropic. We measured a superconducting energy gap of 0.61 meV for current injected along c-axis at 1.5 K. Detailed temperature and magnetic field d…
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From field-angle dependent Andreev reflection spectroscopy on single crystals of La(O,F)BiSeS, which belongs to the recently discovered BiCh 2 (Ch = S, Se) based layered superconductors, we found that the superconductivity in La(O,F)BiSeS is highly anisotropic. We measured a superconducting energy gap of 0.61 meV for current injected along c-axis at 1.5 K. Detailed temperature and magnetic field dependent studies of the gap also reveal the presence of unconventional pairing in La(O,F)BiSeS. We show that the observed anisotropic superconducting properties can be attributed to the anisotropy in the superconducting order parameter with a complex symmetry in superconducting La(O,F)BiSeS.
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Submitted 21 April, 2016;
originally announced April 2016.
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Evidence of a pseudogap driven by competing orders of multi-band origin in the ferromagnetic superconductor Sr$_{0.5}$Ce$_{0.5}$FBiS$_2$
Authors:
Mohammad Aslam,
Arpita Paul,
Gohil S. Thakur,
Sirshendu Gayen,
Ritesh Kumar,
Avtar Singh,
Shekhar Das,
Ashok K. Ganguli,
Umesh V. Waghmare,
Goutam Sheet
Abstract:
From temperature and magnetic field dependent point-contact spectroscopy on the ferromagnetic superconductor Sr$_{0.5}$Ce$_{0.5}$FBiS$_2$ (bulk superconducting $T_c$ = 2.5 K) we observe (a) a pseudogap in the normal state that sustains to a remarkably high temperature of 40 K and (b) two-fold enhancement of $T_c$ upto 5 K in the point-contact geometry. In addition, Andreev reflection spectroscopy…
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From temperature and magnetic field dependent point-contact spectroscopy on the ferromagnetic superconductor Sr$_{0.5}$Ce$_{0.5}$FBiS$_2$ (bulk superconducting $T_c$ = 2.5 K) we observe (a) a pseudogap in the normal state that sustains to a remarkably high temperature of 40 K and (b) two-fold enhancement of $T_c$ upto 5 K in the point-contact geometry. In addition, Andreev reflection spectroscopy reveals a superconducting gap of 6 meV for certain point-contacts suggesting that the mean field $T_c$ of this system could be approximately 40 K, the onset temperature of pseudo-gap. Our results suggest that quantum fluctuations originating from other competing orders in Sr$_{0.5}$Ce$_{0.5}$FBiS$_2$ forbid a global phase coherence at high temperatures thereby suppressing $T_c$. Apart from the known ordering to a ferromagnetic state, our first-principles calculations reveal nesting of a multi-band Fermi surface and a significant electron-phonon coupling that could result in charge density wave-like instabilities.
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Submitted 29 March, 2016;
originally announced March 2016.