25 research outputs found
Correlated states in super-moiré materials with a kernel polynomial quantics tensor cross interpolation algorithm
No full textSuper-moiré materials represent a novel playground to engineer states of matter beyond the possibilities of conventional moiré materials. However, from the computational point of view, understanding correlated matter in these systems requires solving models with several millions of atoms, a formidable task for state-of-the-art methods. Conventional wavefunction methods for correlated matter scale with a cubic power with the number of sites, a major challenge for super-moiré materials. Here, we introduce a methodology capable of solving correlated states in super-moiré materials by combining a kernel polynomial method with a quantics tensor cross interpolation matrix product state algorithm. This strategy leverages a mapping of the super-moiré structure to a many-body Hilbert space, that is efficiently sampled with tensor cross interpolation with matrix product states, where individual evaluations are performed with a Chebyshev kernel polynomial algorithm. We demonstrate this approach with interacting super-moiré systems with up to several millions of atoms, showing its ability to capture correlated states in moiré-of-moiré systems and domain walls between different moiré systems. Our manuscript puts forward a widely applicable methodology to study correlated matter in ultra-long length scales, enabling rationalizing correlated super-moiré phenomena.Peer reviewe
Vuorovaikutusjohtuinen epäkonventionaalinen suprajohtavuus korreloiduissa multiferroisissa materiaaleissa
No full textThis master's thesis studies theoretically the interacting superconducting multiferroic honeycomb lattice. Different analytical and numerical tools are applied to the interacting system to seek out the possible superconducting multiferroic state driven solely by interactions.
As the aim is to produce simultaneously existing electric, magnetic and superconducting orders, they are first studied individually in order to understand how each interaction drives a specific broken symmetry order. Then different combinations of two simultaneous interactions are examined. Finally, all three orders are sought out together by having three different interactions simultaneously in the lattice. As an important point, the lattice is doped from quarter-filling to achieve a metallic state and thus allow the lattice to interact via second-neighbor attractive interaction and drive it towards superconductivity.
The interacting system was constructed and solved using a Python library designed for the exact purpose of computing quantum-lattice tight binding models. The library self-consistently solves the interacting Hamiltonian using mean-field analysis, producing analyzable data. The results are studied via the analysis of bandstructures and broken symmetry order parameters and their phase diagrams. A gap opening in a bandstructure indicates a present broken symmetry order. More precisely, this gap can be used to indicate the presence of superconductivity. The results show that there indeed exists a superconducting state in the interacting multiferroic honeycomb lattice with features pointing towards f-wave superconductivity. The system shows strong coupling between the multiferroic and superconducting orders.Tämä diplomityö tutkii teoreettisesti vuorovaikuttavaa hunajakennohilaa, jossa esiintyy suprajohteisuutta ja multiferroisuutta samanaikaisesti. Työ käyttää erilaisia analyyttisia ja numeerisia työkaluja mahdollista multiferroista suprajohdetilaa etsittäessä. Kyseinen suprajohdetila multiferroisessa hunajakennohilassa yritetään saada esiintymään pelkästään vuorovaikutusten avulla.
Tavoitteena on tuottaa samanaikaisesti esiintyvät rikkonaisen symmetrian tilat, joihin tässä tapauksessa mukaan luetaan magneettinen, sähköinen ja suprajohtava järjestys. Jokainen rikkonaisen symmetrian järjestys tutkitaan ensin yksitellen, sillä systeemiin tuodut vuorovaikutukset ohjaavat kukin tiettyä järjestystä. Näin pystytään ymmärtämään, miten kukin järjestys ilmenee ensin yksin, ja myöhemmin yhdessä muiden järjestysten kanssa samanaikaisesti. Lopuksi kaikki kolme rikkonaisen symmetrian järjestystä saadaan esiintymään hilarakenteessa samanaikaisesti kolmen eri vuorovaikutuksen tuloksena. Hilarakenteen täyttöastetta muunnetaan neljännestäytetystä hilasta hieman matalammaksi, jotta systeemistä saadaan metallinen. Tämä mahdollistaa siten hilassa esiintyvän toisen naapurin vetävän vuorovaikutuksen, ajaen systeemiä suprajohtavuutta kohti.
Vuorovaikutteinen järjestelmä rakennettiin ja ratkaistiin käyttämällä pyqula-nimistä Python-kirjastoa, joka on rakennettu juurikin kvanttihilojen tiukasti sidottujen mallien laskemiseksi ja ratkaisemiseksi. Kirjasto ratkaisee vuorovaikuttavan Hamiltonin operaattorin keskeiskenttäteoriaa hyödyntäen, tuottaen tutkittavaa dataa. Työn tulokset analysoidaan hilojen vyörakenteiden sekä rikkoutuneiden symmetrioiden järjestysparametrien faasikaavioiden avulla. Vyörakenteiden avulla voidaan tunnistaa aukkotilat, jotka toimivat indikaattorina rikkoutuneen symmetrian läsnäolosta. Tarkemmin sanottuna, aukko vyörakenteessa voi itsessään tarkoittaa suprajohtavuutta hilarakenteessa. Tulokset osoittavat suprajohtavan tilan olemassaolon multiferroisessa hunajakennohilarakenteessa. Tilan piirteet osoittavat kohti f-aaltoista suprajohtavuutta. Systeemin järjestysparametrien kytkentöjä tutkiessa hilarakenteen järjestykset osoittavat vahvaa kytkentää multiferroisten ja suprajohtavien järjestysten välillä
Microscopic origin of multiferroic order in monolayer NiI2
No full textKustantaja tarjonnut takautuvaa avoimuutta IOP:n uuden sopimuksen nimissä -> vaihda OA-tyyppi ja tallenna OA-julkaisuThe discovery of multiferroic behavior in monolayer NiI2 provides a new symmetry-broken state in van der Waals monolayers, featuring the simultaneous emergence of helimagnetic order and ferroelectric order at a critical temperature of T = 21 K. However, the microscopic origin of multiferroic order in NiI2 monolayer has not been established, and in particular, the role of non-collinear magnetism and spin–orbit coupling in this compound remains an open problem. Here we reveal the origin of the two-dimensional multiferroicity in NiI2 using first-principles electronic structure methods. We show that the helimagnetic state appears as a consequence of the long-range magnetic exchange interactions, featuring sizable magnetic moments in the iodine atoms. We demonstrate that the electronic density reconstruction accounting for the ferroelectric order emerges from the interplay of non-collinear magnetism and spin–orbit coupling. We demonstrate that the ferroelectric order is controlled by the iodine spin–orbit coupling, and leads to an associated electronically-driven distortion in the lattice. Our results establish the microscopic origin of the multiferroic behavior in monolayer NiI2, putting forward the coexistence of helical magnetic order and ligand spin–orbit coupling as driving forces for multiferroic behavior in two-dimensional materials.Peer reviewe
Moiré-driven multiferroic order in twisted CrCl3, CrBr3 and CrI3 bilayers
No full textLayered van der Waals materials have risen as a powerful platform to engineer artificial competing states of matter. Here we show the emergence of multiferroic order in twisted chromium trihalide bilayers, an order fully driven by the moiré pattern and absent in aligned multilayers. Using a combination of spin models and ab initio calculations, we show that a spin texture is generated in the moiré supercell of the twisted system as a consequence of the competition between stacking-dependent interlayer magnetic exchange and magnetic anisotropy. An electric polarization arises associated with such a non-collinear magnetic state due to the spin–orbit coupling, leading to the emergence of a local ferroelectric order following the moiré. Among the stochiometric trihalides, our results show that twisted CrBr3 bilayers give rise to the strongest multiferroic order. We further show the emergence of a strong magnetoelectric coupling, which allows the electric generation and control of magnetic skyrmions. Our results put forward twisted chromium trihalide bilayers, and in particular CrBr3 bilayers, as a powerful platform to engineer artificial multiferroic materials and electrically tunable topological magnetic textures.Peer reviewe
Ferroelectric valley valves with graphene/MoTe van der Waals heterostructures
No full textFerroelectric van der Waals heterostructures provide a natural platform to
design a variety of electrically controllable devices. In this work, we
demonstrate that AB bilayer graphene encapsulated in MoTe acts as a valley
valve that displays a switchable built-in topological gap, leading to
ferroelectrically driven topological channels. Using a combination of ab initio
calculations and low energy models, we show that the ferroelectric order of
MoTe allows the control of the gap opening in bilayer graphene and leads to
topological channels between different ferroelectric domains. Moreover, we
analyze the effect that the moir\'e modulation between MoTe and graphene
layers has in the topological modes, demonstrating that the edge states are
robust against moir\'e modulations of the ferroelectrically-induced electric
potential. Our results put forward ferroelectric/graphene heterostructures as
versatile platforms to engineer switchable built-in topological channels
without requiring an external electric bias.Comment: 8 pages, 4 figure
Self-doped flat band and spin-triplet superconductivity in monolayer 1T-TaSeTe
No full textTwo-dimensional van der Waals materials have become an established platform to engineer flat bands which can lead to strongly-correlated emergent phenomena. In particular, the family of Ta dichalcogenides in the 1\textit{T} phase presents a star-of-David charge density wave that creates a flat band at the Fermi level. For TaS and TaSe this flat band is at half filling leading to a magnetic insulating phase. In this work, we theoretically demonstrate that ligand substitution in the TaSeTe system produces a transition from the magnetic insulator to a non-magnetic metal in which the flat band gets doped away from half-filling. For the spin-polarized flat band is self-doped and the system becomes a magnetic metal. In this regime, we show that attractive interactions promote three different spin-triplet superconducting phases as a function of , corresponding to a nodal f-wave and two topologically-different chiral p-wave superconducting phases. Our results establish monolayer TaSeTe as a promising platform for correlated flat band physics leading to unconventional superconducting states.6 pages, 4 figures and suplemental materia
Nature of the Unconventional Heavy-Fermion Kondo State in Monolayer CeSiI
No full textCeSiI has been recently isolated in the ultrathin limit, establishing CeSiI as the first intrinsic two-dimensional van der Waals heavy-fermion material up to 85 K. We show that, due to the strong spin–orbit coupling, the local moments develop a multipolar real-space magnetic texture, leading to local pseudospins with a nearly vanishing net moment. To elucidate its Kondo-screened regime, we extract from first-principles the parameters of the Kondo lattice model describing this material. We develop a pseudofermion methodology in combination with ab initio calculations to reveal the nature of the heavy-fermion state in CeSiI. We analyze the competing magnetic interactions leading to an unconventional heavy-fermion order as a function of the magnetic exchange between the localized f-electrons and the strength of the Kondo coupling. Our results show that the magnetic exchange interactions promote an unconventional momentum-dependent Kondo-screened phase, establishing the nature of the heavy-fermion state observed in CeSiI.Peer reviewe
Lethal and sublethal effects of polybrominated diphenyl ethers (PBDEs) for turbot (_Psetta maxima_) early life stage (ELS)
No full textA new toxic menace, polybrominated diphenyl ethers (PBDEs), is being detected in the aquatic environment all over the world. The environmental presence of PBDEs and its entry into the environment as BDE-47 and -99 make quality aquatic toxicity data necessary to assess the aquatic hazard risk of PBDs. This study examines the effects of three PBDE-47 and -99 on embryo and larval stages of the marine flatfish turbot (_Psetta maxima_). The acute toxicity of the three PBDEs was examined and NOEC, LOEC, LC10 and LC50 were calculated. All tested compounds caused lethal as well as nonlethal malformations during embryo development. The effects of PBDEs in the different life stages of turbot were analysed. PBDEs seemed to be teratogenic at concentrations higher than 8.14 and 16.12 µgL-1 for BDE-47 and -99 respectively, leading to prolonged delays in embryo development and consequent death (at 48 h), as well as severe malformations and mortality of larvae. PBDEs showed a higher acute toxicity for embryo-larvae (LC50 for lethal endpoints to embryos 27.35 µg BDE-47 L-1 and 38.28 µg BDE-99 L-1, and 14.13 and 29.64 µg L-1 for BDE-47 and -99, respectively for larvae). Generally, the BDE-47 seemed to cause adverse effects at comparatively low dose rates, whereas much higher doses were needed to cause the same effects with BDE-99. The results of the present study show that the acute toxicity of PBDEs decreases as the degree of bromination increases, since the order of toxicity was BDE-47˃ BDE-99. The major isomers also exhibited a clear toxic potential for turbot ELS although at high concentrations, above solubility saturation, pointing at particulate matter as the main via of uptake for those hydrophobic molecules
Topological multiferroic order in twisted transition metal dichalcogenide bilayers
No full textLayered van der Waals materials have risen as powerful platforms to
artificially engineer correlated states of matter. Here we show the emergence
of a multiferroic order in a twisted dichalcogenide bilayer superlattice at
quarter-filling. We show that the competition between Coulomb interactions
leads to the simultaneous emergence of ferrimagnetic and ferroelectric orders.
We derive the magnetoelectric coupling for this system, which leads to a direct
strong coupling between the charge and spin orders. We show that, due to
intrinsic spin-orbit coupling effects, the electronic structure shows a
non-zero Chern number, thus displaying a topological multiferroic order. We
show that this topological state gives rise to interface modes at the different
magnetic and ferroelectric domains of the multiferroic. We demonstrate that
these topological modes can be tuned with external electric fields as well as
triggered by supermoire effects generated by a substrate. Our results put
forward twisted van der Waals materials as a potential platform to explore
multiferroic symmetry breaking orders and, ultimately, controllable topological
excitations in magnetoelectric domains.Comment: Submission to SciPost, 6 figure
