Spin-polarized Ground State Research Articles

Anomalous Landau Level Gaps Near Magnetic Transitions in Monolayer WSe2

First-order phase transitions produce abrupt changes to the character of both ground and excited electronic states. Here we conduct electronic compressibility measurements to map the spin phase diagram and Landau level (LL) energies of monolayer WSe2 in a magnetic field. We resolve a sequence of first-order phase transitions between completely spin-polarized LLs and states with LLs of both spins. Unexpectedly, the LL gaps are roughly constant over a wide range of magnetic fields below the transitions, which we show reflects spin-polarized ground states with opposite spin excitations. These transitions also extend into compressible regimes, with a sawtooth boundary between full and partial spin polarization. We link these observations to the important influence of LL filling on the exchange energy beyond a smooth density-dependent contribution. Our results show that WSe2 realizes a unique hierarchy of energy scales where such effects induce reentrant magnetic phase transitions tuned by density and magnetic field. Published by the American Physical Society 2024

Detecting fractionalization in critical spin liquids using color centers

Quantum spin liquids are highly entangled ground states of insulating spin systems, in which magnetic ordering is prevented down to the lowest temperatures due to quantum fluctuations. One of the most extraordinary characteristics of quantum spin liquid phases is their ability to support fractionalized, low-energy quasiparticles known as spinons, which carry spin-1/2 but bear no charge. Relaxometry based on color centers in crystalline materials—of which nitrogen-vacancy (NV) centers in diamond are a well-explored example—provides an exciting new platform to probe the spin spectral functions of magnetic materials with both energy and momentum resolution and to search for signatures of these elusive, fractionalized excitations. In this work, we theoretically investigate the color-center relaxometry of two archetypal quantum spin liquids: the two-dimensional U(1) quantum spin liquid with a spinon Fermi surface and the spin-1/2 antiferromagnetic spin chain. The former is characterized by a metallic, spin-split ground state of mobile, interacting spinons, which closely resembles a spin-polarized Fermi liquid ground state but with neutral quasiparticles. We show that the observation of the Stoner continuum and the collective spin wave mode in the spin spectral function would provide a strong evidence for the existence of spinons and fractionalization. In one dimension, mobile spinons form a Luttinger liquid ground state. We show that the spin spectral function exhibits strong features representing the collective density and spin-wave modes, which are broadened in an algebraic fashion with an exponent characterized by the Luttinger parameter. The possibilities of measuring these collective modes and detecting the power-law decay of the spectral weight using NV relaxometry are discussed. We also examine how the transition rates are modified by marginally irrelevant operators in the Heisenberg limit. Published by the American Physical Society 2024

Enhancing Coherence Times of Chromophore-Radical Molecular Qubits and Qudits by Rational Design.

An important criterion for quantum operations is long qubit coherence times. To elucidate the influence of molecular structure on the coherence times of molecular spin qubits and qudits, a series of molecules featuring perylenediimide (PDI) chromophores covalently linked to stable nitroxide radicals were synthesized and investigated by pulse electron paramagnetic resonance spectroscopy. Photoexcitation of PDI in these systems creates an excited quartet state (Q) followed by a spin-polarized doublet ground state (D0), which hold promise as spin qudits and qubits, respectively. By tailoring the molecular structure of these spin qudit/qubit candidates by selective deuteration and eliminating intramolecular motion, coherence times of Tm = 9.1 ± 0.3 and 4.2 ± 0.3 μs at 85 K for D0 and Q, respectively, are achieved. These coherence times represent a nearly 3-fold enhancement compared to those of the initial molecular design. This approach offers a rational structural design protocol for effectively extending coherence times in molecular spin qudits/qubits.

Point Defects in Silicon-Doped β-Ga2O3: Hybrid-DFT Calculations.

In this work, hybrid density functional theory calculations are used to evaluate the structural and electronic properties and formation energies of Si-doped β-Ga2O3. Overall, eight interstitial (Sii) and two substitutional (SiGa) positions are considered. In general, our results indicate that the formation energy of such systems is significantly influenced by the charge state of the defect. It is confirmed that it is energetically more favorable for the substitution process to proceed under Ga-poor growth conditions than under Ga-rich growth conditions. Furthermore, it is confirmed that the formation of SiGaI with a tetrahedral coordination geometry is more favorable than the formation of SiGaII with an octahedral one. Out of all considered interstitial positions, due to the negative formation energy of the Si +3 charge state at i8 and i9 interstitial positions over the wide range of Fermi energy, this type of defect can be spontaneously stable. Finally, due to a local distortion caused by the presence of the interstitial atom as well as its charge state, these systems obtain a spin-polarized ground state with a noticeable magnetic moment.

Nagaoka ferromagnetism in an array of phosphorene quantum dots.

We consider an array of four quantum dots defined in phosphorene containing three excess electrons, i.e., in the conditions of near half filling when itinerant Nagaoka ferromagnetism is expected to appear in a square array with isotropic interdot hopping. The interdot hopping in the array arranged in a square inherits the anisotropy from the form of the phosphorene conduction band. We apply the configuration interaction method for discussion of the appearance and stability of the spin-polarized ground state and discuss the compensation of the effective mass anisotropy by the geometry of the quantum dot array. Our study shows strong stability of Nagaoka ferromagnetism for optimized geometry of the array, with the Nagaoka gap as large as ∼230µeV. A phase diagram for the ground-state spin ordering versus the geometric parameters of the array is presented. We study the suppression of the ferromagnetism in a transition of the [Formula: see text] array to a quasi-1D chain and indicate that the shift of one of the quantum dots away from the array center is enough to transform the system to a quantum dot chain. A shift in the zigzag crystal direction induces the low-spin ground state more effectively than a shift along the armchair direction. We also discuss the robustness of the spin ordering against detuning one of the dots. The ferromagnetic ground-state survives as long as the detuning is not large enough to trap one of the electrons within a single quantum dot (for positive detuning) or remove one of the quantum dots of the accessible energy range (for negative detuning).

Tunable spin and conductance in porphyrin-graphene nanoribbon hybrids

Recently, porphyrin units have been attached to graphene nanoribbons (Por-GNR) enabling a multitude of structures. Here we report first-principles calculations of two prototypical, experimentally feasible, Por-GNR hybrids, one of which displays a small band gap relevant as electrodes in devices. Embedding a Fe atom in the porphyrin causes spin-polarized ground state (S = 1). Using density functional theory and nonequilibrium Green’s function, we examine a 2-terminal setup involving a Fe-Por-GNR between small band gap, Por-GNR electrodes. The coupling between the Fe-d and GNR band states results in a Fano anti-resonance feature in the spin transport, making the conductance highly sensitive to the Fe spin state. We demonstrate how mechanical strain or chemical adsorption on the Fe give rise to spin-crossover to S = 2 and S = 0, directly reflected in the transmission. Our results provide a deep understanding which can open an avenue for carbon-based spintronics and chemical sensing.

Tunable spin and valley excitations of correlated insulators in Γ-valley moiré bands.

Moiré superlattices formed from transition metal dichalcogenides support a variety of quantum electronic phases that are highly tunable using applied electromagnetic fields. While the valley degree of freedom affects optoelectronic properties in the constituent transition metal dichalcogenides, it has yet to be fully explored in moiré systems. Here we establish twisted double-bilayer WSe2 as an experimental platform to study electronic correlations within Γ-valley moiré bands. Through local and global electronic compressibility measurements, we identify charge-ordered phases at multiple integer and fractional moiré fillings. By measuring the magnetic field dependence of their energy gaps and the chemical potential upon doping, we reveal spin-polarized ground states with spin-polaron quasiparticle excitations. In addition, an applied displacement field induces a metal-insulator transition driven by tuning between Γ- and K-valley moiré bands. Our results demonstrate control over the spin and valley character of the correlated ground and excited states in this system.

Two-Dimensional Magnetic Semiconducting Heterostructures of Single-Layer CrI3-CrI2.

Single-layer heterostructures of magnetic materials are unique platforms for studying spin-related phenomena in two dimensions (2D) and have promising applications in spintronics and magnonics. Here, we report the fabrication of 2D magnetic lateral heterostructures consisting of single-layer chromium triiodide (CrI3) and chromium diiodide (CrI2). By carefully adjusting the abundance of iodine based on molecular beam epitaxy, single-layer CrI3-CrI2 heterostructures were grown on Au(111) surfaces with nearly atomic-level seamless boundaries. Two distinct types of interfaces, i.e., zigzag and armchair interfaces, have been identified by means of scanning tunneling microscopy. Our scanning tunneling spectroscopy study combined with density functional theory calculations indicates the existence of spin-polarized ground states below and above the Fermi energy localized at the boundary. Both the armchair and zigzag interfaces exhibit semiconducting nanowire behaviors with different spatial distributions of density of states. Our work presents a novel low-dimensional magnetic system for studying spin-related physics with reduced dimensions and designing advanced spintronic devices.

Optical Spin Polarization of a Narrow-Linewidth Electron-Spin Qubit in a Chromophore/Stable-Radical System.

Photoexcited organic chromophores appended to stable radicals can serve as qubit and/or qudit candidates for quantum information applications. 1,6,7,12-Tetra-(4-tert-butylphenoxy)-perylene-3,4 : 9,10-bis(dicarboximide) (tpPDI) linked to a partially deuterated α,γ-bisdiphenylene-β-phenylallyl radical (BDPA-d16 ) was synthesized and characterized by time-resolved optical and electron paramagnetic resonance (EPR) spectroscopies. Photoexcitation of tpPDI-BDPA-d16 results in ultrafast radical-enhanced intersystem crossing to produce a quartet state (Q) followed by formation of a spin-polarized doublet ground state (D0 ). Pulse-EPR experiments confirmed the spin multiplicity of Q and yielded coherence times of Tm =2.1±0.1 μs and 2.8±0.2 μs for Q and D0 , respectively. BDPA-d16 eliminates the dominant 1 H hyperfine couplings, resulting in a single narrow line for both the Q and D0 states, which enhances the spectral resolution needed for good qubit addressability.

Quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry: Magnetism, correlations, and phase transitions

We describe here a quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry. The simulator consists of a structured lateral gate confining two dimensional electrons in a quantum well into artificial minima arranged in a hexagonal lattice. The sublattice symmetry breaking is generated by forming an artificial triangular graphene quantum dot (ATGQD) with zigzag edges. The resulting extended Hubbard model generates tunable ratio of tunneling strength to electron-electron interactions and of sublattice symmetry with control over shape. The validity of the simulator is confirmed for small systems using mean-field and exact diagonalization many-body approaches which show that the ground state changes from a metallic to an antiferromagnetic (AF) phase by varying the distance between sites or depth of the confining potential. The one-electron spectrum of these triangular dots contains a macroscopically degenerate shell at the Fermi level. The shell persists at the mean-field level for weak interactions (metallic phase) but disappears for strong interactions, in the AF phase. We determine the effects of electron-electron interactions on the ground state, the total spin, and the excitation spectrum as a function of filling of the ATGQD. We find that the half-filled charge neutral shell leads to a partially spin polarized state in both metallic and AF regimes in accordance with Liebs theorem. In both regimes a relatively large gap separates the spin polarized ground state to the first excited many-body state at half filling of the degenerate shell. By adding or removing an electron, this gap drops dramatically, and alternate total spin states emerge with energies nearly degenerate to a spin polarized ground state.

Unveiling the Multiradical Character of the Biphenylene Network and Its Anisotropic Charge Transport.

Recent progress in the on-surface synthesis and characterization of nanomaterials is facilitating the realization of new carbon allotropes, such as nanoporous graphenes, graphynes, and 2D π-conjugated polymers. One of the latest examples is the biphenylene network (BPN), which was recently fabricated on gold and characterized with atomic precision. This gapless 2D organic material presents uncommon metallic conduction, which could help develop innovative carbon-based electronics. Here, using first principles calculations and quantum transport simulations, we provide new insights into some fundamental properties of BPN, which are key for its further technological exploitation. We predict that BPN hosts an unprecedented spin-polarized multiradical ground state, which has important implications for the chemical reactivity of the 2D material under practical use conditions. The associated electronic band gap is highly sensitive to perturbations, as seen in finite temperature (300 K) molecular dynamics simulations, but the multiradical character remains stable. Furthermore, BPN is found to host in-plane anisotropic (spin-polarized) electrical transport, rooted in its intrinsic structural features, which suggests potential device functionality of interest for both nanoelectronics and spintronics.

Paramagnetism of carbyne nanocrystals

In terms of carbon-atom hybridization, well-established forms of carbon are diamond with three-dimensional sp3-hybridization and graphite with two-dimensional sp2-hybridization which have been known as the first and second carbons and utilized for millennia. In fact, there is the third carbon with one-dimensional sp-hybridization, i.e. so-called carbyne. Recently, nanocrystals of the third carbon have been synthesized in the laboratory and they are called white carbon because these synthesized carbyne nanocrystals (CNCs) look white powders. Here, for the first time, the paramagnetism of CNCs is reported. Experimental observations show notable paramagnetism but no magnetic ordering at any given temperature. The moment concentration is estimated to be one moment per approximately 10,000 carbon atoms. These magnetic moments are probably originated from the collective magnetism phenomenon, which is attributed to hydrogen atom adsorbed on the carbon chain and vacancy in CNCs according to density functional theory-based first principle calculations. The localized defect wave favors a spin-polarized ground state and forms a local magnetic moment, promoting a long-range magnetic coupling between local magnetic moment, finally, results in a collective magnetism. This understanding about magnetism and its origin in the sp-hybridized carbon material can be instructive to interpretation of magnetism of other carbon allotropes.

Finite-thickness effect and spin polarization of the even-denominator fractional quantum Hall states

The spin-polarized even-denominator fractional quantum Hall (FQH) states in the second Landau level (LL), i.e. 5/2 and 7/2, may possess novel quasi-particle excitations obeying non-Abelian statistics. However, the spin polarization of the 7/2 FQH state has not been investigated experimentally and the spin polarization of the 5/2 FQH state from tilted field experiments remains controversial. Using a piezo-driven sample rotator with the lowest electron temperature down to 25 mK, we studied the energy gap of the even-denominator FQH states in the second LL by precise control of the tilted angles with a resolution less than 0.1{\deg}. We observed two different energy gap dependences on the in-plane magnetic field for 5/2, 7/2, other FQH states (7/3 and 8/3) in the second LL and reentrant integer quantum Hall (RIQH) states in the third LL. Though the transition fields vary from states, their corresponding in-plane magnetic lengths are comparable to the quantum well thickness of the sample, which may result from the influence of the finite-thickness effect. At low in-plane magnetic fields, before the conjectured finite-thickness effect starts to dominate, the energy gaps of both 5/2 and 7/2 states show a non-decreasing behavior, supporting spin-polarized ground states. Our results also suggest that the 7/3, 8/3 FQH states, and the RIQH states in the third LL are spin-polarized or partially spin-polarized.

Ab initio study of optoelectronic and magnetic properties of Mn-doped ZnS with and without vacancy defects

The effect of vacancy defects on optoelectronic and magnetic properties of Mn-doped ZnS have been systematically investigated using first principle approaches. A single Mn substitution at Zn site induces a spin-polarized ground state in pure ZnS with total magnetization 5 . Our results for magnetic coupling show that the coupling between Mn spins in pure Mn doped ZnS is antiferromagnetic under the super-exchange mechanism. The existence of native defects has a great influence on the magnetic ground state of Mn-doped ZnS. In particular, a p -type defect such as Zn vacancy play a crucial role in stabilizing ferromagnetic ground state while n-type defect, such as S vacancy, has no effect on the magnetic ground state i.e. the interaction between two Mn spins with S-vacancy remain antiferromagnetic. Furthermore, optical properties such as dielectric functions, absorption coeffiecients, reflectivity and transmissitivity for Mn doped systems with and without vacancy defect were also studied, and we found that an absorption peak was obtained in the infrared region which is attributed to the defect states introduced by Zn vacancy in the system. In a S-vacancy defect system, the peaks in the near infrared and visible region are due to donor states introduced by S vacancy defect and these peaks are produced by electrons flipping from a spin up state to a spin down state. Finally, we also correlated the magnetic interactions with the d–d optical transition in pure Mn-doped ZnS and found that the d–d transitions during optical absorptions are red shifted and blue shifted in FM and AFM coupled Mn ions pair, which is in good agreement with the experimental observations. This study may help to understand the behavior of optical and magnetic properties of DMS under vacancy defects.

First principles study on hydrogen storage in yttrium doped graphyne: Role of acetylene linkage in enhancing hydrogen storage

Through Density Functional Theory Simulations we predict that a Ytrrium atom attached on graphyne surface can adsorb up to a maximum of 9 molecular hydrogens (H2), with a uniform binding energy of ∼0.3 eV/H2 and an average desorption temperature of around 400 K (ideal for fuel cell applications), leading to 10 wt% of hydrogen, substantially higher than the requirement by DoE. The higher hydrogen wt% in Y doped graphyne compared to Y doped Single Walled Carbon Nanotubes (SWNT) and graphene is due to the presence of sp hybridized C atoms (in the acetylene linkage) supplying additional in-plane px-py orbitals leading to π (π*) bonding (antibonding) states. Charge transfer from metal to carbon nanostructure results in a redistribution of s, p, d orbitals of the metal leading to a non - spin polarized ground state in Y doped graphyne, due to the presence of the acetylene linkage, whereas Y doped SWNT and graphene remain magnetic like the isolated metal atom. In the non-magnetic graphyne + Y system, the net charge transfer from Y to successive H2 molecules is less than in magnetic Y + graphene and Y + SWNT systems, enabling Y + graphyne to store a larger number of H2 molecules. Furthermore, our ab initio MD simulations show that the system is stable even at room temperature and there is no dissociation of H2 molecules, enabling the system to achieve 100% desorption. So Y doped graphyne is found to be a promising hydrogen storage device with high wt%, 100% recyclability and desirable desorption temperature.

Functionalization of monolayer AsP phases by adatoms: a first-principles study

In this paper we investigated the adsorption selected single adatoms on a two-dimensional (2D) monolayer of AsP in buckled honeycomb as well as asymmetric washboard structures by using spin-polarized density functional theory. We found that adatoms are bounded to the surface of buckled and asymmetric washboard AsP phases and also introduced localized states in diverse locations of the energy gap. In addition, resonance states in the band continua of AsP phases are induced by the presence of the adatoms and hence modify electronic and magnetic properties. Adsorbed adatoms, namely group-VA adatoms N, P, As, Sb and Bi, can attribute permanent magnetic moments to both phases of monolayer AsP structures. This situation may attribute half-metallic character to ML-AsP phases covered by group-VA adatoms. While single C atom adsorbed to buckled AsP structure had spin-polarized magnetic ground state, asymmetric washboard AsP with a C atom is nonmagnetic as well as other group IV elements. Our results show that the electronic and magnetic properties of monolayer AsP phases can be modified by the adsorption of adatoms in order to various applications.

Comparison of Green’s functions for transition metal atoms using self-energy functional theory and coupled-cluster singles and doubles (CCSD)

We demonstrate in the present study that self-consistent calculations based on the self-energy functional theory (SFT) are possible for the electronic structure of realistic systems in the context of quantum chemistry. We describe the procedure of a self-consistent SFT calculation in detail and perform the calculations for isolated 3d transition metal atoms from V to Cu as a preliminary study. We compare the one-particle Green's functions obtained in this way and those obtained from the coupled-cluster singles and doubles method. Although the SFT calculation starts from the spin-unpolarized Hartree-Fock state for each of the target systems, the self-consistency loop correctly leads to degenerate spin-polarized ground states. We examine the spectral functions in detail to find their commonalities and differences among the atoms by paying attention to the characteristics of the two approaches. It is demonstrated via the two approaches that calculations based on the density functional theory (DFT) can fail in predicting the orbital energy spectra for spherically symmetric systems. It is found that the two methods are quite reliable and useful beyond DFT.

Structural, electronic and magnetic properties of Mn-doped and vacancy-doped 2D LiZnP

The electronic and magnetic properties of Mn-doped 2D LiZnP with and without vacancy are investigated within the framework of density functional theory. Our results indicate that the pristine 2D LiZnP still exhibit semiconductor character with a narrower band gap about 1.3 eV due to change of crystal structure. The Mn-doped 2D LiZnP favors spin-polarized ground states. The magnetic coupling results show that the Mn-doped 2D LiZnP exist the ferromagnetic stable state, which is mediated by the stronger interaction between Mn-3d and P-2p state. Both Li and Zn vacancies acting as acceptor defect increase the carrier concentration and thus enhance the magnetism. Among them, Li vacancies system can result in antiferromagnetic coupling among Mn impurities, while Zn vacancies system enhances the ferromagnetic stable state.

Mechanism of Magnetic Coupling in Carrier-Doped SnO Nanosheets

Two-dimensional materials continue to fascinate, for both their rich fundamental physics and their multifarious potential for applications. In such layered semiconductors, although the interlayer interactions are weak, they contribute significantly to overall electronic behavior. For example, this study shows that stable room-temperature ferromagnetism in bilayer SnO depends on the balance between hole doping and the interlayer interaction of lone-pair electrons. With its inherently flat structure and half-metallic, spin-polarized ground state, this compound is especially interesting for spintronic nanodevices.

Investigations on magnetic properties of Cr-doped LiZnP by first principle calculations

The electronic structures and magnetic properties for Cr doped LiZnP system are studied by first principles calculations. The results indicate that Cr doped LiZnP structures with Cr concentration 6.25 at.% favors spin polarized ground states and the system should be magnetic. The magnetic coupling results show that the system exists in the ferromagnetic (FM) stable state and the FM interactions in the Cr-Cr pair can be attributed to the p-d exchange interactions as Cr↑-P↓-Cr↑. VLi act as donor in Li(Zn,Cr)P host, which increasing the carrier concentration and thus lead to the FM state more stable. VZn has less influence on FM stability, and VP weakens FM stability of the system.