Dynamic molecular switches with hysteretic negative differential conductance emulating synaptic behaviour.

To realize molecular-scale electrical operations beyond the von Neumann bottleneck, new types of multifunctional switches are needed that mimic self-learning or neuromorphic computing by dynamically toggling between multiple operations that depend on their past. Here, we report a molecule that switches from high to low conductance states with massive negative memristive behaviour that depends on the drive speed and number of past switching events, with all the measurements fully modelled using atomistic and analytical models. This dynamic molecular switch emulates synaptic behavior and Pavlovian learning, all within a 2.4-nm-thick layer that is three orders of magnitude thinner than a neuronal synapse. The dynamic molecular switch provides all the fundamental logic gates necessary for deep learning because of its time-domain and voltage-dependent plasticity. The synapse-mimicking multifunctional dynamic molecular switch represents an adaptable molecular-scale hardware operable in solid-state devices, and opens a pathway to simplify dynamic complex electrical operations encoded within a single ultracompact component.

Electric-field-driven dual-functional molecular switches in tunnel junctions.

To avoid crosstalk and suppress leakage currents in resistive random access memories (RRAMs), a resistive switch and a current rectifier (diode) are usually combined in series in a one diode-one resistor (1D-1R) RRAM. However, this complicates the design of next-generation RRAM, increases the footprint of devices and increases the operating voltage as the potential drops over two consecutive junctions1. Here, we report a molecular tunnel junction based on molecules that provide an unprecedented dual functionality of diode and variable resistor, resulting in a molecular-scale 1D-1R RRAM with a current rectification ratio of 2.5 × 104 and resistive on/off ratio of 6.7 × 103, and a low drive voltage of 0.89 V. The switching relies on dimerization of redox units, resulting in hybridization of molecular orbitals accompanied by directional ion migration. This electric-field-driven molecular switch operating in the tunnelling regime enables a class of molecular devices where multiple electronic functions are preprogrammed inside a single molecular layer with a thickness of only 2 nm.

Three-leaf quantum interference clovers in a trigonal single-molecule magnet.

We report on a single-molecule magnet where the spatial arrangement of three manganese ions and their spin-orbit coupling tensor orientations result in threefold angular modulations of the magnetization tunneling rates and quantum interference patterns that mimic the form of a three-leaf clover. Although expected in all quantum tunneling of magnetization resonances for a trigonal molecular symmetry, the threefold modulation only appears at resonances for which a longitudinal magnetic field is applied (i.e., resonance numbers |k|>0). A sixfold transverse field modulation observed at resonance k = 0 manifests as a direct consequence of a threefold corrugation of the spin-orbit coupling energy landscape, creating an effective longitudinal field which varies the resonance condition in the presence of a transverse field. The observations allow for an association between the trigonal distortion of the local spin-orbit interactions and the spatial disposition of the constituent ions, a finding that can be extrapolated to other systems where spin-orbit coupling plays a significant role.

Water-stable manganese(IV) complex of a N2O4-donor non-Schiff-base ligand: synthesis, structure, and multifrequency high-field electron paramagnetic resonance studies.

A novel water-stable (t1/2 ∼ 6.8 days) mononuclear manganese(IV) complex of a hexacoordinating non-Schiff-base ligand (H4L) with N2O4-donor atoms has been synthesized and characterized crystallographically. High-frequency electron paramagnetic resonance experiments performed on a single crystal reveal a manganese(IV) ion with an S = 3/2 ground spin state that displays a large single-ion anisotropy, setting the record of mononuclear manganese(IV) complexes reported so far. In addition, spin-echo experiments reveal a spin-spin relaxation time T2 ∼ 500 ns.

A 220 GHz-1.1 THz continuous frequency and polarization tunable quasi-optical electron paramagnetic resonance spectroscopic system.

Here, we describe a custom-designed quasi-optical system continuously operating in the frequency range 220 GHz to 1.1 THz with a temperature range of 5-300 K and magnetic fields up to 9 T capable of polarization rotation in both transmitter and receiver arms at any given frequency within the range through a unique double Martin-Puplett interferometry approach. The system employs focusing lenses to amplify the microwave power at the sample position and recollimate the beam to the transmission branch. The cryostat and split coil magnets are furnished with five optical access ports from all three major directions to the sample sitting on a two-axes rotatable sample holder capable of performing arbitrary rotations with respect to the field direction, enabling broad accessibility to experimental geometries. Initial results from test measurements on antiferromagnetic MnF2 single crystals are included to verify the operation of the system.

Stable Universal 1- and 2-Input Single-Molecule Logic Gates.

Controllable single-molecule logic operations will enable development of reliable ultra-minimalistic circuit elements for high-density computing but require stable currents from multiple orthogonal inputs in molecular junctions. Utilizing the two unique adjacent conductive molecular orbitals (MOs) of gated Au/S-(CH2 )3 -Fc-(CH2 )9 -S/Au (Fc = ferrocene) single-electron transistors (≈2 nm), a stable single-electron logic calculator (SELC) is presented, which allows real-time modulation of output current as a function of orthogonal input bias (Vb ) and gate (Vg ) voltages. Reliable and low-voltage (ǀVb ǀ ≤ 80 mV, ǀVg ǀ ≤ 2 V) operations of the SELC depend upon the unambiguous association of current resonances with energy shifts of the MOs (which show an invariable, small energy separation of ≈100 meV) in response to the changes of voltages, which is confirmed by electron-transport calculations. Stable multi-logic operations based on the SELC modulated current conversions between the two resonances and Coulomb blockade regimes are demonstrated via the implementation of all universal 1-input (YES/NOT/PASS_1/PASS_0) and 2-input (AND/XOR/OR/NAND/NOR/INT/XNOR) logic gates.

Cationic Mn4 single-molecule magnet with a sterically isolated core.

The synthesis, structure, and magnetic properties of a ligand-modified Mn(4) dicubane single-molecule magnet (SMM), [Mn(4)(Bet)(4)(mdea)(2)(mdeaH)(2)](BPh(4))(4), are presented, where the cationic SMM units are significantly separated from neighboring molecules in the crystal lattice. There are no cocrystallized solvate molecules, making it an ideal candidate for single-crystal magnetization hysteresis and high-frequency electron paramagnetic resonance studies. Increased control over intermolecular interactions in such materials is a crucial factor in the future application of SMMs.

A single atom change turns insulating saturated wires into molecular conductors.

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, ß, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of ß from 0.75 to 0.25 Å-1. Impedance measurements show tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in ß. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. Here, we demonstrate experimentally that [Formula: see text], suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

Bias-Polarity-Dependent Direct and Inverted Marcus Charge Transport Affecting Rectification in a Redox-Active Molecular Junction.

This paper describes the transition from the normal to inverted Marcus region in solid-state tunnel junctions consisting of self-assembled monolayers of benzotetrathiafulvalene (BTTF), and how this transition determines the performance of a molecular diode. Temperature-dependent normalized differential conductance analyses indicate the participation of the HOMO (highest occupied molecular orbital) at large negative bias, which follows typical thermally activated hopping behavior associated with the normal Marcus regime. In contrast, hopping involving the HOMO dominates the mechanism of charge transport at positive bias, yet it is nearly activationless indicating the junction operates in the inverted Marcus region. Thus, within the same junction it is possible to switch between Marcus and inverted Marcus regimes by changing the bias polarity. Consequently, the current only decreases with decreasing temperature at negative bias when hopping is "frozen out," but not at positive bias resulting in a 30-fold increase in the molecular rectification efficiency. These results indicate that the charge transport in the inverted Marcus region is readily accessible in junctions with redox molecules in the weak coupling regime and control over different hopping regimes can be used to improve junction performance.

Large spin-state changes in isostructural cyanate- and azide-bridged Mn(3)(III)Mn(2)(II) single-molecule magnets.

We prepared three structurally related Mn(3)(III)Mn(2)(II) complexes that possess S approximately 1-11 spin ground states as a result of variations in the geometry and identity of mu(2)-eta(1):eta(1) bridging groups. These complexes function as single-molecule magnets yet demonstrate other interesting behavior such as quasi-classical magnetization hysteresis and comparable magnetization reversal barriers (U(eff)).

Ferromagnetic ordering and simultaneous fast magnetization tunneling in a Ni4 single-molecule magnet.

Low-temperature heat capacity and oriented single-crystal field-cooled and zero-field-cooled magnetization data for the single-molecule magnet [Ni(hmp)(dmb)Cl](4) are presented that indicate the presence of ferromagnetic ordering at approximately 300 mK, which has little effect on the magnetization relaxation rates.

Subterahertz spin pumping from an insulating antiferromagnet.

Spin-transfer torque and spin Hall effects combined with their reciprocal phenomena, spin pumping and inverse spin Hall effects (ISHEs), enable the reading and control of magnetic moments in spintronics. The direct observation of these effects remains elusive in antiferromagnetic-based devices. We report subterahertz spin pumping at the interface of the uniaxial insulating antiferromagnet manganese difluoride and platinum. The measured ISHE voltage arising from spin-charge conversion in the platinum layer depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized subterahertz irradiation. Our results open the door to the controlled generation of coherent, pure spin currents at terahertz frequencies.

Nanomodulation of molecular nanomagnets.

Detailed synthetic, structural, and magnetic characterizations for a family of six [Mn(3)Zn(2)](13+) complexes are presented. These complexes have planar [Mn(3)(III)-(mu(3)-oxo)](7+) core magnetic units and have formulas represented by [cation](3)[Mn(3)Zn(2)(R-salox)(3)O(N(3))(6)X(2)], where [cation](+) = [NEt(4)](3)(+) or [AsPh(4)](3)(+); R = H or Me; and X = Cl(-), Br(-), I(-), or N(3)(-). Least-squares fits to the magnetic susceptibility data for these complexes indicate large negative values of the axial zero field splitting (ZFS) parameter D (approximately -1.1 K) and spin ground states ranging from a highly spin-mixed S approximately 1 to a reasonably isolated S = 6 (DeltaE(S = 5) = 69.2 K). The strength and magnitude of the intramolecular exchange interactions have been observed to change with the crystal packing as a result of systematic variations in the co-crystallizing cation, terminal ion, and oximate ligand. Alternating current susceptibility data were collected from 1.8-7 K at 10-997 Hz, revealing strong frequency-dependent peaks in the out-of-phase susceptibility (chi''(M)) for ferromagnetic S = 6 complexes 1, 2, and 6. Fitting of these data to the Arrhenius equation gave U(eff) = 44.0 K and tau(0) = 3.8 x 10(-8) s for [NEt(4)](3)[Mn(3)Zn(2)(salox)(3)O(N(3))(6)Cl(2)] (1), and U(eff) = 45.6 K and tau(0) = 2.1 x 10(-7) s for [NEt(4)](3)[Mn(3)Zn(2)(Me-salox)(3)O(N(3))(6)Cl(2)] (6). The enhanced relaxation behavior in complex 6 is associated with stronger ferromagnetic exchange interactions and a more isolated S = 6 ground state than in 1 and 2. Comprehensive high-frequency electron paramagnetic resonance (HFEPR) experiments were conducted on single crystals of complexes 1, 2, and 6, revealing sharp absorption peaks and allowing for the precise determination of ZFS parameters. Similar experiments on [AsPh(4)](3)[Mn(3)Zn(2)(salox)(3)O(N(3))(6)Cl(2)] (4) resulted in the observation of a broad absorption peak, consistent with the highly spin-mixed ground state. Single crystal magnetization hysteresis measurements on complexes 1 and 2 indicate SMM behavior via temperature- and sweep-rate dependent hysteresis loops and the observance of very sharp quantum tunneling resonances. Additionally, the Hamiltonian parameters derived from the magnetic data, HFEPR, and hysteresis measurements are in good agreement and highlight the relationships between superexchange, spin-orbit interactions, and the varied relaxation behavior in these complexes.

Sub-Kelvin (100 mK) time resolved electron paramagnetic resonance spectroscopy for studies of quantum dynamics of low-dimensional spin systems at low frequencies and magnetic fields.

This article presents a time-resolved electron paramagnetic resonance spectrometry setup designed to work at frequencies below 20 GHz and temperatures down to 50 mK. The setup consists of an on-chip microstrip resonator (Q < 100) placed in a dilution cryostat located within a superconducting 3D vector magnet. A housemade spin echo circuitry controlled by a microwave network analyzer, a pulse pattern generator, and an oscilloscope connects to the microstrip through a series of copper, stainless steel, and superconducting semirigid coaxial lines which are thermally anchored to the different cooling stages of the fridge by means of power attenuators, circulators, and a cryogenic amplifier. Spin echo experiments were performed at a 0.5-T magnetic field on a spin 1 2 paramagnetic coal marker sample mounted on a 15 GHz microstrip resonator at temperatures ranging from 100 to 800 mK. The results show an increase in echo signal intensity as temperature is decreased until saturation as theoretically expected in reaching 99% spin polarization at 100 mK. Our technique allows tuning of the spin system in the pure-state regime and minimizing dipolar fluctuations, which are the main contribution to decoherence in solid-state samples of single-molecule magnets (SMMs) - molecular spin systems that are currently being tested for applications in quantum computation. The achievement of full spin polarization at 100 mK will allow for coherent control over the time evolution of spin systems without the need for large magnetic fields (commonly used to polarize the dipolar bath at higher temperatures) and high frequencies.

Single-molecule-magnet behavior and spin changes affected by crystal packing effects.

Five Mn 3Zn 2 heterometallic complexes have been synthesized and structurally and magnetically characterized. Spin ground states up to S = 6 have been observed for these complexes and are shown to depend on the cocrystallizing cation and on the terminal ligand. Large axial zero-field interactions ( D = -1.16 K) are the result of near-parallel alignment of the Mn (III) Jahn-Teller axes. High-frequency electron paramagnetic resonance, single-crystal magnetization hysteresis, and alternating current susceptibility measurements are presented to characterize [NEt 4] 3[Mn 3Zn 2(salox) 3O(N 3) 6X 2] [X (-) = Cl (-) ( 1), Br (-) ( 2)] and [AsPh 4] 3[Mn 3Zn 2(salox) 3O(N 3) 6Cl 2] ( 3) and reveal that 1 and 2 are single-molecule magnets ( U eff = 44 K), while 3 is not.

Molecular wheels: new Mn12 complexes as single-molecule magnets.

The preparation, structure and magnetic properties of three new wheel-shaped dodecanuclear manganese complexes, [Mn12(Adea)8(CH3COO)14] x 7 CH3CN (1 x 7CH3CN), [Mn12(Edea)8(CH3CH2COO)14] (2) and [Mn12(Edea)8(CH3COO)2(CH3CH2COO)12] (3), are reported, where Adea(2-) and Edea(2-) are dianions of the N-allyl diethanolamine and the N-ethyl diethanolamine ligands, respectively. Each complex has six Mn(II) and six Mn(III) ions alternating in a wheel-shaped topology, with eight n-substituted diethanolamine dianions. All variable-temperature direct current (DC) magnetic susceptibility data were collected in 1, 0.1, or 0.01 T fields and in the 1.8-300 K temperature range. Heat capacity data, collected in applied fields of 0-9 T and in the 1.8-100 K temperature range, indicate the absence of a phase-transition due to long-range magnetic ordering for 1 and 3. Variable-temperature, variable-field DC magnetic susceptibility data were obtained in the 1.8-10 K and 0.1-5 T ranges. All complexes show out-of-phase signals in the AC susceptibility measurements, collected in a 50-997 Hz frequency range and in a 1.8-4.6 K temperature range. Extrapolation to 0 K of the in-phase AC susceptibility data collected at 50 Hz indicates an S = 7 ground state for 1, 2, and 3. Magnetization hysteresis data were collected on a single crystal of 1 in the 0.27-0.9 K range and on single crystals of 2 and 3 in the 0.1-0.9 K temperature range. Discrete steps in the magnetization curves associated with resonant quantum tunneling of magnetization (QTM) confirm these complexes to be single-molecule magnets. The appearance of extra QTM resonances on the magnetic hysteresis of 1 is a result of a weak coupling between two Mn ions at opposite ends of the wheel, dividing the molecule into two ferromagnetic exchange-coupled S = 7/2 halves. The absence of these features on 2 and 3, which behave as rigid spin S = 7 units, is a consequence of different interatomic distances.

Wheel-shaped Mn16 single-molecule magnets.

The syntheses, structures, and magnetic properties of two new single-stranded hexadecanuclear manganese wheels [Mn16(CH3COO)8(CH3CH2CH2COO)8(teaH)12] x 10 MeCN (1 x 10 MeCN) and [Mn16((CH3)2CHCOO)16(teaH)12] x 4 CHCl3 (2 x 4 CHCl3), where teaH(2-) is the dianion of triethanolamine, are reported. 1 crystallizes in the tetragonal I4(1)/a space group [a = b = 33.519(4) A and c = 16.659(2) A]. 2 crystallizes in the monoclinic C2/c space group [a = 21.473(5), b = 26.819(6), c = 35.186(7), and beta = 93.447(5) degrees]. Both complexes consist of 8 Mn(II) and 8 Mn(III) ions alternating in a wheel-shaped topology with 12 monoprotonated triethanolamine ligands. Variable-temperature direct current (DC) magnetic susceptibility data were collected in 1 T, 0.1 and 0.01 T fields, and in the 1.8-300 K temperature range for 1 and 2. Variable-temperature variable-field DC magnetic susceptibility data were obtained in the 1.8-10 K and 0.1-5 T ranges and least-squares fitting of these reduced magnetization versus H/T data indicates a S = 13 ground-state for 1 and 2. Single-crystal magnetization hysteresis measurements were performed in a 0.04-1 K temperature range for complex 2. Hysteresis loops were observed that showed a temperature dependence, which indicates that 2 exhibits magnetization relaxation and is a SMM. Both 1 and 2 show frequency-dependent out-of-phase signals in the AC susceptibility measurements, collected in a temperature range of 1.8-5 K and in the frequency range of 50-10,000 Hz. Extrapolation of the in-phase component of the AC susceptibility data to 0 K indicates an S = 12 ground state for 1 and an S = 11 ground-state for 2. Complex 1 has the highest-spin ground state reported to date for a single-stranded manganese wheel and is likely to be an SMM based on a frequency-dependent out-of-phase signal in the AC susceptibility. The AC susceptibility as well as magnetization hysteresis data for 2 confirm that this species is an SMM.

How to distinguish between interacting and noninteracting molecules in tunnel junctions.

Recent experiments demonstrate a temperature control of the electric conduction through a ferrocene-based molecular junction. Here we examine the results in view of determining means to distinguish between transport through single-particle molecular levels or via transport channels split by Coulomb repulsion. Both transport mechanisms are similar in molecular junctions given the similarities between molecular intralevel energies and the charging energy. We propose an experimentally testable way to identify the main transport process. By applying a magnetic field to the molecule, we observe that an interacting theory predicts a shift of the conductance resonances of the molecule whereas in the noninteracting case each resonance is split into two peaks. The interaction model works well in explaining our experimental results obtained in a ferrocene-based single-molecule junction, where the charge degeneracy peaks shift (but do not split) under the action of an applied 7-Tesla magnetic field. This method is useful for a proper characterization of the transport properties of molecular tunnel junctions.

Molecular diodes with rectification ratios exceeding 105 driven by electrostatic interactions.

Molecular diodes operating in the tunnelling regime are intrinsically limited to a maximum rectification ratio R of ∼103. To enhance this rectification ratio to values comparable to those of conventional diodes (R ≥ 105) an alternative mechanism of rectification is therefore required. Here, we report a molecular diode with R = 6.3 × 105 based on self-assembled monolayers with Fc-C≡C-Fc (Fc, ferrocenyl) termini. The number of molecules (n(V)) involved in the charge transport changes with the polarity of the applied bias. More specifically, n(V) increases at forward bias because of an attractive electrostatic force between the positively charged Fc units and the negatively charged top electrode, but remains constant at reverse bias when the Fc units are neutral and interact weakly with the positively charged electrode. We successfully model this mechanism using molecular dynamics calculations.