# Professor Lesley F Cohen

Faculty of Natural SciencesDepartment of Physics

Professor of Solid State Physics

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### Contact

+44 (0)20 7594 7598l.cohen CV

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### Assistant

Mrs Carolyn Dale +44 (0)20 7594 7579

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### Location

1111Blackett LaboratorySouth Kensington Campus

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## Publications

Publication Type
Year
to

434 results found

Cohen L, Johnson F, 2022, Antiferromagnetic domain orientation in Mn3NiN by scanning Anomalous Nernst Effect microscopy, Applied Physics Letters, ISSN: 0003-6951

Journal article

Bennett TLR, Alshammari M, Au-Yong S, Almutlg A, Wang X, Wilkinson LA, Albrecht T, Jarvis SP, Cohen LF, Ismael A, Lambert CJ, Robinson BJ, Long NJet al., 2022, Multi-component self-assembled molecular-electronic films: towards new high-performance thermoelectric systems, CHEMICAL SCIENCE, Vol: 13, Pages: 5176-5185, ISSN: 2041-6520

Journal article

Bader SD, Lake RK, Cohen LF, 2022, Applied Physics Letters 2022 60th Anniversary Editorial, Applied Physics Letters, Vol: 120, Pages: 110401-110401, ISSN: 0003-6951

Journal article

Bower R, Wells MP, Johnson F, Kilmurray R, Doiron B, Calì E, Mallia G, Zou B, Mihai AP, Harrison NM, Fearn S, Oulton R, Alford NM, Cohen LF, Petrov PKet al., 2021, Tunable double epsilon-near-zero behavior in niobium oxynitride thin films, Applied Surface Science, Vol: 569, Pages: 150912-150912, ISSN: 0169-4332

Journal article

Johnson F, Boldrin D, Zemen J, Pesquera D, Kim J, Moya X, Zhang H, Singh HK, Samathrakis I, Cohen Let al., 2021, Strain dependence of Berry-phase-induced anomalous Hall effect in the non-collinear antiferro-magnet Mn3NiN, Applied Physics Letters, Vol: 119, ISSN: 0003-6951

The anomalous Hall effect (AHE) has been shown to be present in certain non-collinear antiferromagnets due to their symmetry-breaking magnetic structure, and its magnitude is dependent primarily on the non-zero components of the Berry curvature. In the non-collinear antiferromagnet Mn3NiN, the Berry phase contribution has been predicted to have strong strain dependence, although in practice, direct observation may be obscured by other strain-related influences—for instance, magnetic phase transitions mediated by strain. To unravel the various contributions, we examine the thickness and temperature dependence of the AHE for films grown on the piezoelectric substrate BaTiO3. We observe a systematic reduction in TN due to increased compressive strain as film thickness is reduced and a linear decrease in the AHE magnitude as the films are cooled from their ferrimagnetic phase above TN to their antiferromagnetic phase below. At 190 K, we applied an electric field across a 0.5 mm thick BaTiO3 substrate with a 50 nm thick Mn3NiN film grown on top and we demonstrate that at the coercive field of the piezoelectric substrate, the tensile in-plane strain is estimated to be of the order of 0.15%, producing a 20% change in AHE. Furthermore, we show that this change is, indeed, dominated by the intrinsic strain dependence of the Berry curvature.

Journal article

Boldrin D, Cohen L, Mendive-Tapia E, Zemen J, Staunton JB, Gomes AM, Ghivelder L, Halpin J, Gibbs A, Aznar A, Tamarit J-L, Lloveras P, Moya X, Cohen LFet al., 2021, Barocaloric properties of quaternary Mn3(Zn,In)N for room temperature refrigeration applications, Physical Review B: Condensed Matter and Materials Physics, Vol: 104, Pages: 1-7, ISSN: 1098-0121

The magnetically frustrated manganese nitride antiperovskite family display significant changesof entropy under hydrostatic pressure that can be useful for the emerging field of barocaloric cool-ing. Here we show that barocaloric properties of metallic antiperovskite Mn nitrides can be tai-lored for room temperature application through quaternary alloying.We find an enhanced en-tropy change of ∆St= 37 J K−1kg−1at theTt= 300 K antiferromagnetic transition of quater-nary Mn3Zn0.5In0.5N relative to the ternary end-members.The pressure-driven barocaloric entropychange of Mn3Zn0.5In0.5N reaches ∆SBCE= 20 J K−1kg−1in 2.9 kbar.Our results open up a largephase space where new compounds with improved barocaloric properties may be found.

Journal article

Wang X, Ismael A, Almutlg A, Alshammari M, Al-Jobory A, Alshehab A, Bennett TLR, Wilkinson LA, Cohen LF, Long NJ, Robinson BJ, Lambert Cet al., 2021, Optimised power harvesting by controlling the pressure applied to molecular junctions, Chemical Science, Vol: 12, Pages: 5230-5235, ISSN: 2041-6520

A major potential advantage of creating thermoelectric devices using self-assembled molecular layers is their mechanical flexibility. Previous reports have discussed the advantage of this flexibility from the perspective of facile skin attachment and the ability to avoid mechanical deformation. In this work, we demonstrate that the thermoelectric properties of such molecular devices can be controlled by taking advantage of their mechanical flexibility. The thermoelectric properties of self-assembled monolayers (SAMs) fabricated from thiol terminated molecules were measured with a modified AFM system, and the conformation of the SAMs was controlled by regulating the loading force between the organic thin film and the probe, which changes the tilt angle at the metal-molecule interface. We tracked the thermopower shift vs. the tilt angle of the SAM and showed that changes in both the electrical conductivity and Seebeck coefficient combine to optimize the power factor at a specific angle. This optimization of thermoelectric performance via applied pressure is confirmed through the use of theoretical calculations and is expected to be a general method for optimising the power factor of SAMs.

Journal article

Zverev VI, Gimaev RR, Miyanaga T, Vaulin AA, Gubkin AF, Kovalev BB, dos Santos AM, Lovell E, Cohen LF, Zarkevich NAet al., 2021, Peculiarities of the phase transformation dynamics in bulk FeRh based alloys from magnetic and structural measurements, Journal of Magnetism and Magnetic Materials, Vol: 522, Pages: 1-10, ISSN: 0304-8853

We analyze coexistence of antiferromagnetic and ferromagnetic phases in bulk iron-rhodium and its alloys with palladium, Fe50,4Rh49,6, Fe49,7Rh47,4Pd2,9 and Fe48,3Rh46,8Pd4,9, using neutron diffraction, magnetization and scanning Hall probe imaging. Temperature dependencies of the lattice parameters, AFM and FM phase weight fractions, and Fe magnetic moment values were obtained on cooling and heating across the AFM-FM transition. Substantial thermomagnetic hysteresis for the phases’ weight fractions and a relatively narrow one for the unit cell volume has been observed on cooling-heating. A clear dependence of hysteretic behavior on Pd concentration has been traced. Additional direct magnetic measurements of the spatial distribution of the phase transition are acquired using scanning Hall probe microscopy, which reveals the length scale of the phase coexistence and the spatial progression of the transition in the presence of external magnetic field. Also, the magnetic phase diagram has been constructed for a series of Pd-doped FeRh alloys.

Journal article

Komori S, Devine-Stoneman JM, Ohnishi K, Yang G, Devizorova Z, Mironov S, Montiel X, Olde Olthof LAB, Cohen L, Kurebayashi H, Blamire MG, Buzdin AI, Robinson JWAet al., 2021, Spin-orbit coupling suppression and singlet-state blocking of spin-triplet Cooper pairs, Science Advances, Vol: 7, Pages: 1-7, ISSN: 2375-2548

An inhomogeneous magnetic exchange field at a superconductor/ferromagnet interface converts spin-singlet Cooper pairs to a spin-aligned (i.e. spin-polarized) triplet state. Although the decay envelope of such triplet pairs within ferromagnetic materials is well studied, little is known about their decay in non-magnetic metals and superconductors, and in particular in presence of spin-orbit coupling. Here we report devices in which triplet supercurrents are created and are injected into the s-wave superconductor Nb. In the normal state of Nb, triplet pairs decay over a distance of 5 nm, which is an order of magnitude smaller than the decay of zero spin singlet pairs due to the spin-orbit coupling interacting with the spin associated with a triplet supercurrent. In the superconducting state of Nb, triplet supercurrents are blocked by the lack of available equilibrium states in the singlet superconducting gap. The results offer new insight into the dynamics between s-wave singlet and triplet states.

Journal article

Mendonca AA, Ghivelder L, Bernardo PL, Gu H, James RD, Cohen L, Gomes AMet al., 2020, Experimentally correlating thermal hysteresis and phase compatibility in multifunctional Heusler alloys, Physical Review Materials, Vol: 4, ISSN: 2475-9953

Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first-order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural, and magnetic properties of the Ni2Mn1−xCuxGa0.84Al0.16 Heusler alloys. The alloys present a thermal hysteresis reduction of ∼60% when the Cu content in the compound varies from x=0.10 to x=0.25, with a minimum hysteresis width of 6 K being achieved. We applied the geometric nonlinear theory of martensite to address the phase compatibility, quantified by the parameter λ2, the middle eigenvalue of the transformation stretch tensor, and found that the minimum of hysteresis is associated with a better crystallographic compatibility (λ2 closer to 1) between the austenite and martensite phases. In addition, we show that the valleylike properties of hysteresis found in the Ni2Mn1−xCuxGa0.84Al0.16 compounds is present in several other alloys in the literature. These results provide pathways to understand as well as to master the phase compatibility and ultimately achieve a low thermal hysteresis in multifunctional Heusler alloys.

Journal article

Cohen LF, 2020, To boldly go: new frontiers for APL, Applied Physics Letters, Vol: 117, ISSN: 0003-6951

Journal article

Jeon K-R, Montiel X, Komori S, Ciccarelli C, Haigh J, Kurebayashi H, Cohen L, Chan AK, Stenning KD, Lee C-M, Blamire MG, Robinson JWAet al., 2020, Tunable pure spin supercurrents and the demonstration 3 of their gateability in a spin-wave device, Physical Review X, Vol: 10, ISSN: 2160-3308

Recent ferromagnetic resonance experiments and theory of Pt/Nb/Ni8Fe2 proximity-coupled structures strongly suggest that spin-orbit coupling (SOC) in Pt in conjunction with a magnetic exchange field in Ni8Fe2 are the essential ingredients to generate a pure spin supercurrent channel in Nb. Here, by substituting Pt for a perpendicularly magnetized Pt/Co/Pt spin-sink, we are able to demonstrate the role of SOC, and show that pure spin supercurrent pumping efficiency across Nb is tunable by controlling the magnetization direction of Co. By inserting a Cu spacer with weak SOC between Nb and Pt/(Co/Pt) spin-sink, we also prove that Rashba-type SOC is key for forming and transmitting pure spin supercurrents across Nb. Finally, by engineering these properties within a single multilayer structure, we demonstrate a prototype superconducting spin-wave (SW) device in which lateral SW propagation is gateable via the opening or closing of a vertical pure spin supercurrent channel in Nb.

Journal article

Cohen L, 2020, Tuning the thermoelectrical properties of anthracene-based self-assembled monolayers, Chemical Science, Vol: 11, Pages: 6836-6841, ISSN: 2041-6520

It is known that the electrical conductance of single molecules can be controlled in a deterministic manner by chemically varying their anchor groups to external electrodes. Here, by employing synthetic methodologies to vary the terminal anchor groups around aromatic anthracene cores, and by forming self-assembled monolayers (SAMs) of the resulting molecules, we demonstrate that this method of control can be translated into cross-plane SAM-on-gold molecular films. The cross-plane conductance of SAMs formed from anthracene-based molecules with four different combinations of anchors are measured to differ by a factor of approximately 3 in agreement with theoretical predictions. We also demonstrate that the Seebeck coefficient of such films can be boosted by more than an order of magnitude by an appropriate choice of anchor groups and that both positive and negative Seebeck coefficients can be realised. This demonstration that the thermoelectric properties of SAMs are controlled by their anchor groups represents a critical step towards functional ultra-thin-film devices for future molecular-scale electronics.

Journal article

Wang X, Bennett TLR, Ismael A, Wilkinson LA, Hamill J, White AJP, Grace IM, Kolosov OV, Albrecht T, Robinson BJ, Long NJ, Cohen LF, Lambert CJet al., 2020, Scale-up of room-temperature constructive quantum interference from single molecules to self-assembled molecular-electronic films, Journal of the American Chemical Society, Vol: 142, Pages: 8555-8560, ISSN: 0002-7863

The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up of QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on CQI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ∼50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step toward functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.

Journal article

Doiron B, Gusken NA, Lauri A, Li Y, Mihai A, Matsui T, Bower R, Huettenhoffer L, Regoutz A, Forno SD, Fearn S, Petrov PK, Cortes E, Cohen LF, Alford NM, Lischner J, Petrov P, Maier SA, Oulton RFet al., 2020, Hot Carrier Optoelectronics with Titanium Nitride, Lasers and Electro-Optics Society Annual Meeting-LEOS, ISSN: 1092-8081

© 2020 OSA. Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.

Conference paper

Guo L, Lovell E, Tang Q, Boldrin DC, Tang CC, Day SJ, Cohen LF, Ryan MPet al., 2020, Fine control of Curie temperature of magnetocaloric alloys La(Fe,Co,Si)(13) using electrolytic hydriding, Scripta Materialia, Vol: 175, Pages: 33-37, ISSN: 1359-6462

This work demonstrates precision control of hydrogen content in La(Fe,Co,Si)13Hδ for the development of environmentally friendly magnetocaloric-based cooling technologies, using an electrolytic hydriding technique. We show the Curie temperature, a critical parameter which directly governs the temperature window of effective cooling, can be varied easily and reproducibly in 1 K steps within the range 274 K to 402 K. Importantly, both partially (up to 10%) and fully hydrided compositions retain favorable entropy change values comparable to that of the base composition. Crucially, we show in these second-order phase transition compounds, partial hydriding is stable and not susceptible against phase separation.

Journal article

Cohen LF, 2020, Applied Physics Letters welcomes papers in quantum technologies, Applied Physics Letters, Vol: 116, ISSN: 0003-6951

Journal article

Doiron B, Güsken NA, Lauri A, Li Y, Mihai A, Matsui T, Bower R, Huettenhoffer L, Regoutz A, Forno SD, Fearn S, Petrov PK, Cortés E, Cohen LF, Alford NM, Lischner J, Petrov P, Maier SA, Oulton RFet al., 2020, Hot carrier optoelectronics with titanium nitride

Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.

Conference paper

Gusken NA, Lauri A, Li Y, Jacassi A, Matsui T, Doiron B, Bower R, Regoutz A, Mihai A, Petrov PK, Oulton RF, Cohen LF, Maier SAet al., 2020, IR hot carrier based photodetection in titanium nitride oxide thin film-Si junctions, MRS Advances, Vol: 5, Pages: 1843-1850, ISSN: 2059-8521

Hot carrier based methods constitute a valuable approach for efficient and silicon compatible sub-bandgap photodetection. Although, hot electron excitation and transfer have been studied extensively on traditional materials such as Au and Ti, reports on alternative materials such as titanium nitride (TiN) are rare. Here, we perform hot hole photodetection measurements on a p-Si/metal thin film junction using Ti, Au and TiN. This material is of interest as it constitutes a refractory alternative to Au which is an important property for plasmonic applications where high field intensities can occur. In contrast to Au, a TiN/Si junction does not suffer from metal diffusion into the Si, which eases the integration with current Si-fabrication techniques. This work shows that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction in the IR, allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Via a comparison between TiN and other commonly used materials such as Au, the origin of this comparably high photoresponse can be traced back to be directly linked to a thin TiO2-x interfacial layer allowing for a distinct hot-hole transfer mechanism. Moreover, the fabrication of TiN nanodisk arrays is demonstrated which bears great promise to further boost the device efficiency.

Journal article

Cohen L, Boldrin D, Johnson F, Thompson R, Mihai AP, Zou B, Griffiths J, Gubeljak P, Ormandy KL, Manuel P, Khalyavin DD, Ouladdiaf B, Petrov P, Branford W, Cohen LFet al., 2019, The biaxial strain dependence of magnetic order in spin frustrated mn3nin thin films, Advanced Functional Materials, Vol: 29, ISSN: 1616-301X

Multi-component magnetic phase diagrams are a key property of functional materials for a variety of uses, such as manipulation of magnetisation for energy efficient memory, data storage and cooling applications. Strong spin-lattice coupling extends this functionality further by allowing electric-field-control of magnetisation via strain coupling with a piezoelectric . Here we explore the magnetic phase diagram of piezomagnetic Mn3NiN thin films, with a frustrated non-collinear antiferromagnetic (AFM) structure, as a function of the growth induced biaxial strain. Under compressive strain the films support a canted AFM state with large coercivity of the transverse anomalous Hall resistivity, ρxy, at low temperature, that transforms at a well-defined Néel transition temperature (TN) into a soft ferrimagnetic-like (FIM) state at high temperatures. In stark contrast, under tensile strain the low temperature canted AFM phase transitions to a state where ρxy is an order of magnitude smaller and therefore consistent with a low magnetisation phase. Neutron scattering confirms that the high temperature FIM-like phase of compressively strained films is magnetically ordered and the transition at TN is 1st-order. Our results open the field towards future exploration of electric-field driven piezospintronic and thin film caloric cooling applications in both Mn3NiN itself and the broader Mn3AN family.

Journal article

Cohen L, Boldrin D, 2019, Anomalous Hall effect in noncollinear antiferromagnetic Mn 3NiN thin films, Physical Review Materials, Vol: 3, ISSN: 2475-9953

We have studied the anomalous Hall effect (AHE) in strained thin lms of the frustrated anti-ferromagnet Mn3NiN. The AHE does not follow the conventional relationships with magnetizationor longitudinal conductivity and is enhanced relative to that expected from the magnetization inthe antiferromagnetic state belowTN= 260 K. This enhancement is consistent with origins fromthe non-collinear antiferromagnetic structure, as the latter is closely related to that found in Mn3Irand Mn3Pt where a large AHE is induced by the Berry curvature. As the Berry phase inducedAHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of thechemically exible Mn3AN structure.

Journal article

Dion T, Arroo DM, Yamanoi K, Kimura T, Gartside JC, Cohen LF, Kurebayashi H, Branford WRet al., 2019, Tunable magnetization dynamics in artificial spin ice via shape anisotropy modification, Physical Review B, Vol: 100, ISSN: 2469-9950

Ferromagnetic resonance (FMR) is performed on kagome artificial spin ice (ASI) formed of disconnected Ni80 Fe20 nanowires. Here we break the threefold angular symmetry of the kagome lattice by altering the coercive field of each sublattice via shape anisotropy modification. This allows for distinct high-frequency responses when a magnetic field is aligned along each sublattice and additionally enables simultaneous spin-wave resonances to be excited in all nanowire sublattices, unachievable in conventional kagome ASI. The different coercive field of each sublattice allows selective magnetic switching via global field, unlocking novel microstates inaccessible in homogeneous-nanowire ASI. The distinct spin-wave spectra of these states are detected experimentally via FMR and linked to underlying microstates using micromagnetic simulation.

Journal article

Ouyang M, Boldrin P, Maher R, Chen X, Liu X, Cohen L, Brandon Net al., 2019, A mechanistic study of the interactions between methane and nickel supported on doped ceria, Applied Catalysis B: Environmental, Vol: 248, Pages: 332-340, ISSN: 0926-3373

A novel combined method using modified methane pulses and in-situ Raman spectroscopy together with mass spectrometry is applied to impregnated Ni/gadolinium-doped ceria (CGO). The partial oxidation of methane is deduced to proceed via a Mars-van-Krevelen type mechanism composed of initial methane decomposition together with carbon oxidation by oxygen from CGO. The critical role of the ceria surface and the bulk oxygen in the reaction is defined in detail. Oxygen is a necessary reactant in the reaction, as well as inhibiting carbon deposition. Oxygen spill-over is the driving force for the carbon oxidation and the ceria surface oxygen is resupplied by bulk oxygen after depletion. Bulk migration of oxygen to the surface is the rate-determining step. We also demonstrate that the ceria oxygen stoichiometry significantly affects the type of reaction and the rate of reaction between methane and Ni/CGO: The total oxidation of methane happens only when the oxygen stoichiometry is high while the oxygen spill-over rate decreases with decreasing oxygen stoichiometry, which reduces the rate of carbon elimination and results in reduction in the rate of methane oxidation. This work lays out a comprehensive evaluation methodology and provides important insights for future design of methane oxidation catalysts for solid oxide fuel cells, and more widely for methane reforming with different oxidants (steam, CO2, NO2 etc).

Journal article

Cohen LF, 2019, View from the Bridge, Applied Physics Letters, Vol: 115, Pages: 1-1, ISSN: 1077-3118

Journal article

Shautsova V, Gusken NA, Sidiropoulos T, Xiao X, Black NCG, Gilbertson AM, Giannini V, Maier SA, Cohen LF, Oulton RFet al., 2019, Plasmonic photo-thermo-electric effect in graphene, Conference on Lasers and Electro-Optics (CLEO), Publisher: IEEE, ISSN: 2160-8989

We present a novel photo-thermo-electric effect in graphene photo-detectors established by hot electrons concentration gradients at plasmonic contacts. Our description is crucial for an in depth understanding of graphene-based photo detection devices.

Conference paper

Gusken NA, Lauri A, Li Y, Matsui T, Doiron B, Bower R, Regoutz A, Mihai A, Petrov PK, Oulton RF, Cohen LF, Maier SAet al., 2019, TiO2-x-enhanced IR hot carrier based photodetection in metal thin film-si junctions, ACS Photonics, Vol: 6, Pages: 953-960, ISSN: 2330-4022

We investigate titanium nitride (TiN) thin film coatings on silicon for CMOS-compatible sub-bandgap charge separation upon incident illumination, which is a key feature in the vast field of on-chip photodetection and related integrated photonic devices. Titanium nitride of tunable oxidation distributions serves as an adjustable broadband light absorber with high mechanical robustness and strong chemical resistivity. Backside-illuminated TiN on p-type Si (pSi) constitutes a self-powered and refractory alternative for photodetection, providing a photoresponsivity of about ∼1 mA/W at 1250 nm and zero bias while outperforming conventional metal coatings such as gold (Au). Our study discloses that the enhanced photoresponse of TiN/pSi in the near-infrared spectral range is directly linked to trap states in an ultrathin TiO2–x interfacial interlayer that forms between TiN and Si. We show that a pSi substrate in conjunction with a few nanometer thick amorphous TiO2–x film can serve as a platform for photocurrent enhancement of various other metals such as Au and Ti. Moreover, the photoresponse of Au on a TiO2–x/pSi platform can be increased to about 4 mA/W under 0.45 V reverse bias at 1250 nm, allowing for controlled photoswitching. A clear deviation from the typically assumed Fowler-like response is observed, and an alternative mechanism is proposed to account for the metal/semiconductor TiO2–x interlayer, capable of facilitating hole transport.

Journal article

Jeon K-R, Ciccarelli C, Kurebayashi H, Cohen LF, Komori S, Robinson JWA, Blamire MGet al., 2019, Abrikosov vortex nucleation and its detrimental effect on superconducting spin pumping in Pt/Nb/Ni80Fe20/Nb/Pt proximity structures, Physical Review B (Condensed Matter), Vol: 99, ISSN: 0163-1829

We report Abrikosov vortex nucleation in Pt/Nb/Ni80Fe20/Nb/Pt proximity-coupled structures under oblique ferromagnetic resonance that turns out to be detrimental to superconducting spin pumping. By measuring an out-of-plane field-angle θH dependence and comparing with Pt-absent control samples, we show that as θH increases, the degree of enhancement (suppression) of spin pumping efficiency in the superconducting state for the Pt-present (Pt-absent) sample diminishes and it reverts to the normal state value at θH=90∘. This can be explained in terms of a substantial out-of-plane component of the resonance field for the Ni80Fe20 layer (with in-plane magnetization anisotropy and high aspect ratio) that approaches the upper critical field of the Nb, turning a large fraction of the singlet superconductor volume into the normal state.

Journal article

Sundar S, Salem-Sugui S, Chattopadhyay MK, Roy SB, Sharath Chandra LS, Cohen L, Ghivelder Let al., 2019, Study of Nb0:18Re0:82 non-centrosymmetric superconductor in the normal and superconducting states, Superconductor Science and Technology, Vol: 32, Pages: 1-11, ISSN: 0953-2048

We examine the evidence for multiband superconductivity and non s-wave pairing in the non-centrosymmetric superconductor Nb0.18Re0.82, using electrical transport, magnetization and spe-cific heat measurements. In the normal state, the evolution of electrical resistivity with temperatureand magnetic field support the phonon assisted interband scattering and multiband picture. Inthe superconducting state, the temperature dependence of the upper critical field,Hc2(T), is foundlinear and can not be described within the Werthamer, Helfand and Hohenberg (WHH) model overthe whole temperature range measured. In addition, the observedHc2(0) exceeds the Pauli limit,suggesting non-s-wave pairing. Interestingly, the Kadowaki-Woods ratio and Uemura plot reveala behavior in Nb0.18Re0.82which is similar to that found in unconventional superconductors. Thenormalized superfluid density (ρs), estimated using the temperature dependence of the lower criticalfield,Hc1(T), is well explained with the help of the multiband description. Phase-fluctuation analysisconducted on the reversible magnetization data, reveals a significant deviation from the mean-fieldconventional s-wave behavior. This trend is interpreted in terms of a non s-wave spin-triplet com-ponent in the pairing symmetry as might be anticipated in a non-centrosymmetric superconductorwhere anti-symmetric spin-orbit coupling plays a dominant role. Recently, time reversal symmetrybreaking (TRSB) observed in Nb0.18Re0.82supports this picture.

Journal article

Guo L, Lovell E, Wilson N, Burdett P, Cohen LF, Ryan MPet al., 2019, The electrochemical behaviour of magnetocaloric alloys La(Fe,Mn,Si)13Hx under magnetic field conditions, Chemical Communications, Vol: 55, Pages: 3642-3645, ISSN: 1359-7345

The degradation mechanism of La(Fe,Mn,Si)13Hx has been examined under conditions representative of the complex operating parameters of a refrigeration cycle. The magnetic field effects are found to be dominated by magneto-transport and are most significant when the material is in its paramagnetic state - resulting in significantly accelerated corrosion rates.

Journal article

Sundar S, Salem Sugui Jr S, Lovell E, Vanstone A, Cohen LF, Gong D, Zhang R, Lu X, Luo H, Ghivelder Let al., 2019, Doping dependence of the second magnetization peak, critical current density and pinning mechanism in BaFe2−xNixAs2 Pnictide superconductors, ACS Applied Electronic Materials, Vol: 1, Pages: 179-188, ISSN: 2637-6113

A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density ($J_c$) and the vortex-pinning behaviour in these compounds. The observed $J_c$ is higher than the threshold limit (10$^5$ A/cm$^2$) considered for the technological potential and even greater than 1 MA/cm$^2$ for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen $et$ $al$, which suggest the dominant role of $\delta l$-type pinning.

Journal article

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