Physics

News and events

Landau Seminar Series 2016 -17

Wednesday 26 July 2017 - 2.00pm, SCH.0.13 (Schofield Building)

Novel architectures for graphene spintronics, by Dr Ivan Marun, University of Manchester

Abstract: The field of graphene spintronics has evolved in less than a decade from a tantalising possibility to a mature and strong research community. Several fundamental questions remain open, whose answers require us to push beyond the traditional design boundaries of spintronic architectures. First, the elimination of extrinsic sources of spin relaxation is the key to realizing the exceptional intrinsic spin transport performance of graphene. Towards this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture that allows us to separately investigate the roles of substrate and polymer residues on spin relaxation. The results suggests that the limit in current devices is due to a magnetic resonance. Second, we probe the role of proximity-induced exchange coupling in graphene due to ferromagnetic metals, using vertical spin transport junctions. Finally, we introduce thermoelectric circuitry to evidence the presence of linear-regime nonlocal signals due to heat currents. We show how the latter can mimic the presence of anisotropic spin relaxation in Hanle experiments.

Wednesday 28 June 2017 - 2.00pm, N.1.12 (Haslegrave Building)

Qualitative Analysis of Quantum States and Observables, by Professor Artur Sowa, Department of Mathematics and Statistics, University of Saskatchewan

The method of harmonic analysis is broadly applied in classical signal processing. The central idea is that signals can be decomposed into basic modes and classified according to their modal energy distribution profiles. By comparison Quantum Mechanics starts with a description of the basic modes (eigenstates) before addressing the more complex superpositions and composites. However, the composite quantum states typically have complex high-dimensional structure seemingly recalcitrant to qualitative descriptions. 

A well-established remedial approach is based upon the Wigner transform. However, the Wigner transform works best when representing certain special types of states (Gaussian, coherent, or squeezed) and is less informative in other situations. In the era of quantum technologies 2.0, when we design quantum-state manipulating devices and quantum metamaterials, [1-2], and ask fundamental questions about the qualities of prevalent quantum states, [3], it is beneficial to have alternative analytic tools. 

I will discuss such an alternative, namely the Q-transform, [4], which provides a rigorous way of representing quantum observables in the form of doubly-periodic real functions and distributions. In particular, it furnishes a natural concept of Sobolev classes of observables and, through that, yields qualitative regularity-type results for a variety of quantum flows (both unitary and non-unitary). 

The Q-transform bridges quantum theory with harmonic analysis, and makes it possible to adapt and reinterpret some of the contents of the latter. In particular, one can give a quantum interpretation to the phenomenon of broadband redundancy (BR), [5]. The BR results from an observation that there exist ensembles of spread spectrum waveforms which average to a pure tone; accordingly, one can construct a family of broadly diverse observables with a predesigned average. Among targeted applications of the BR is quantum tomography, where it is natural to look for optimal ensembles of measurements. 

The Q-transform should also prove useful in regard to the problem of quantumness and speedup in quantum information processing. Where much progress has been achieved, e.g. [6-8], the Q-transform should yield additional insight into the possibility of efficient classical simulation of some quantum algorithms. 

1. A.L. Rakhmanov, A.M. Zagoskin, S. Savel'ev, and F. Nori, Phys. Rev. B 77, 144507 (2008) 

2. A. M. Zagoskin, Quantum Engineering, Cambridge University Press (2011) 

3. W. H. Zurek, Nature Physics 5, pp. 181-188 (2009) 

4. A. Sowa, arXiv: 1609.01712 [quant-ph] 

5. A. Sowa, arXiv: 1603.03667 [math.GM] 

6. R.W. Spekkens, PRL 101, 020401 (2008) 

7. A. Mari and J. Eisert, PRL 109, 230503 (2012) 

8. L. Kocia, Y. Huang, and P. Love, arXiv: 1703.04630 [quant-ph] 

Email:sowa@math.usask.ca, web site: http://math.usask.ca/~sowa/

Wednesday 21 June 2017 - 2.30pm, SMB.0.14 (Stewart Mason Building)

Tuesday 13 June 2017 - 2.00pm, SCH.0.13 (Schofield Building)

Propulsion and controlled steering of magnetic nanohelices, by Maria Michiko Alcanzare, Department of Applied Physics, Aalto University

"Propulsion and controlled steering of magnetic nanohelices"

Externally controlled motion of micro and nanomotors in a fluid environment constitutes a promising tool in biosensing and targeted delivery. In particular, recent experiments have demonstrated that fuel-free propulsion and three-dimensional capture and release of microscale cargoes can be achieved through the application of external magnetic fields on magnetic helically shaped structures [1]. The magnetic interaction between helices and the rotating field induces a torque that rotates and propels them via the coupled rotational-translational motion. Experiments, however, show that controlled motion remains a challenge at the nanoscale due to Brownian motion [2]. In this work, we employ a quantitatively accurate simulation methodology to demonstrate ultra-fast propulsion of nanohelices, and controlled propulsion and steering at the nanoscale [3]. 

 

[1] S. Tottori, L. Zhang, F. Qiu, K. K. Krawczyk, A. Franco-Obregón, and B. J. Nelson, Magnetic helical micromachines: fabrication, controlled swimming, and cargo transport, Advanced Materials 24, 811 (2012); T.-Y. Huang, M. S. Sakar, A. Mao, A. J. Petruska, F. Qiu, X.-B. Chen, S. Kennedy, D. Mooney, and B. J. Nelson, 3D Printed Microtransporters: Compound Micromachines for Spatiotemporally Controlled Delivery of Therapeutic Agents, Advanced Materials 27, 6644 (2015); S. Sanchez, A. A. Solovev, S. Schulze, and O. G. Schmidt, Controlled manipulation of multiple cells using catalytic microbots, Chemical Communications 47, 698 (2011).

[2] D. Schamel, A. G. Mark, J. G. Gibbs, C. Miksch, K. I. Morozov, A. M. Leshansky, and P. Fischer, Nanopropellers and their actuation in complex viscoelastic media, ACS Nano 8, 8794 (2014).

[3] M. M. T. Alcanzare, V. Thakore, S. T. T. Ollila, M. Karttunen, T. Ala-Nissila. Controlled propulsion and separation of helical particles at the nanoscale.Soft Matter, 13, 11 , 2148–2154 (2017); M. M. T. Alcanzare, M. Karttunen and T. Ala-Nissila. Propulsion and controlled steering of magnetic nanohelices. arXiv:1702.01989

Monday 12 June 2017 - 2.00pm, N.1.12 (Haslegrave Building)

Non-Markovian Effects, Quantum Jumps and Fluctuation Relations in Driven Quantum Systems, by Tapio Ala-Nissilä, Aalto University and Loughborough University

"Work, Quantum Jumps and Fluctuation Relations in Driven Quantum Systems"

Abstract:
Definition and measurement of work done on a driven open quantum system remains a major challenge in quantum thermodynamics. The stochastic mapping of an open quantum system to the Lindblad maste equation and its unraveling by quantum jumps offers a powerful way to define and calculate thermodynamics of work and the related fluctuation relations [1]. However, several approximations must be made
including that of Markovianity of the dynamics. A calorimetric detection method has recently been proposed by J. Pekola and collaborators [2] as a feasible experimental scheme to measure work and fluctuation relations. However, the detection requires a finite size for the environment, which influences the system dynamics. This process cannot be modeled with the standard Markovian stochastic approaches. We develop a quantum jump model suitable for systems coupled to a finite-size environment and discuss its application to work measurement and fluctuation relations [3]. We also discuss the consequences of non-Markovianity in entropy production in strongly coupled systems [4].

1. S. Suomela, J. Salmilehto, I.G. Savenko, T. Ala-Nissila, and M. Mottonen, Phys. Rev. E 91, 022126 (2016).
2. J.P. Pekola, P. Solinas, A. Shnirman, and D.V. Averin, New J. of Phys. 15, 115006 (2013).
3. S. Suomela, A. Kutvonen, and T. Ala-Nissila, Phys. Rev.  E 93, 062106 (2016); S. Suomela, R. Sampaio, and T. Ala-Nissila, Phys. Rev. E 94, 032138 (2016); J.P. Pekola, S. Suomela and Y.M. Galperin, J. Low Temp.Phys. (2016).
4. A. Kutvonen, T. Ala-Nissila, and J. Pekola, Phys. Rev. E 92, 012107 (2015).

Wednesday 03 May 2017 - 2.30pm, SCH.013 (Schofield Building)

Electron Doped Strontium Iridates: A 5d Analogue of Cuprates?, by Felix Baumberger, DQMP, University of Geneva and Swiss Light Source, Paul Scherrer Institute

The insulating ground state of the layered perovskite iridates is often described in a single band pseudospin - 1/2 Hubbard model. Iridates were thus proposed as analogues to the cuprates and as such, a potential platform for engineering high-temperature superconductivity under electron doping [1].

In this seminar, I will discuss our recent work on lightly doped single crystals of the single and bilayer perovskites Sr2IrO4 and Sr3Ir2O7 [2,3]. Using La substitution to dope electrons in the IrO2 plane we induce an unusual metallic state that indeed shares many similarities with cuprates. In particular, our ARPES data on (Sr(1-x)Lax)2IrO4 show Fermi arcs arising from nodal quasiparticles coexisting with an antinodal pseudogap. Moreover, we find evidence for short-range unidirectional order [4] and persistent high-energy magnetic fluctuations within the pseudogap phase [5]. Despite these similarities with cuprates, our samples do not show signs of superconductivity down to 200 mK.

 

References

[1] F. Wang and T. Senthil, Phys. Rev. Lett. 106, 136402 (2011)

[2] A. de la Torre et al., Phys. Rev. Lett. 113, 256402 (2014)

[3] A. de la Torre et al., Phys. Rev. Lett. 115, 176402 (2015)

[4] I. Battisti et al. Nat. Phys. 13, 21 (2017)

[5] D. Pincini et al. arXiv:1703.09051

Wednesday 26 April 2017 - 2.30pm, SCH.013 (Schofield Building)

Friday 21 April 2017 - 2.00pm, SCH.0.13 Schofield Building

Thursday 6 April, 2017 - 2.00pm, W2.19 (Sir David Davies Building)

Thermal and electrical quantum Hall effects in ferromagnet/topological insulator/ferromagnet junction, by V. Kagalovsky, Shamoon College of Engineering, Beer-Sheva, Israel

We present a theoretical description for a class of experimental setups thatmeasure quantumHall coefficients in ferromagnet/topological insulator/ferromagnet (FM-TI-FM) junctions. We predict that, varying the magnetization direction in ferromagnets, one can change the induced Hall voltage and transverse temperature gradient from the maximal values, corresponding to the quantized Hall coefficients, down to their complete suppression to zero. We provide detailed analysis of thermal and electrical Hall resistances as functions of the magnetization directions in ferromagnets, the spin scattering time in TI, and geometrical positions of FM leads and measurement contacts. We find a special symmetric configuration of the experimental setup, at which the quantum Hall coefficients are independent of spin scattering.

Wednesday 15 February, 2017 - 2.00pm, W2.19 (Sir David Davies Building)

Eigenvalue problems for non-local Schrodinger operators, by Dr Jozsef Lorinczi, Mathematical Sciences, Loughborough Universiy

The (semi-)relativistic Hamiltonian of a quantum particle with zero or non-zero rest mass is an example of a non-local Schrödinger operator, much studied in mathematical physics. Replacing the kinetic term of the relativistic Hamiltonian by another pseudo-differential operator gives rise to a class of non-local Schrödinger operators, which describe other phenomena (encountered, for instance, in anomalous transport). In this talk I will discuss spectral properties of such operators. First I will present two examples for which the eigenvalue problem can be solved explicitly. Next I will discuss explicit cases for which resonances occur. Finally, I will review some results on the spatial decay of eigenfunctions by using a Feynman-Kac path integral representation, and explain how this description relates with a problem of statistical mechanics. 

Wednesday 25 January, 2017 - 2.00pm, W2.19 (Sir David Davies Building)

Do we really see chemical bonds?, by Professor P Moriarty, Nottingham

Exceptionally clear images of intramolecular structure can now be attained in dynamic force microscopy (DFM) through the combination of tip-apex control and operation in what has become known as the “Pauli exclusion regime” of the tip-sample interaction [1-4]. (See, for example, Fig.1. This figure, taken from Ref. 4, shows a DFM image of a 2D assembly of NTCDI molecules alongside a ball-and-stick model of the molecule where blue spheres represent nitrogen atoms, and red spheres are oxygen.)

My particular focus will be on the interpretation of Pauli’s principle in the context of interatomic and intermolecular interactions, what this means for the quantitative analysis of ultrahigh resolution force microscopy data, and whether or not we really see chemical bonds in DFM images [5,6].

[1] L Gross, et al., Science, 325 (2009) 1110

[2] Dimas G. de Oteyza, et al., Science, 340 (2013) 1434

[3] Jun Zhang, et al., Science 342 (2013) 611

[4] A Sweetman, et al., Nature Comm. 5 (2014) 3931

[5] P Hapala, et al., Phys Rev B 90, 085421 (2014); Sampsa K. Hämäläinen et al, Phys. Rev. Lett. 113, 186102 (2014)

[6] S. P. Jarvis, et al., Phys. Rev. B 92, 241405(R) (2015)

See also https://muircheart.wordpress.com/2014/08/15/perplexed-pauli/

Tuesday 22 November, 2016 - 3.00pm, W2.19 (Sir David Davies Building)

Energy band topology and the thermodynamics in weakly anisotropic superconductivity, by Professor Tugrul Hakioglu, Institute of Theory and Applied Physics

Abstract: In noncentrosymmetric superconductors (NCSs), the inversion symmetry (IS) is most commonly broken by an antisymmetric spin-orbit coupling (SOC) removing the spin degeneracy and splitting the Fermi surface (FS) into two branches. A two component condensate is then produced with a doublet pair potential mixing an even singlet and an odd triplet. When the triplet and the singlet strengths are comparable, the pair potential can have rich nodes. The angular line nodes (ALNs) are associated with strong anisotropy and they are widely studied in the literature. When the anisotropy is not strong, they can be replaced by other types of nodes in closed or open forms affecting the low temperature properties.


Here, we focus on the weakly anisotropic case and the line nodes in the superconducting plane which become circular in the limit of full isotropy. We study the topology of these radial line nodes (RLNs) and show that it is characterized by the Z 2 classification similar to the Quantum-Spin-Hall Insulators. From the thermodynamic perspective, the RLNs cause, even in the topological phases, an exponentially suppressed low temperature behaviour which can be mistaken by nodeless s-wave pairing, thus, providing an explanation to a number of recent experiments with contraversial pairing symmetries. In the rare case when the RLN is on the Fermi surface, the exponential suppression is replaced by a linear temperature dependence. The RLNs are difficult to detect, and for this reason, they may have escaped experimental attention. We demonstrate that Andreev conductance measurements with clean interfaces can efficiently probe the weakly anisotropic samples where the RLNs are expected to be found.



Wednesday 09 November, 2016 - 3.00pm, W2.19 (Sir David Davies Building)

Networks and Quantum Coherent Feedback, by Sergei Fedotov, Manchester

During this seminar we will review the Hudson-Parthasarathy quantum stochastic calculus and the input-output theory for open quantum systems (known as the "SLH'' formalism), and show how it may be extended to give a theory of quantum feedback networks. We will give an overview of feedback techniques, including current directions in quantum coherent feedback control.

Friday 4 November, 2016 - 3.00pm, KG.1.09 (Keith Green Building)

Wednesday 2 November, 2016 - 3.00pm, W2.19 (Sir David Davies Building)

Sr2RuO4 and Sr3Ru2O7 Under Uniaxial Pressure, by Clifford Hicks, Dresden

A major area of interest in condensed matter physics is the way that electrons in correlated electron materials can self-organise at low temperatures into ordered states. The related correlated-electron materials Sr2RuO4 and Sr3Ru2O7 are especially well-known, because both can be grown with exceptionally low levels of disorder, and because both host unconventional electronic phases at low temperature: unconventional superconductivity in the case of Sr2RuO4, and a state with magnetic order in the case of Sr3Ru2O7. Both materials have a tetragonal crystal structure, and, as I will report in this seminar, the ordered states in both compounds respond very sensitively to uniaxial pressure that lifts this tetragonal point-group symmetry. The transition temperature of the superconductivity of Sr2RuO4 is found to pass through a dramatic peak, at which it is more than double its zero-pressure value, and the c-axis upper critical field is enhanced by a factor of twenty. This feature is found to be at a strain consistent with driving one of the Fermi surfaces through a topological (Lifshitz) transition. In Sr3Ru2O7, the resistivity within the ordered state is found to become highly anistropic with small lattice distortions: by perturbing the ordered state, an orthorhombic distortion of only 0.1% induces a ~100% resistivity anisotropy

Wednesday 26 October, 2016 - 3.00pm, W2.19 (Sir David Davies Building)

Wednesday 19 October, 2016 - 3.00pm, W2.19 (Sir David Davies Building)

Resonant tunneling in twisted graphene-boron nitride transistors, by Mark Greenaway, Loughborough

Multilayer transistors based on graphene and other van der Waals crystals [1-8] exhibit many interesting physical properties: high on-off current switching ratios, mechanical flexibility, photoresponsivity, light emission, and resonant tunnelling with gate voltage-tuneable negative differential conductance at room temperature. Here, I will give a brief review of our recent work on electron tunneling between the graphene layers of these devices. I will demonstrate the strong sensitivity of in-plane momentum conserving tunnel transitions to any small misalignment or twist angle between the crystalline lattices of the two graphene electrodes [3-5]. For devices where there is a large misalignment between the graphene lattices, I will show how phonon emission can determine the tunnel current [6]. Finally, I will explain how an applied magnetic field can be used to reveal the effects of chirality, which is a unique feature of electron dynamics in graphene and related materials [7,8].

[1] K.N. Novoselov et al., Science 353, 461 (2016).
[2] T. Georgiou et al., Nat. Nanotechnol2, 100 (2013).
[3] L. Britnell et al., Nat. Commun4, 1794 (2013).
[4] A. Mishchenko et al., Nat. Nanotechnol9, 808 (2014).

[5] J. Gaskell et al., Appl. Phys. Lett. 107, 103105 (2015).

[6] E.E. Vdovin et al., Phys. Rev. Lett.116, 186603 (2016).
[7] M.T. Greenaway et al., Nat. Phys11, 1057 (2015).

[8] J.R. Wallbank et al., Science353, 575 (2016).

Wednesday 5 October, 2016 - 3.00pm, W2.19 (Sir David Davies Building)

Resonating valence bond theory of the spin-1/2 kagome antiferromagnet, by Ioannis Roussochatzakis, University of Minnesota

Abstract:
Recent studies of highly frustrated antiferromagnets (AFMs) have demonstrated the qualitative impact of virtual, longer-range singlet excitations on the effective RVB tunneling parameters of the low energy sector of the problem [1,2]. Here, I will discuss the current state of affairs on the RVB description of the spin-1/2 kagome AFM, including new results that shed light on the valence bond crystals that compete with the spin liquid [3].

[1] I. Rousochatzakis, Y. Wan, O. Tchernyshyov, and F. Mila, PRB 90, 100406(R) (2014)

[2] A. Ralko and I. Rousochatzakis, PRL 115, 167202 (2015) 

[3] in preparation.