Semester and Master's Projects

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We offer a wide menu of projects for semester and diploma work in experimental solid state physics. Some of these are carried out in our laboratory on the ETH campus. Others involve experiments at neutron scattering and user facilities available at Paul Scherrer Institut (30 km from ETH).

Since our program is a very active one, the list on this page is most certainly not complete. Please do not hesitate to contact  for the latest information.

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Semester/Master's Project: composite magnon-phonon excitations in a quantum spin ladder

Spectrum of (C7H10N)2CuBr4 with arrows showing magnon-phonon excitations.  
Spectrum of (C7H10N)2CuBr4 with arrows showing magnon-phonon excitations.

Contact: or

Phonons are quasiparticles in solids, corresponding to collective excitations of the lattice. Similarly, magnons correspond to collective magnetic excitations. Usually, they interact weakly and can be treated separately. In some cases, however, they interact strongly resulting in more complex behavior. For example, in light spectroscopy experiments such effects would manifest in excitations, where magnons and phonons are created together forming a composite excitation.
We have performed preliminary light scattering measurements on a strong-leg quantum spin ladder (C7H10N)2CuBr4 [1] and have found indications of such excitations. This provides a unique opportunity to study magnon-phonon coupling in quantum magnets.
A student carrying out this project will perform Raman spectroscopy experiments on (C7H10N)2CuBr4. The phonon spectrum will be distorted and the effect on the magnon-phonon excitation will be measured. A simple way to perturb the phonon spectrum is to replace H by D in the compound and see what effect it has on magnetism. Interesting parallels with isotope effect in superconductivity can be drawn here [2].
The project can be potentially extended into a Master's thesis by including an alternative way to perturb the lattice by using hydrostatic pressure. It is significantly more challenging experimentally, but allows studying gradual changes in the spectrum and hence, a more detailed understanding of the coupling mechanisms.
A student working part of the NSM group will learn experimental skills of X-ray diffraction and Raman spectroscopy, as well as data processing techniques. We expect the research to contribute to a high-profile journal publication.

[1] Schmidiger et al., Phys. Rev. Lett. 108, 167201 (2012)
[2] recent example – Drozdov et al., Nature 525, 73 (2015)

Master's Project: dielectric properties of quantum magnets

Magnetocapacitive effect in Sul-Cu2Cl4 at ultralow temperatures (left) and the corresponding phase diagram (right). Each phase diagram point comes from the capacitive lambda-anomaly. See Ref. [4] for more information on this particular study.  
Magnetocapacitive effect in Sul-Cu2Cl4 at ultralow temperatures (left) and the corresponding phase diagram (right). Each phase diagram point comes from the capacitive lambda-anomaly. See Ref. [4] for more information on this particular study. 

Contact: or

Combining nontrivial  magnetic and electric properties within one material has always been a tricky problem, as they require a completely different type of symmetry breaking. Nonetheless, as it has been discovered about ten years ago, certain types of magnetic ordering do also trigger the electric polarization [1]. Naturally, such magnetic ordering also manifests itself in the dielectric properties of the material. This opens an exciting new way to learn something about the magnetic properties of exotic quantum antiferromagnets by just looking at their dielectric susceptibility [2, 3]. Needless to say, 'looking at properties' in the context of quantum magnetism means  a combination of ultralow-temperature environment and high static magnetic fields, which is available right here, in our lab at ETH Hönggerberg campus.

Some first steps on this way have already been done with a highly frustrated S=1/2 material Sul-Cu2Cl4 (see figure). The careful examination of magnetocapacitive effect in this compound allowed us to obtain a brilliant quality phase diagram [4]. However, we already have a number of other candidate materials, for which a rich interplay between the electric polarization and quantum magnetism is expected. The aim of this Master's project would be to pursue those effects by using the unique setup created in NSM lab at ETH. The results obtained during this study are expected to be an essential contribution to a high-profile journal publication.

[1] M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)
[2] F. Schrettle, S Krohns, P. Lunkenheimer et al., Phys. Rev. B 87, 121105 (2013)
[3] J. W. Kim, S. Khim, S. H. Chun et al., Nature Commun. 5, 4419 (2014)
[4] K. Yu. Povarov, A. Reichert, E. Wulf, A. Zheludev, Phys. Rev. B 92, 140410(R) (2015)

Semester Project: effects of site disorder in a Heisenberg spin-ladder

Phase diagram of spin-chain compound K2CuSO4Cl2, obtained by specific heat measurement.  
Phase diagram of spin-chain compound K2CuSO4Cl2, obtained by specific heat measurement.

Contact:

"A good effective low-energy theory is worth all of quantum Monte-Carlo with Las Vegas thrown in," - once claimed famous P. W. Anderson [1]. This is especially true for the case of Luttinger Liquid (LL) theory, a one-dimensional analogue of Fermi Liquid paradigm [2]. The goal of the current research is the study of the phase diagram of slightly non-ideal 1D quantum system, a real spin-chain compound in our case (an example is shown in the figure below), which is expected to have some universal properties [3].

A student, working as a member of young and dynamic team, will learn state-of-art methods of dealing with ultra-low temperatures, high magnetic fields and calorimetric measurements under such a conditions. All the experiments will be conducted in NS&M Group laboratory at ETH Honngerberg campus. This perspective Semester project has all chances for turning into a more profound Master's project study, and we naturally expect it to contribute to a high-profile journal publication.

[1] P. W. Anderson and P. A. Casey, J. Phys.: Condens. Matter 22 164201 (2010)
[2] T. Giamarchi, "Quantum physics in one dimension", Clarendon Press (2004)
[3] H. J. Schulz, Phys. Rev. Lett. 77 2790 (1996)

Master's Project: phase diagram of a non-ideal Luttinger Liquid

Inelastic neutron scattering spectrum and structure of DIMPY.  
Inelastic neutron scattering spectrum and structure of DIMPY.

Contact:   or Dr. David Schmidiger

Even though their ground states exhibit no long range order, low-dimensional quantum magnets reveal a cornucopia of both unexpected, exotic and theoretically very exciting physical phenomena. Among them, the Heisenberg S=1/2 spin ladder belongs to the most important and most studied model systems [1]. In zero-field, it remains in a complex many-body singlet ground state, a spin-liquid. The excitations with lowest-energy are sharp magnons carrying S = 1 and they exhibit a spin gap (fig 1a). In applied fields, these spin-liquids undergo a quantum phase transition to a novel exotic state which is nearly ordered, a Luttinger-Liquid. The sharp magnon excitations deconfine into a continuum of fractional S=1/2 spinon excitations and the gap vanishes (fig 1b).

The aim of the present work is to study the effect of site-disorder in a real Heisenberg spin-ladder. We focus on the recently discovered material (C7H10N)2CuBr4 (DIMPY, [2]) which was shown to be an almost perfect realization of the Heisenberg spin-ladder Hamiltonian [3,4]. In this material, the spin S=1/2 is due to the copper Cu2+ ions and site-disorder is introducted by random subsitution of Cu2+ with non-magnetic Zn2+. These 'missing spins' in the spin-ladder are predicted to affect the physics strongly. For example, low-energy degrees of freedom are released leading to localized states, 'spin islands' [5,6].  The phase diagram and thermodynamic response functions are hence strongly influenced by these low-energy quantum states.

We want to study these effects by careful magneto-thermodynamic studies. In our laboratory, we have grown a series of (C7H10N)2Cu1-xZnxBr4 crystals with 0<x<0.08. The aim of this work is to study the magnet-thermodynamic properties of Zn-disordered symples by specific heat and magnetization measurements at very low temperatures (down to 50 mK) and high magnetic fields (up to 14 Tesla) and to compare the results to measurements performed in the pure sample. As a Masters project, this project has many advantages. You will learn the use of cryogenic equipment and several commonly used experimental techniques. This

study is part of a larger project with various planned complementary experiments using different techniques both in- and outside our research group. It is hence likely, that the results of this thesis yield a nice publication in a peer-reviewed journal. Finally, this project has the potential of growing into a nice PhD study in the field of quantum magnetism, involving neutron spectroscopy (at our spectrometer at Paul Scherrer Institut), Raman spectroscopy and Muon Spin Rotation studies. Collaborations may include EPFL, Uni Geneva, Institut Laue Langevin andLaboratoire National des Champs Magnétiques Intenses (France) as well as Oak Ridge National Laboratory (USA).

[1]  E. Dagotto, and T. M. Rice, Science 271, 618 (1996)
[2] A. Shapiro et al., Journal of the American Chemical Society 129, 952 (2007)
[3] D. Schmidiger et al., Phys. Rev. Lett. 108, 167201 (2012)
[4] D. Schmidiger et al., Phys. Rev. Lett. 111, 107202 (2013)
[5] H.-J. Mikeska et al., Phys. Rev. B 55, 2955 (1997)
[6] A. Furusaki et al., Phys. Rev. B 52, 15930 (1995)

 
 
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25.06.2017
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