Neutron Scattering and Magnetism
Laboratory for Solid State Physics · ETH Zurich

Join us

We offer exciting projects for semester and master's work in experimental solid state physics. This is real hands-on research, not an exercise: some of the projects are carried out in our laboratory on the ETH Hönggerberg campus, others involve experiments at the Paul Scherrer Institut (30 km from ETH) or at large-scale neutron scattering facilities worldwide. Quite often the data collected or analyzed by a student become part of a peer-reviewed publication from the group, and then the student is, of course, a co-author.

Which projects are actually on offer in any given semester is very fluid and cannot be planned more than a few months in advance. Nor do we want to advertise our immediate plans online to our competitors. Most importantly, we always try to match the project to the background and interests of the student who will execute it. For all these reasons, if you are interested in joining us, the best thing to do is to email Prof. A. Zheludev directly and discuss the options. Just as a reference, here is an admittedly somewhat random collection of projects that were successfully carried out in our group over the years.

If you are still charting your course through the ETH physics curriculum, have a look at your path to solid state physics.

Phase diagram of a ℤ2 vortex crystal candidate

The measured magnetic phase diagram of our vortex crystal candidate material

The measured magnetic phase diagram of our candidate material in a magnetic field along the c axis, with three distinct incommensurate phases (IC1–IC3): J. Nagl et al., ℤ2 vortex crystal candidate in the triangular quantum antiferromagnet , npj Quantum Mater. (2026); arXiv:2512.01793.

Completed in 2026

For decades, the antiferromagnet on a triangular lattice has remained a paradigm of quantum magnetism due to the wide range of exotic phases it exhibits. Recently, theoretical work has pointed to this geometry as a platform for realizing a new, exotic quantum phase: a ℤ2 vortex crystal. This quantum phase emerges when topological defects condense into a lattice, and is yet to be experimentally observed.

We have recently synthesized a new family of triangular lattice quantum antiferromagnets. The unique combination of frustrated geometry, strong spin-orbit coupling and Kitaev interactions in these materials makes them ideal candidates for the ℤ2 vortex crystal phase. Initial work on a Ru-based member of this family has already revealed a rich phase diagram, with phases that could correspond to this ℤ2 vortex crystal. However, our efforts now turn to a different material in this family that is even more likely to exhibit this ℤ2 vortex crystal phase.

In this semester project, you will conduct state-of-the-art calorimetric and magnetic measurements on these new quantum materials. The measurements will be carried out at ultra-low temperatures (down to 50 mK) and high magnetic fields (up to 14 T). You will investigate the thermodynamic properties of the material, allowing you to map out the different quantum phases in the magnetic phase diagram of this material. This will be a pivotal step towards understanding the complex magnetic behavior of these materials. Your project is likely to contribute to a publication in a high-profile journal.

Quantum phases on the triangular lattice

The anisotropic triangular lattice Theoretical phase diagram of the anisotropic triangular lattice antiferromagnet

(a) The spatially anisotropic triangular lattice with competing exchange couplings J and J′. (b) Theoretical phase diagram of the anisotropic triangular-lattice antiferromagnet in an applied magnetic field.

Completed in 2025

Quantum antiferromagnets on a triangular lattice pose one of the oldest and most influential paradigms in quantum magnetism [1]. The highly frustrated lattice geometry tends to inhibit classical long-range order and instead promotes exotic quantum phases of matter. While the simple S=1/2 Heisenberg system has been studied extensively, even a modest exchange anisotropy may dramatically affect the physics and lead to novel ground states or excitations [2], many of which have yet to be realized in experiment.

We have recently synthesized a new family of quantum antiferromagnets on the triangular lattice, which seems to combine a small spatial anisotropy in the triangular plane with the spin-space anisotropies inherent to heavy 4d/5d transition metals with strong spin-orbit coupling. In this master's project, you will learn state-of-the-art techniques of performing calorimetric and magnetic measurements at ultra-low temperatures (down to 50 mK) and high magnetic fields (up to 14 T). You will investigate the thermodynamics of one of these new quantum materials, allowing you to map out its magnetic phase diagram. This should give us a solid basis for understanding the complex magnetic behavior of these materials, likely contributing to a high-profile journal publication.

Symmetry reduction in multiferroic quantum spin chains

Dielectric constant of Rb2Cu2Mo3O12 Dielectric constant of Cs2Cu2Mo3O12

Changes in the dielectric constant of Rb2Cu2Mo3O12 (left) and Cs2Cu2Mo3O12 (right) show strong magnetoelectric coupling. In Cs2Cu2Mo3O12 a signature of magnon-mediated dipolar relaxation is present throughout the phase diagram.

Completed in 2023

The ferro-antiferromagnetic spin chains A2Cu2(MoO3)4 (with A = Rb or Cs) show extremely rich physics due to frustration. In addition to complex magnetism, a strong magnetoelectric coupling opens the way to a variety of exotic magnetoelectric phenomena. Both systems display magnetic Bose-Einstein condensation of magnons, where usually inaccessible critical exponents have been measured [1]. However, despite the structural similarities, measurements of the dielectric permittivity show qualitative differences between them. Cs2Cu2Mo3O12 shows modulations of the permittivity with a strong magnetic-field dependence both in the long-range ordered phase and in the paramagnetic phase. This effect has been interpreted as a newly observed relaxation of low-energy electric dipoles mediated by critical magnons [2]. This effect is absent in Rb2Cu2Mo3O12 and its origin is still unclear.

In this master's project, you will be part of a detailed study of the structure of these two magnetic systems. You will take part in a series of synchrotron experiments at large-scale facilities such as ESRF (Grenoble, France) or Diamond (Oxfordshire, UK). Using a combination of single-crystal and polycrystal techniques, the structure of this family of magnets will be thoroughly investigated in the search for an elusive lattice symmetry reduction. These experiments will clarify the different magnetic behavior of the compounds, and pinpoint the microscopic origin of the novel coupling mechanism found in Cs2Cu2Mo3O12. This project will most likely contribute to a publication in a high-impact journal.

Development of a low-temperature magnetic susceptibility probe

AC susceptibility of Dy2Ti2O7 at millikelvin temperatures Design and photograph of the superconducting susceptometer coil

(a) AC susceptibility demonstrates the extremely slow dynamics in the spin-ice compound Dy2Ti2O7 down to millikelvin temperatures. (b,c) The susceptometer design is based on a custom-made superconducting coil.

Completed in 2023

Magnetic susceptibility is an essential quantity to study in condensed matter systems. It provides invaluable information on the microscopic properties of materials, ranging from magnetic insulators, to spin glasses and even superconductors. Beyond static bulk properties, AC susceptibility gives also access to dynamical processes, like relaxation mechanisms [1]. As such, it is a widespread technique down to ~2 K and many commercial setups are on the market. However, state-of-the-art research in quantum magnetism demands extreme experimental conditions that are at odds with commercial solutions. Extremely versatile low-power, high-throughput devices are required to access the millikelvin regime in the presence of strong magnetic fields (> 10 T). We took up this endeavor and developed our own custom platform for AC susceptibility measurements.

In this master's project, you will develop a custom-made low-temperature susceptometer. The design will be based on a compact superconducting coil [2], specifically designed in collaboration with industry for this project. Several approaches will be taken to enhance the sensitivity of the device while keeping it low-power. In addition, you will gain insight into several areas of magnetism; various quantum materials are already available and urge for low-temperature AC susceptibility measurements. Finally, your work will contribute to establishing a cutting-edge measurement technique and will likely be part of several publications in high-impact journals.

Anomalous magnetoelastic coupling at quantum phase transitions

magnetostriction

Preliminary measurements of magnetostriction (a) and sound velocity (b) in Cs2CoBr4, as well as the phase diagram (c), all with a magnetic field applied along b.

Completed in 2023

In magnetically frustrated systems, competing interactions give rise to a plethora of exotic magnetic phases. Even at zero temperature, these can be accessed by tuning an external control parameter like magnetic field or pressure, leading to significant renormalization of thermodynamic properties. Case in point, the magnetoelastic coupling, an often neglected interaction between the spin and lattice degrees of freedom, can exhibit anomalous behavior close to such quantum phase transitions.

The goal of this master's thesis shall be an investigation of the magnetoelastic properties of Cs2CoBr4, a novel frustrated quantum material realizing a complex phase diagram with five distinct magnetic phases [1,2]. You will complete commissioning of our recently established dilatometric and ultrasonic measurement setups by carrying out systematic high-precision measurements at cryogenic temperatures and high magnetic fields. You will determine the magnetostriction, changes in sample length in response to a field, and the sound velocity, the speed of propagating lattice vibrations, of Cs2CoBr4 as a function of external parameters. This will elucidate how the lattice is affected by magnetic correlations and vice versa, giving us insights into the complex magnetism of this material and most likely resulting in a high-profile journal publication.

Triangular lattice quantum spin liquid candidates

A valence-bond state of a frustrated triangular lattice antiferromagnet

A valence-bond state of a frustrated triangular lattice antiferromagnet [1].

Completed in 2022

Quantum spin liquids (QSLs) [1,2] are novel collective quantum spin states where long-range magnetic order has been suppressed by quantum fluctuations. Theorists have predicted that QSLs exhibit many interesting properties, such as fractionalized spin excitations. This may allow quantum mechanical encryption and transportation of information that can be protected against environmental influences. Experimentalists have devoted great efforts to searching for QSL states in several different candidate materials, mostly geometrically frustrated quantum magnets. However, solid, unambiguous evidence for these exotic states has not been found yet.

We have recently grown several new QSL candidate materials based on triangular lattice quantum magnets. In this project, you will perform thermodynamic and magnetic measurements on these materials at ultra-low temperatures and high magnetic fields, to characterize their ground state and potential magnetic-field-induced phase transitions. The ultimate goal is to look for experimental signatures of a QSL state.

A presaturation state in a triangular lattice system

br

Magnetic phase diagram of Cs2CoBr4 with (a) magnetic field along a and (b) field along b [1].

Completed in 2022

Spin-density wave (SDW) states are the epitome of quantum magnetism. Semiclassical models of spin insulators forbid their existence. More sophisticated quantum field theories predict their realization in particular triangular lattice systems, where coupling antiferromagnetic chains in a frustrated zigzag manner boosts longitudinal correlations, leading to the SDW order.

We have found the world's first experimental realization of a 2D SDW in Cs2CoBr4 at low field [1,2]. Surprisingly, one of the high-field "pre-saturation" states bears scattering signatures of possibly another SDW state. Its peculiarity is the extreme robustness: it appears to exist for any orientation of the magnetic field.

Clarifying this observation is the goal of this semester project. You will perform angle-dependent thermodynamic measurements at ultra-low temperatures and high magnetic fields. This will give us insight into the stability of the phase diagram. Combining these data with our scattering results will clarify the origin of this presaturation state and its angle dependency.

Phase diagram of a kagome quantum antiferromagnet

The frustrated kagome lattice

The frustrated kagome lattice [1]

Completed in 2022

Geometric frustration remains one of the key topics in quantum magnetism, as it typically inhibits classical long-range order and instead promotes exotic quantum phases of matter. Perhaps the most archetypical of models to study magnetic frustration is the S = 1/2 kagome antiferromagnet, realizing a corner-sharing lattice of triangles. According to long-standing theoretical predictions, it should realize the highly sought-after quantum spin liquid ground state [1], which is characterized by long-ranged entanglement between spins without any spontaneous symmetry breaking, as well as fractionalized excitations. However, the hunt for experimentally suitable model materials is still ongoing, as most candidates studied so far are plagued by effects of structural disorder or additional terms in the Hamiltonian. Therefore, the recently discovered disorder-free family of rare-earth based kagome lattice antiferromagnets RE3BWO9 [2] holds tremendous promise. Several species show no signs of ordering down to the lowest temperatures, making them prime candidates to study spin liquid physics.

In this semester project, you will investigate the thermodynamic properties of a single member of the RE3BWO9 family of frustrated quantum magnets. You will learn state-of-the-art techniques of performing calorimetric and magnetic measurements at ultra-low temperatures (down to 50 mK) and high magnetic fields (up to 14 T). This will allow you to map out the entire phase diagram, marking an important step in the study of these exciting new quantum materials. The results obtained during this study are expected to be an essential contribution to a high-profile journal publication.

Spin Hamiltonian in linear frustrated ferro-antiferromagnets

Magnetic phase diagram of Rb2Cu2(MoO3)4 Magnetic phase diagram of Cs2Cu2(MoO3)4

Magnetic phase diagram of (a) Rb2Cu2(MoO3)4 [1] and (b) Cs2Cu2(MoO3)4 [2] under magnetic fields applied along their chain direction.

Completed in 2021

One-dimensional magnetic systems are a fantastic playground for the study of quantum phases of matter. Adding competing interactions results in a zoo of unusual quantum phases, since any classical order is often washed out. In this regard, the spin chain molybdates A2Cu2(MoO3)4 (with A = Rb or Cs) are especially interesting. Despite their similar crystal structure, the subtle interplay between exchange interactions results in very different magnetic ground states. In the Rb-based compound a quantum paramagnetic ground state has been reported [1] and very peculiar dielectric properties have been found. The Cs counterpart realizes an antiferromagnetic ground state where a handful of phases have been observed under magnetic fields, including an intriguing presaturation phase for which spin-multipolar condensates (such as spin-nematics) have been predicted [2].

In this master's project you will dive into the characterization of the spin excitation spectrum in these systems. For that you will get involved in inelastic neutron scattering experiments at NIST or Oak Ridge in the USA. Then, you will deduce an effective spin Hamiltonian for these materials based on a thorough analysis of the data. This study will provide insight on the intimate differences between these magnetic systems. It will unveil the complex interactions taking place in the presumed simple Rb2Cu2(MoO3)4 spin chain and certainly draw conclusions about the realization of spin-nematicity in the Cs system. This project will also most probably contribute to the publication of a paper in a high-impact journal.

Magnetic excitations in high magnetic fields

Enlarged view: PD_FL

Magnetic phase diagram of SrZnVO(PO4)2. The novel presaturation (PS) phase appears on top of the classical canted columnar antiferromagnetic (CAF) state just below complete magnetic saturation. Is this phase a spin-nematic?

Completed in 2021

Classical magnets with competing ferromagnetic and antiferromagnetic interactions may switch from one type of magnetic structure to the other depending on their relative strength. In quantum magnets such a direct ferro-antiferro transition is prevented by zero-point spin fluctuations. Instead, a novel and often very exotic quantum phase must appear between the two classical states. The effect is enhanced in strong magnetic fields [1]. The most sought-after state of this kind is the so-called quantum spin-nematic [2].

In this master's project you will investigate a recently discovered pre-saturation phase in the frustrated quantum ferro-antiferromagnet SrZnVO(PO4)2. You will participate in a week-long high-magnetic-field inelastic neutron scattering experiment in Grenoble, France. You will then perform a very sophisticated analysis of the collected data in order to determine the magnetic excitation spectrum. This study will most certainly clarify the nature of the new high-field phase and hopefully prove it to be a quantum spin nematic, a first for this class of systems. The project has a high chance of success and will likely result in a nice paper in a high-profile physics journal.

Magnetometry at ultra-low temperatures

Torque magnetometer
Torque magnetometer as used in our group [4].

Completed in 2018

Magnetism is an important player in many of the exciting topics in condensed matter physics: just think of unconventional superconductivity or exotic quantum states in magnetic insulators, to name only two fields of current research.

Naturally, measuring uniform magnetization is one of the first steps in any experimental study of a material where magnetism is relevant. Down to a few kelvin there are several techniques routinely used to obtain precise measurements of magnetization. However, at temperatures below 2 K, measuring magnetization becomes very difficult.

In our group we have already built a "proof-of-concept" Faraday force magnetometer that enables measurements of magnetization down to 0.05 K [1]. The present project encompasses the development, testing and commissioning of a reliable experimental setup for magnetization measurements at these ultra-low temperatures and in high magnetic fields.

Once working, this setup will be one of only a few in the world, and we have samples of several interesting materials where measurements of magnetization at these low temperatures would be extremely valuable, in particular for the study of quantum phase transitions, i.e., zero-temperature transitions between different ground states [e.g., 2,3].

A student working on this project will participate in all aspects of experimental condensed matter physics. First you will learn skills in designing and building measurement equipment and ultra-low temperature experiments. Then you will make use of the new measurement setup to investigate magnetization near quantum critical points in magnetic insulators featuring quantum phase transitions.

Quantum criticality in a magnetic insulator

Enlarged view: DTN

Magnetic excitation spectrum measured by inelastic neutron scattering for pure DTN (a) and for 6% and 21% bromine substituted DTNX (b,c).

Completed in 2017

The compound NiCl2·4SC(NH2)2 (a.k.a. DTN) is one of the very frequently studied magnetic insulators. It contains Ni2+ ions which carry a spin 1. These spins then interact in a very well defined and "simple" manner, making DTN an excellent model system in the field of quantum magnetism.

Initially DTN became famous for a magnetic field induced antiferromagnetic phase which has been understood as a Bose-Einstein condensate of magnons [1]. More recently, Ni(Cl1−xBrx)2·4SC(NH2)2 "DTNX" samples were synthesized with some of the chlorine atoms substituted by bromine. This strongly alters the interaction between the spin-1 moments. Thus it opens the possibility to tune the spin interactions by varying the bromine content in DTNX [2]. At the same time, this opens the question how the introduced disorder affects the magnetism. Here, there is the tantalizing possibility of experimentally studying a Bose glass state, an exotic quantum state of bosons in the presence of a random potential [3].

In our group we have succeeded in growing large high-quality single crystals of both pure and bromine substituted DTN. This opens the possibility of a variety of exciting experiments, such as: inelastic neutron scattering experiments to investigate a quantum phase transition occurring at approximately 15% bromine substitution; or measurements of specific heat or magnetization at high magnetic fields and very low temperatures to investigate possible Bose glass physics.

As you can see, although long studied, there is a lot to be learned from this compound and exotic physics to be explored. On top of that, high-quality samples are ready and waiting to be studied! This project can be easily adjusted in scope and will make an excellent semester or master's project and might even grow into an interesting PhD study in the field of quantum magnetism.

A magnetic field induced quantum phase transition at its upper critical dimension

Enlarged view: Specific heat scaling from BPCB paper

Scaling of specific heat near the magnetic field induced quantum phase transition in (C5H12N)2CuBr4. (a) Magnetic specific heat as measured and (b) the same data for T < 0.5 K plotted in scaled variables.

Completed in 2019

Quantum critical points (QCPs) occur at zero temperature as a function of some non-thermal parameter when a system's ground state abruptly changes. At first glance this seems like a rather abstract situation and certainly not relevant experimentally. However, it turns out that the presence of a QCP can strongly affect physical properties in a wide parameter range and in particular at elevated temperatures [1].

Magnetic insulators offer particularly suitable model systems for the experimental study of such phase transitions. A prominent example are magnetic field induced transitions from a non-magnetic state at low fields to a magnetized state at high fields.

Previously we have investigated such transitions in the quasi-one-dimensional spin ladder compound (C5H12N)2CuBr4 [2]. We have observed striking scaling behavior of specific heat data (figure). By means of inelastic neutron scattering experiments we have also studied in detail the critical dynamics near the transition.

In the present project we plan to investigate the compound (C5H6N2F)2CuCl4 [3]. This compound exhibits precisely the same magnetic field induced quantum phase transition. However, it has a layered structure and offers the possibility to study this QCP in two dimensions. This makes a huge difference, as d=2 corresponds to the upper critical dimension for this particular transition, and experimentally we are entering uncharted territory.

A student working on this project will perform measurements of specific heat and magnetization at low temperatures (down to 50 mK) and high magnetic fields (up to 14 T) in our laboratory at ETH. To study the quantum critical dynamics you will take part in neutron scattering experiments performed at large-scale neutron facilities. We expect this research to contribute to a 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.

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.

Completed in 2016

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 the 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 the 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 as 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.

Dielectric properties of quantum magnets

Enlarged view: Magnetocapacitive effect in Sul-Cu2Cl4 at ultralow temperatures (left) and the corresponding phase diagram (right).

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.

Completed in 2016

Combining nontrivial magnetic and electric properties within one material has always been a tricky problem, as they require completely different types of symmetry breaking. Nonetheless, as was discovered about ten years ago, certain types of magnetic ordering do also trigger an 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 an ultralow-temperature environment and high static magnetic fields, which is available right here, in our lab on the ETH Hönggerberg campus.

Some first steps on this way have already been taken with the highly frustrated S=1/2 material Sul-Cu2Cl4 (see figure). The careful examination of the 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 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 the NSM lab at ETH. The results obtained during this study are expected to be an essential contribution to a high-profile journal publication.

Effects of site disorder in a Heisenberg spin ladder

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

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

Completed in 2016

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

A student, working as a member of a young and dynamic team, will learn state-of-the-art methods of dealing with ultra-low temperatures, high magnetic fields and calorimetric measurements under such conditions. All the experiments will be conducted in the NSM group laboratory on the ETH Hönggerberg campus. This semester project has all the chances of turning into a more profound master's project study, and we naturally expect it to contribute to a high-profile journal publication.

Phase diagram of a non-ideal Luttinger liquid

Enlarged view: Inelastic neutron scattering spectrum and structure of DIMPY.

Inelastic neutron scattering spectrum and structure of DIMPY.

Completed in 2015

Even though their ground states exhibit no long-range order, low-dimensional quantum magnets reveal a cornucopia of 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 lowest-energy excitations are sharp magnons carrying S = 1 and they exhibit a spin gap. 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.

The aim of the present work is to study the effect of site disorder in a real Heisenberg spin ladder. We focus on the 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 introduced by random substitution 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 magneto-thermodynamic properties of the Zn-disordered samples by specific heat and magnetization measurements at very low temperatures (down to 50 mK) and high magnetic fields (up to 14 T) and to compare the results to measurements performed on the pure sample. 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 will 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 the Paul Scherrer Institut), Raman spectroscopy and muon spin rotation studies. Collaborations may include EPFL, the University of Geneva, Institut Laue-Langevin and Laboratoire National des Champs Magnétiques Intenses (France), as well as Oak Ridge National Laboratory (USA).