Quantum phase transitions (QPT) on the border of magnetism have provided a fertile hunting ground for the discovery of new states of matter, for example; the marginal Fermi Liquid and non Fermi Liquid states as well high T$_C$ cuprate and magnetically mediated superconductivity. In this thesis I present work on three materials in which it may be possible to tune the system through a magnetic QPT with the application of hydrostatic pressure. Although the details of the underlying physics are different in each of the materials, they are linked by the possibility of finding new states on the border of magnetism. Applying hydrostatic pressure, we have suppressed the ferromagnetic (FM) transition in metallic Fe$_2$P to very low temperature and to a potential QPT. Counter-intuitive broadening of the magnetic hysteresis leading up to the FM-AFM QPT may well be a crucial clue as to the nature of the model needed to understand this phase transition. A sharp increase in the quasi-particle scattering cross-section as well as the residual resistivity accompany a departure from the quadratic temperature dependence of the resistivity. This possible deviation from Fermi liquid behaviour is stable over a significant range of temperature. The unexplained upturn in the resistivity of CeGe that accompanies the AFM transition was studied under pressure. Pressure increased the residual resistivity as well as decreasing the relative size of the upturn, but had a moderate effect on the Neel temperature. The insensitivity of the N$\acute e$el temperature to pressure has been compared to its relative sensitivity to applied feld. The existence of the upturn and its evolution with pressure and applied feld can reasonably be argued to be due to the details of the electron band structure in the system. By applying pressure we have drastically reduced the resistivity of the insulating antiferromagnet NiPS$_3$. Concurrent work on FePS$_3$ has shown metallisation under pressure. It seems reasonable to speculate that NiPS$_3$ may also metallise at higher pressure. The energy gap is narrowed in both materials as pressure is increased. Magnetisation measurements have revealed a low temperature upturn indicating some possible ferromagnetic component or proximity to another magnetic state. A peak in the magnetisation is also seen at 45K in zero-feld cooled measurements. Both of these features point to a system with a complex magnetic ground state.

A major area of interest in condensed matter physics over the past decades has been the emergence of new states of matter from strongly correlated electron systems. A few limited examples would be the emergence of unconventional superconductivity in the high-T$_c$ superconductors and heavy-fermion systems, the appearance of the skyrmion magnetic vortex state in MnSi and magnetically mediated superconductivity in UGe$_2$. While detailed studies of many of the emergent phases have been made, there are still many gaps in understanding of the underlying states and mechanisms that allow them to form. This work aims to add to knowledge of the basic physics behind such states, and the changes within them as they are tuned to approach new phases. The cubic perovskite material SrTiO$_3$ has been studied for many decades and is well-documented to be an incipient ferroelectric, theorised to exist in the absence of any tuning in the proximity of a ferroelectric quantum critical point. This work presents the first high-precision dielectric measurements under hydrostatic pressure carried out on a quantum critical ferroelectric, leading to a full pressure-temperature phase diagram for SrTiO$_3$. The influence of quantum critical fluctuations is seen to diminish as the system is tuned away from the quantum critical point and a novel low temperature phase is shown to be emergent from it. The Néel Temperature of the two-dimensional antiferromagnet FePS$_3$ was found to increase linearly with applied hydrostatic pressure. Evidence of an insulator-metal transition is also presented, and an unexplained upturn in the resistivity at low temperatures in the metallic phase.

We present a comprehensive study of a low-dimensional spin-half quantum antiferromagnet, the J-Q model, in the presence of an external (Zeeman) magnetic field using numerical methods, chiefly stochastic series expansion quantum Monte Carlo with directed loop updates and quantum replica exchange. The J-Q model is a many-body Hamiltonian acting on a lattice of localized spin-half degrees of freedom; it augments the Heisenberg exchange with a four-spin interaction of strength Q. This model has been extensively studied at zero field, where the Q term drives a quantum phase transition from a Néel-like state to a valence-bond solid (a nonmagnetic state consisting of a long-range-ordered arrangement of local singlet bonds between sites). This transition is believed to be an example of deconfined quantum criticality, where the excitations are spinons—exotic spin-half bosons. We study the J-Q model in the presence of a magnetic field in both one and two dimensions. In one dimension, there is metamagnetism above a critical coupling ratio (Q/J)min. Metamagnetism is a first-order quantum phase transition characterized by discontinuities in the magnetization as a function of field (magnetization jumps). We derive an exact expression for (Q/J)min = 2/9, and show that the metamagnetism is caused by the onset of attractive interactions between magnons (flipped spins on a polarized background). We predict that the same mechanisms will produce metamagnetism in the unfrustrated antiferromagnetic J1-J2 model with anisotropy. Below (Q/J)min, the saturation transition is continuous and we show that it is governed by the expected zero-scale-factor universality. In two dimensions, we also find metamagnetism above a critical coupling ratio (Q/J)min=0.417, caused by the same mechanism as in the one-dimensional case. In two dimensions we also show evidence of an anomalous temperature dependence of specific heat arising from field-induced Bose-Einstein condensation of spinons at the deconfined quantum critical point. / 2019-11-06T00:00:00Z

In the repertoire of an experimental condensed matter physicist, the ability to tune continuously through features in the electronic structure and to selectively break point-group symmetries are both valuable techniques. The experimental technique at the heart of this dissertation, uniaxial stress, can do both such things. The thesis will start with a thorough discussion of our new technique, which was continually developed over the course of this work, presenting both its unique capabilities and also some guidance on the best working practices, before moving on to describe results obtained on two different strongly correlated electron materials. The first, Sr2RuO4, is an unconventional superconductor, whose order parameter has long been speculated to be odd-parity. Of interest to us is the close proximity of one of its three Fermi surfaces to a Van Hove singularity (VHs). Our results strongly suggest that we have been able to traverse the VHs, inducing a topological Lifshitz transition. T[sub]c is enhanced by a factor ~2.3 and measurements of H[sub](c2) open the possibility that optimally strained Sr2RuO4 has an even-parity, rather than odd-parity, order parameter. Measurements of the normal state properties show that quasiparticle scattering is increased across all the bands and in all directions, and effects of quantum criticality are observed around the suspected Lifshitz transition. Sr3Ru2O7 has a metamagnetic quantum critical endpoint, which in highly pure samples is masked by a novel phase. Weak in-plane magnetic fields are well-known to induce strong resistive anisotropy in the novel phase, leading to speculation that a spontaneous, electronically driven lowering of symmetry occurs. Using magnetic susceptibility and resistivity measurements we can show that in-plane anisotropic strain also reveals the strong susceptibility to electronic anisotropy. However, the phase diagram that these pressure measurements reveal is consistent only with large but finite susceptibility, and not with spontaneous symmetry reduction.

In a many-body fermionic system, the suppression of continuous transitions to absolute zero can result in a low temperature quantum fluid which deviates strongly from typical metallic behaviour; unconventional superconductivity can be induced by the strange metal region surrounding the zero-temperature phase transition. In this thesis we focus on three systems which demonstrate a highly tunable phase transition, with the aim of pushing them toward the border of a zero-temperature phase transition, and potentially superconductivity. CeAgSb2 is a uniaxial 4f ferromagnet, where physical pressure or a transverse field may be used to tune the magnetic transition towards T = 0 K. Our investigations, however, did not reveal the presence of superconductivity. It is likely that the field tuned transition does not correspond to a true critical point, whilst the high pressure region may be occupied by an antiferromagnetic phase, with the true critical point at higher pressures. However, other interesting features emerge in the electrical resistivity and AC-susceptibility, along with novel thermodynamic signatures linking the magnetisation to the specific heat. The doping series Lu(1-x)YxFe2Ge2 shows an antiferromagnetic transition which is suppressed to absolute zero at a critical concentration x_c=0.2. YFe2Ge2 displays anomalous low temperature behaviour consistent with the proximity to quantum critical fluctuations, along with a superconducting transition which appears in the electrical resistivity beneath a critical temperature of T_c ~ 1.7 K. Using low temperature DC magnetisation measurements, we show that this is a bulk effect, and that the superconductivity in YFe2Ge2 is of type-II. The thermodynamic and BCS properties of the superconducting phase are analysed in line with the parameters we extract experimentally. The superconducting 3-4-13 stannides (Ca,Sr)3Ir4Sn13 show a high temperature structural transition which may be suppressed by the application of hydrostatic pressure or effective chemical pressure. A superconducting dome is found, which appears to peak near where the structural transition extrapolates to zero temperature. Anomalous exponents are seen in the electrical resistivity over a wide temperature range. We investigate the influence of pressure on the superconducting critical temperature in Ca3Ir4Sn13 and the related compound Co3Ca4Sn13, along with an analysis of the upper critical field and flux-line phenomena in Ca3Ir4Sn13 and Sr3Ir4Sn13.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21554号 / 理博第4461号 / 新制||理||1640(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 前野 悦輝, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22238号 / 理博第4552号 / 新制||理||1654(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 石田 憲二, 教授 寺嶋 孝仁 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Cette thèse expérimentale explore les propriétés du point critique de la phase pseudogap dans le diagramme de phase des cuprates supraconducteurs. Dans une première partie, j’expose un état de l’art sur les connaissances du diagramme de phases température-dopage (T-p) de ces systèmes. Des études récentes montrent une chute importante de la densité de porteurs électroniques au voisinage du point critique suggérant une reconstruction de la surface Fermi. Pour comprendre la nature exacte de la transition de phases liée à cette reconstruction, j’ai réalisé des mesures complémentaires de transport thermique et de chaleur spécifique sous champ magnétique intense sur les familles La1.8-xSrxEu0.2CuO4 et La1.6-xSrxNd0.4CuO4.Dans une deuxième partie, après une introduction théorique sur la chaleur spécifique et le transport thermique, je détaille comment ces deux grandeurs ont été mesurées. En particulier, une technique originale de mesures de la chaleur spécifique a été mise au point pour combiner haute résolution et précision absolue en champ magnétique intense et basse température. Différents modèles thermiques et électroniques ont été développés pour comprendre et analyser les mesures et ont permis d’optimiser les différents montages de chaleur spécifique selon les gammes de température.Dans une troisième partie, je présente l’ensemble des résultats obtenus en transport thermique et chaleur spécifique. Le transport thermique confirme la chute de la densité de porteur dans l’état normal (sans supraconductivité) des cuprates déjà observée en transport électrique sous champ intense. Par ailleurs, j ‘ai montré que cette chute existe également au sein de la phase supraconductrice (à champ magnétique nul), montrant qu’elle n’est influencée ni par la présence de la supraconductivité ni par le champ magnétique. Dans l’état normal, la loi de Wiedemann-Franz est respectée prouvant le caractère métallique de la phase pseudogap.La chaleur spécifique électronique montre un comportement non classique à proximité du point critique. Ce comportement anormal est caractérisé par une dépendance logarithmique en fonction de la température au dopage critique p* correspondant à la chute du nombre de porteurs. De plus, ces mesures suggèrent une divergence de la masse effective à p* en fonction du dopage. Ces deux observations sont la signature d’un point critique quantique localisé à T = 0 et p = p* dont l’origine est discutée dans la dernière partie. Les différentes classes d’universalités possibles sont discutées et une comparaison avec d’autres composés (fermions lourds, pnictures) possédant un point critique quantique est présentée. / This experimental PhD thesis explores the properties of the pseudogap critical point in the phase diagram of superconducting cuprates. In a first part, I present a state of the art on the knowledge of the temperature-doping (T-p) phase diagram of these systems. Recent studies show a dramatic drop in the electronic carrier density near the critical point, suggesting a Fermi surface reconstruction. To understand the exact nature of the phase transition related to this reconstruction, I performed complementary high magnetic field measurements of thermal transport and specific heat on La1.8-xSrxEu0.2CuO4 and La1.6-xSrxNd0.4CuO4 cuprates.In a second part, after a theoretical introduction on specific heat and thermal transport, I detail how these two quantities were measured. In particular, an original technique for measuring specific heat has been developed to combine high resolution and absolute accuracy in high magnetic field and low temperature. Different thermal and electronic models have been developed to understand and analyze the measurements in order to optimize the different set-ups according to the temperature range.In a third part, I present the results obtained in thermal transport and specific heat. Thermal transport confirms the drop in carrier density in the normal state (without superconductivity) of cuprates, already observed in high magnetic field electrical transport. Moreover, this drop also exists within the superconducting phase (in zero magnetic field), showing that it is neither influenced by the presence of superconductivity nor by the magnetic field. In the normal state, the Wiedemann-Franz low is satisfied, proving the metallic character of the pseudogap phase.Electronic specific heat shows non-classical behavior in the vicinity of the critical point. This abnormal behavior is characterized by a logarithmic dependence as a function of temperature at the critical doping p *, corresponding to the drop in the carrier density. Moreover, these measurements suggest a divergence of the effective mass at p * as a function of doping. These two observations are the signature of a quantum critical point located at T = 0 and p = p *, whose origin is discussed in the last part. I discuss the possible universality classes, and I compare with others compounds (heavy fermions, pnictides) which present a quantum critical point.

Materials in proximity to quantum critical points (QCPs) experience strong fluctuations in the order parameter associated with the transition and often, as a result, display interesting properties. In this dissertation, we have used a variety of experimental probes such as Shubnikov-de Haas quantum oscillations, thermal conductivity and heat capacity, to better understand two such materials — $A_3T_4$Sn$_{13}$ and YFe$_2$Ge$_2$. $A_3T_4$Sn$_{13}$ ($A$ = Ca, Sr; $T$ = Ir, Rh) is a family of quasi-skutterudite superconductors with moderate $T_c$’s between 4 and 8 K. Although the superconductivity is believed to be phonon-mediated with s-wave pairing symmetry, an unusual second-order structural transition makes this material family fascinating to study. Whether this structural transition is a result of three distortions with perpendicular wavevectors resulting in a cubic-to-cubic transformation, or each wavevector acting independently giving rise to cubic-to-tetragonal transformations and formation of twinned domains is a disputed issue. We have measured quantum oscillations in the resistivity of Sr3Ir4Sn13 and compared it to density functional theory (DFT) calculations for both scenarios. Our results strongly suggest that the former interpretation is correct. The structural transition temperature $T^*$ in $A_3T_4$Sn$_{13}$ can be suppressed to zero by tuning with physical or chemical pressure. In (Ca$_x$Sr$_{1−x}$)$_3$Rh$_4$Sn$_13$, the quantum critical point can be accessed purely by chemical substitution at x ~ 0.9. In the vicinity of the QCP, we expect large fluctuations of the order parameter at low temperatures, which for a structural transition could manifest as a structural disorder. We have measured thermal conductivity at temperatures much lower than $T_c$ and found that it is well described by a single power law with suppressed exponents near the QCP. The heat capacity, however, remains ~ $T^3$. After excluding conventional phonon scattering mechanisms, we propose the possibility of intrinsic quasi-static spatial disorder that is related to the structural QCP. YFe$_2$Ge$_2$ is closely linked to the “122” family of iron-based superconductors like KFe$_2$As$_2$, although it has a significantly lower $T_c$ ~ 1 K. It has a rather three-dimensional Fermi surface which closely resembles that of KFe$_2$As$_2$ in the pressure-induced collapsed tetragonal phase. YFe$_2$Ge$_2$ is in proximity to several types of magnetic order which are predicted by DFT calculations to have lower energy than the non-spin polarised case. Even though YFe$_2$Ge$_2$ is non-magnetic, its superconductivity could be strongly affected by magnetic fluctuations. Through a collaboration with researchers at the University of Waterloo, we have measured the thermal conductivity of YFe$_2$Ge$_2$ down to millikelvin temperatures and up to 2.5 T in field. Our results suggest that YFe$_2$Ge$_2$ is a nodal superconductor. This result could assist in the explanation of the unconventional superconductivity in iron-based superconductors.