Superconductivity in TiSe$_2$ emerges when the charge density wave (CDW) order is suppressed under pressure or doping. Recent theoretical and experimental studies suggest that a Lifshitz transition plays a key role in stabilizing the superconducting phase. Here, we present muon spin resonance measurements of pressurized TiSe$_2$, revealing a two-gap superconducting state. Our results indicate that the smaller gap contributes unexpectedly strongly to the total superfluid density. This effect is consistent with an enhanced density of states in a newly formed Fermi surface pocket at the Lifshitz transition. These findings provide microscopic insight into the interplay between CDW suppression, Fermi surface reconstruction, and multi-gap superconductivity in TiSe$_2$, demonstrating how pressure-induced changes in electronic structure can shape superconducting properties in layered materials. 

Layered transition metal dichalcogenides (TMDs) are model systems to investigate the interplay between superconductivity and the charge density wave (CDW) order. Here, we use muon spin rotation and relaxation ($\mu^+$SR) to probe the superconducting ground state of polycrystalline 2H-TaS$_2$, which hosts a CDW transition at 76~K and superconductivity below 1~K. $\mu^+$SR measurements, conducted down to 0.27~K, reveal a fully gapped $s$-wave superconducting state consistent with Bardeen-Cooper-Schrieffer (BCS) theory. Fits to the temperature dependence of the depolarization rate and Knight shift measurements confirm spin-singlet pairing. Crucially, no evidence of time-reversal symmetry breaking (TRSB) is observed, distinguishing 2H-TaS$_2$ from polymorphs like 4Hb-TaS$_2$, where TRSB and unconventional superconductivity have been reported. These findings firmly establish 2H-TaS$_2$ as a canonical BCS superconductor and prove essential for understanding the diverse electronic ground states that emerge in structurally distinct TMD polymorphs. 

We report a pressure-induced quantum phase transition in the spin-ladder compound (C$_5$H$_9$NH$_3$)$_2$CuBr$4$ (Cu-CPA), investigated using SQUID magnetometry, muon spin relaxation (\musr), and inelastic neutron scattering (INS). At ambient pressure, Cu-CPA realizes a quantum spin-liquid (QSL) state characteristic of an antiferromagnetic (AFM) strong-leg spin ladder. Under hydrostatic pressure, we identify an abrupt transition at $p_{\rm c} \approx 2.35$~kbar into a ferromagnetically (FM) ordered ground state, marked by a sharp increase in magnetization and supported by Curie–Weiss analysis. \musr\ and SQUID measurements reveal additional low-temperature transitions, while INS confirms a double-leg ladder excitation spectrum with two small spin gaps ($\Delta_1 = 0.53$~meV, $\Delta_2 = 0.37$~meV) that are expected to collapse under modest pressure. Unlike the gradual gap closure typical of AFM liquids, Cu-CPA exhibits a discontinuous pressure response, pointing towards a pressure-driven structural modification or a shift of AFM excitations in $Q$-space. These findings establish Cu-CPA as a rare and fascinating example of a low-dimensional quantum magnet undergoing a pressure-tuned transition from an AFM spin-liquid to a FM ordered state, highlighting new routes to pressure-controlled quantum states.

Charge-density waves (CDWs) in layered transition-metal dichalcogenides (TMDs) emerge from the coupled evolution of lattice vibrations and electronic states. In 2H-TaS$_2$, however, the microscopic origin of the CDW transition has remained unsettled. Here we combine inelastic X-ray scattering (IXS), angle-resolved photoemission spectroscopy (ARPES), and density-functional theory (DFT) to uncover a mechanism that deviates from the canonical soft-mode scenario. IXS reveals a momentum-localized Kohn anomaly at $q_{\mathrm{CDW}}$ that softens strongly but saturates at $\sim 4$~meV, defining a narrow precursor regime $\sim 3$~K above $T_{\mathrm{CDW}} = 77$~K. In contrast, ARPES shows a sharp electronic response at the transition: a sizeable CDW gap of $\Delta = 95 \pm 9$~meV opens abruptly below $T_{\mathrm{CDW}}$, while no pseudogap is detected above. Harmonic calculations reproduce the ordering vector but overestimate the transition scale, highlighting the importance of anharmonic fluctuations, phonon mode mixing, and electronic feedback in truncating the phonon collapse. Together, these results identify 2H-TaS$_2$ as a distinct archetype of CDW order, where an incomplete lattice softening coexists with a strong-coupling electronic gap, in sharp contrast to the complete phonon collapse recently reported in 2H-TaSe$_2$. More broadly, they demonstrate how electron–phonon coupling and lattice dynamics cooperate to generate diverse CDW pathways in TMDs.

For quantum systems or materials, a common procedure for probing their behaviour is to tune electronic/magnetic properties using external parameters, e.g. temperature, magnetic field or pressure. Pressure application as an external stimuli is a widely used tool, where the sample in question is inserted into a pressure cell providing a hydrostatic pressure condition. Such device causes some practical problems when using in Muon Spin Rotation/Relaxation (µ+SR) experiments as a large proportion of the muons will be implanted in the pressure cell rather than in the sample, resulting in a higher background signal. This issue gets further amplified when the temperature dependent response from the sample is much smaller than that of the pressure cell,which may cause the sample response to be lost in the background and cause difficulties in aligning the sample within the beam. To tackle this issue, we have used pySRIM [1] to construct a practical and helpful simulation tool for calculating muon stopping fractions, specifically for the pressure cell setup at the µE1 beamline using the GPD spectrometer at the Paul Scherrer Institute, with the use of TRIM simulations. The program is used to estimate the number of muon stopping in both the sample and the pressure cell at a given momentum. The simultion tool is programmed into a GUI, making it accessible to user to approximate prior to their experiments at GPD what fractions will belong to the sample and the pressure cell in their fitting procedure.

Low-dimensional quantum magnets are a versatile materials platform for studying the emergent many-body physics and collective excitations that can arise even in systems with only short-range interactions. Understanding their low-temperature structure and spin Hamiltonian is key to explaining their magnetic properties, including unconventional quantum phases, phase transitions, and excited states. We study the metal-organic coordination compound (C5H9NH3)2CuBr4 and its deuterated counterpart, which upon its discovery was identified as a candidate two-leg quantum (S=12) spin ladder in the strong-leg coupling regime. By growing large single crystals and probing them with both bulk and microscopic techniques, we deduce that two previously unknown structural phase transitions take place between 136 and 113 K. The low-temperature structure has a monoclinic unit cell that gives rise to two inequivalent spin ladders. We further confirm the absence of long-range magnetic order down to 30 mK and investigate the implications of this two-ladder structure for the magnetic properties of (C5H9NH3)2CuBr4 by analyzing our own specific-heat and susceptibility data.