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1. Unconventional superconducting phase diagram of monolayer ${\mathrm{WTe}}_2$

   https://doi.org/10.1103/PhysRevResearch.7.013224  

   Song, Tiancheng; Jia, Yanyu; Yu, Guo; Tang, Yue; Uzan, Ayelet J; Zheng, Zhaoyi Joy; Guan, Haosen; Onyszczak, Michael; Singha, Ratnadwip; Gui, Xin; et al (February 2025, Physical Review Research)

   The existence of a quantum critical point (QCP) and fluctuations around it are believed to be important for understanding the phase diagram in unconventional superconductors such as cuprates, iron pnictides, and heavy fermion superconductors. However, the QCP is usually buried deep within the superconducting dome and is difficult to investigate. The connection between quantum critical fluctuations and superconductivity remains an outstanding problem in condensed matter. Here combining both electrical transport and Nernst experiments, we explicitly demonstrate the onset of superconductivity at an unconventional QCP in gate-tuned monolayer tungsten ditelluride $\left({\mathrm{WTe}}_2\right)$, with features incompatible with the conventional Bardeen-Cooper-Schrieffer scenario. The results lead to a superconducting phase diagram that is distinguished from other known superconductors. Two distinct gate-tuned quantum phase transitions are observed at the ends of the superconducting dome. We find that quantum fluctuations around the QCP of the underdoped regime are essential for understanding how the monolayer superconductivity is established. The unconventional phase diagram we report here illustrates a previously unknown relation between superconductivity and QCP. Published by the American Physical Society2025 

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2. Quantum simulation of the spin- $\frac{1}{2}$ XYZ model using solid-state spin centers

   https://doi.org/10.1103/PhysRevB.110.014413  

   Losey, Troy; Candido, Denis R; Zhang, Jin; Meurice, Y; Flatté, Michael E; Tsai, S-W (July 2024, Physical Review B)

   In this work we propose a solid-state platform for creating quantum simulators based on implanted spin centers in semiconductors. We show that under the presence of an external magnetic field, an array of 𝑆=1 spin centers interacting through magnetic dipole-dipole interaction can be mapped into an effective spin-half system equivalent to the XYZ model in an external magnetic field. Interestingly, this system presents a wide range of quantum phases and critical behaviors that can be controlled via magnetic field and orientational arrangement of the spin centers. We demonstrate our interacting spin chain can be tuned to both isotropic Heisenberg model and transverse-field Ising universality class. Notably, our model contains a line where the system is in a critical floating phase that terminates at Berezinskii-Kosterlitz-Thouless and Pokrovsky-Talapov transition points. We propose this system as a solid-state quantum simulator for the floating phase based on spin centers. 

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3. Field-induced quantum critical point in the itinerant antiferromagnet Ti3Cu4

   https://doi.org/10.1038/s42005-022-00901-7  

   Moya, Jaime M.; Hallas, Alannah M.; Loganathan, Vaideesh; Huang, C.-L.; Kish, Lazar L.; Aczel, Adam A.; Beare, J.; Cai, Y.; Luke, G. M.; Weickert, Franziska; et al (December 2022, Communications Physics)

   Abstract New phases of matter emerge at the edge of magnetic instabilities, which can occur in materials with moments that are localized, itinerant or intermediate between these extremes. In local moment systems, such as heavy fermions, the magnetism can be tuned towards a zero-temperature transition at a quantum critical point (QCP) via pressure, chemical doping, and, rarely, magnetic field. By contrast, in itinerant moment systems, QCPs are more rare, and they are induced by pressure or doping; there are no known examples of field induced transitions. This means that no universal behaviour has been established across the whole itinerant-to-local moment range—a substantial gap in our knowledge of quantum criticality. Here we report an itinerant antiferromagnet, Ti 3 Cu 4 , that can be tuned to a QCP by a small magnetic field. We see signatures of quantum criticality and the associated non-Fermi liquid behaviour in thermodynamic and transport measurements, while band structure calculations point to an orbital-selective, spin density wave ground state, a consequence of the square net structural motif in Ti 3 Cu 4 . Ti 3 Cu 4 thus provides a platform for the comparison and generalisation of quantum critical behaviour across the whole spectrum of magnetism. 

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4. Tuning magnetic confinement of spin-triplet superconductivity

   https://doi.org/10.1038/s41535-020-00270-w  

   Lin, Wen-Chen; Campbell, Daniel J.; Ran, Sheng; Liu, I-Lin; Kim, Hyunsoo; Nevidomskyy, Andriy H.; Graf, David; Butch, Nicholas P.; Paglione, Johnpierre (December 2020, npj Quantum Materials)

   null (Ed.)

   Abstract Electrical magnetoresistance and tunnel diode oscillator measurements were performed under external magnetic fields up to 41 T applied along the crystallographic b axis (hard axis) of UTe 2 as a function of temperature and applied pressures up to 18.8 kbar. In this work, we track the field-induced first-order transition between superconducting and magnetic field-polarized phases as a function of applied pressure, showing suppression of the transition with increasing pressure until the demise of superconductivity near 16 kbar and the appearance of a pressure-induced ferromagnetic-like ground state that is distinct from the field-polarized phase and stable at zero field. Together with evidence for the evolution of a second superconducting phase and its upper critical field with pressure, we examine the confinement of superconductivity by two orthogonal magnetic phases and the implications for understanding the boundaries of triplet superconductivity. 

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5. A Quantum Critical Line Bounds the High Field Metamagnetic Transition Surface in UTe₂

   https://doi.org/10.1103/PhysRevX.15.021019  

   Wu, Z; Weinberger, T I; Hickey, A J; Chichinadze, D V; Shaffer, D; Cabala, A; Chen, H; Long, M; Brumm, T J; Xie, W; et al (April 2025, Physical Review X)

   Quantum critical phenomena are widely studied across various materials families, from high-temperature superconductors to magnetic insulators. They occur when a thermodynamic phase transition is suppressed to zero temperature as a function of some tuning parameter such as pressure or magnetic field. This generally yields a point of instability—a so-called quantum critical point—at which the phase transition is driven exclusively by quantum fluctuations. Here, we show that the heavy fermion metamagnet ${\mathrm{UTe}}_2$possesses a quantum phase transition at extreme magnetic field strengths of over 70 T. Rather than terminating at one singular point, we find that the phase boundary is sensitive to magnetic field components in each of the three Cartesian axes of magnetic field space. This results in the transition surface being bounded by a continuous ring of quantum critical points, the locus of which forms an extended line of quantum criticality—a novel form of quantum critical phase boundary. Within this quantum critical line sits a magnetic field-induced superconducting state in a toroidal shape, which persists to fields over 70 T. We model our data by a phenomenological free energy expansion and show how a quantum critical line—rather than a more conventional singular point of instability—anchors the remarkable high magnetic field phase landscape of ${\mathrm{UTe}}_2$. 

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