Limited coincidence between ultrahigh-field superconductivity and line of metamagnetic endpoints in UTe$_2$

Abstract

The field-dependent magnetization of UTe$_2$ was measured through the metamagnetic transition at a variety of field angles, tracking how the step in magnetization evolves with fields tilted away from the $b$ axis. For fields oriented within the $ab$ plane, jumps in both $M_a$ and $M_b$ vanish approximately 18° away from the $b$ axis. From contactless conductivity measurements, we find that the halo-like high-field superconducting region extends to the $ab$ plane, where it exists only within a very narrow ($<$1°) angular range near the termination of the metamagnetic phase boundary and extends beyond the highest measured field of 73 T. As the field orientation tilts towards the $c$ axis, the superconducting and metamagnetic phase boundaries no longer coincide and exhibit distinct trends.

Figures (14)

• Figure 1: (a) Normalized magnetization jump along $b$ as a function of angle comparing field rotations in the $ab$ and $bc$ plane. Dashed lines are guides to the eye. Dotted line shows $\cos(\theta)$. (b) A comparison of magnetization jumps along $a$ and $b$ as a function of angle for rotations in the $ab$ plane. Dashed lines are guides to the eye. (c) $\mathrm{d}M_b/\mathrm{d}H$ versus field strength for sample S1 with field pulses performed at various angles within the $ab$ plane. (d) $\mathrm{d}M_b/\mathrm{d}H$ for sample S1 for fields within the $bc$ plane. (e) $\mathrm{d}M_a/\mathrm{d}H$ for sample S2 for fields within the $ab$ plane. (e) $\mathrm{d}M_b/\mathrm{d}H$ for sample S4 for fields within the $bc$ plane. For (c-f), all curve labels indicate the angle $\theta$ between the applied magnetic field and the $b$ axis, and all curves have been offset by an amount proportional to $\theta$ for visual clarity. (g) Illustration of the compensated coil susceptometry measurement. In the case of samples S1, S3, and S4, the coil is oriented such that it measures the magnetization component along $b$ as the sample is rotated so that its $b$ axis is at an angle $\theta$ from the applied field. (h) In the case of sample S2, the coil is oriented such that it measures the magnetization component along $a$ as the sample is rotated so that its $b$ axis is at an angle $\theta$ from the applied field.
• Figure 2: (a) PDO frequency as a function of field strength for pulses performed at various angles $\theta_a$ within the $ab$ plane. (b) Field angle-dependent phase diagram for fields applied within the $ab$ plane. (c) PDO frequency as a function of field strength for pulses performed at various angles $\theta_a$ away from the $bc$ plane, at fixed $\theta_{bc} =$ 45°. (d) Field angle-dependent phase diagram for fields applied at various angles $\theta_a$ away from the $bc$ plane, at fixed $\theta_{bc} =$ 45°. In (b) and (d), gray points indicate angles at which a metamagnetic transition is observed in magnetometry; blue (red) points from PDO samples indicate angles at which a superconducting transition (step-like metamagnetic transition) is observed.
• Figure 3: (a) High-field phase diagram for UTe$_2$ at 0.6 K as a function of $\theta_a$ and $\theta_{bc}$ based on magnetization and PDO measurements. PDO data shown are a compilation of PDO measurements from this report (samples P2 and P3; filled symbols) and from Ref. Lewin2024high (samples P1 and P2; empty symbols). Blue (red) points indicate angles where a superconducting transition (step-like metamagnetic transition) occurs. Gray points indicate angles at which a peak in $\mathrm{d}M_i/\mathrm{d}H$ was observed in magnetometry. Open black symbols indicate angles at which a partial peak was observed but was cut off at our maximum measurement field of 73 T. Blue and red shaded regions are guides to the eye for the SC$_{\rm{FP}}$ region and the $\Delta M > 0$ region described in the text. Inset is a zoom-in on select points in the $ab$ plane. Data points highlighted in white indicate angles at which temperature-dependent PDO measurements were performed; the labels a, b, and c correspond to the subfigures of Fig. \ref{['fig:temps']}. (b) Illustration of the 0.6 K phase diagram of UTe$_2$ as a function of $\theta_a$, $\theta_{bc}$, and magnetic field strength, based on the measurements shown in (a) as well as Ref. LewinFieldangle2024Lewin2024high. Away from the $ab$ plane, the boundary of the $\Delta M > 0$ region does not coincide with the field angles at which the SC$_{\rm{FP}}$ phase exists.
• Figure 4: Temperature-dependent PDO measurements of (a) sample P2 at $\theta_{bc}$ = 0° and $\theta_a$ = 18.5°; (b) sample P3 at $\theta_{bc}$ = 35° and $\theta_a$ = 0$\pm$0.2°; and (c) sample P3 at $\theta_{bc}$ = 35° and $\theta_a$ = 12.3$\pm$0.1°. The data points highlighted in white in Fig. \ref{['fig:phasediag']}a indicate the field angles at which these measurements were performed, in context of the overall phase diagram.
• Figure 5: (a) High-field phase diagram for UTe$_2$ at 0.6 K as a function of $H_a$ and $H_c$ based on magnetization and PDO measurements. All symbols are the same as those in Fig. \ref{['fig:phasediag']}a. (b) An illustration of the 0.6 K phase diagram of UTe$_2$ as a function of magnetic field components, based on the measurements shown in (a) as well as Ref. LewinFieldangle2024Lewin2024high.