Pressure-induced superconductivity in topological insulator Ge2Bi2Te5 and the evolution with Mn doping

This study demonstrates that high pressure induces superconductivity in the topological insulator Ge2Bi2Te5, while Mn doping introduces antiferromagnetism that competes with and suppresses this superconducting state, offering a new platform to explore the interplay between band topology, magnetism, and superconductivity.

The Gist

Imagine a material called Ge₂Bi₂Te₅ as a special kind of "electronic highway." Under normal conditions, this highway is a topological insulator: the middle of the road is blocked (insulating), but the edges are wide open and super-fast (conducting). Scientists love these materials because they might hold the keys to future quantum computers.

However, this specific highway has a secret superpower waiting to be unlocked: superconductivity. This is a state where electricity flows with absolutely zero resistance, like a car driving on a frictionless track. The problem? It doesn't happen naturally.

Here is the story of how the researchers in this paper unlocked that power and what happens when they try to mix in a new ingredient.

1. The Pressure Cooker Experiment

The researchers decided to squeeze the material. Think of the material as a sponge. When you squeeze a sponge, its internal structure changes. In this case, they used a Diamond Anvil Cell, which is essentially a high-tech vice made of diamonds that can crush a tiny crystal with immense force (up to 57 times the pressure of the atmosphere).

• The Result: As they squeezed the Ge₂Bi₂Te₅ harder, something magical happened. At a specific pressure (about 23 gigapascals), the material turned into a superconductor.
• The "Dome" Shape: The superconductivity didn't just appear and stay the same. It acted like a hill or a dome.
  • At low pressure, nothing happened.
  • As pressure increased, the temperature at which it became superconducting (called $T_c$) rose, reaching a peak of 7.6 Kelvin (about -265°C).
  • If they squeezed it too hard, the superconductivity started to fade away again.

2. The "Mn" Ingredient: A Spoiler in the System

Next, the scientists tried to mix a new ingredient into the highway: Manganese (Mn). Think of Mn as a rowdy construction crew trying to build a wall across the road.

• At Normal Pressure: Adding Mn didn't just change the traffic; it stopped the flow entirely. It introduced antiferromagnetism. In simple terms, the electrons started spinning in opposite directions in a rigid pattern, effectively locking the material into a magnetic state.
• Under Pressure: When they squeezed the Mn-doped samples, the story changed dramatically.
  • Low Mn (25%): The material did become superconducting, but it was a weak version. The "hill" of superconductivity was flattened. The peak temperature dropped from 7.6 K down to just 2.3 K, and it took much more pressure to get there.
  • High Mn (49%): The "construction crew" was too strong. Even when they squeezed the material as hard as they could (65 GPa), the superconductivity never appeared. The magnetic order completely blocked the superconducting state.

3. The Great Rivalry: Magnetism vs. Superconductivity

The paper reveals a clear rivalry between two forces in this material:

• Magnetism (caused by Mn) wants to organize the electrons into a rigid, spinning pattern.
• Superconductivity wants the electrons to pair up and flow freely without resistance.

The researchers found that these two forces are competitive. When the magnetic "crew" is strong (high Mn), they win, and superconductivity is crushed. When the magnetic influence is weak or absent (pure Ge₂Bi₂Te₅), pressure can force the material to become a superconductor.

4. The Big Picture

The team compared their findings with other similar materials (a family called $mAX \cdot nB_2X_3$). They noticed a pattern:

• Non-magnetic members of this family usually become superconductors under pressure, reaching peak temperatures between 6 K and 8.5 K.
• Magnetic members usually struggle to become superconductors. If they do, the temperature is very low (around 2 K), and it requires extreme pressure.

In summary: This paper shows that by squeezing a topological insulator, you can turn it into a superconductor. However, if you try to add magnetic elements (Mn) to the mix, they act like a "spoiler" that fights against superconductivity, making it much harder to achieve. This gives scientists a new playground to study how magnetism and superconductivity fight for control in these exotic quantum materials.

Technical Summary

Technical Summary: Pressure-Induced Superconductivity in Topological Insulator Ge$_2$Bi$_2$Te$_5$ and the Evolution with Mn Doping

Problem Statement
While topological insulators (TIs) are characterized by insulating bulk states and robust, gapless surface states, the integration of intrinsic magnetism or superconductivity (SC) into these materials is essential for realizing exotic quantum states such as the quantum anomalous Hall effect and topological superconductivity (TSC). However, intrinsic magnetic TIs or TSC materials are scarce. Although the layered pseudobinary chalcogenide family ($m$AX $\cdot$ $n$B$_2$X$_3$) exhibits nontrivial band topology and has shown pressure-induced SC in non-magnetic members, magnetic members of this family largely lack superconducting phases. Specifically, the interplay between band topology, magnetism, and SC in the Ge$_2$Bi$_2$Te$_5$ system, and how Mn doping alters this relationship, remains an open question.

Methodology
The study employed a systematic investigation of single crystals of (Ge$_{1-x}$Mn$_x$)$_2$Bi$_2$Te$_5$ with Mn concentrations ($x$) ranging from 0 to 0.49.

• Sample Synthesis: Pure Ge$_2$Bi$_2$Te$_5$ ($x=0$) crystals were grown via the self-flux method. Mn-doped crystals ($x=0.25$ and $x=0.49$) were synthesized using the chemical vapor transport (CVT) method due to the difficulty of flux synthesis for these compositions.
• Characterization: Structural properties were verified using single-crystal X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) spectroscopy. Magnetic properties were measured via magnetization (ZFC/FC modes), and electrical transport properties were analyzed using the van der Pauw method.
• High-Pressure Experiments: In-situ high-pressure electrical transport measurements were conducted up to ~65 GPa using a diamond anvil cell (DAC) within a Physical Property Measurement System (PPMS). Pressure was calibrated using ruby luminescence.

Key Results

1. Structural and Magnetic Properties:

   • All samples crystallize in a layered rhombohedral structure (space group $P\bar{3}m1$). Increasing Mn content leads to a systematic decrease in the $c$-axial lattice parameter due to the smaller ionic radius of Mn compared to Ge.
   • While pure Ge$_2$Bi$_2$Te$_5$ is paramagnetic at ambient pressure, Mn doping induces antiferromagnetic (AFM) order. The Néel temperature ($T_N$) increases from ~6.0 K ($x=0.25$) to ~11.8 K ($x=0.49$).
   • All samples exhibit metallic behavior, with a resistivity kink observed in doped samples near $T_N$, attributed to the suppression of spin scattering upon entering the AFM state.

2. Pressure-Induced Superconductivity in Pure Ge$_2$Bi$_2$Te$_5$ ($x=0$):

   • Application of pressure induces superconductivity. SC emerges above ~3.8 GPa.
   • The critical temperature ($T_c$) follows a dome-shaped phase diagram, reaching a maximum $T_c$ of 7.6 K at 23.0 GPa.
   • Above 23 GPa, $T_c$ decreases monotonically.
   • Upper critical field ($\mu_0H_{c2}$) measurements indicate that the orbital depairing mechanism dominates, as the Pauli limiting field is significantly higher than the measured $\mu_0H_{c2}(0)$.

3. Effect of Mn Doping on Superconductivity:

   • $x=0.25$: The introduction of AFM order strongly suppresses SC. Superconductivity is only induced at much higher pressures (>19 GPa) and exhibits a significantly reduced $T_c$ of approximately 2.3 K, which remains relatively constant (forming a plateau) up to 61.5 GPa. The upper critical field is drastically reduced compared to the undoped sample.
   • $x=0.49$: In the highly doped sample with a higher $T_N$ (~12 K), no superconducting transition was observed up to the maximum pressure of 65.3 GPa.

Key Contributions

• Discovery of SC in Ge$_2$Bi$_2$Te$_5$: This work reports the first observation of pressure-induced superconductivity in the 2AX $\cdot$ B$_2$X$_3$-type material Ge$_2$Bi$_2$Te$_5$, establishing it as a new platform for studying topological superconductivity.
• Demonstration of AFM-SC Competition: The study provides clear experimental evidence that AFM order and SC are competitive in this material system. The introduction of Mn doping, which stabilizes AFM order, significantly suppresses the pressure-induced SC, reducing the maximum $T_c$ from 7.6 K to ~2.3 K and eventually eliminating it entirely at high doping levels.
• Systematic Phase Diagram: The authors constructed an electronic phase diagram mapping the evolution of SC and AFM order as a function of both pressure and Mn concentration, highlighting the trade-off between magnetic interactions and Cooper pair formation.

Significance and Claims
The paper claims that these findings provide a new experimental platform for investigating the interplay among band topology, magnetism, and superconductivity. The results suggest that within the $m$AX $\cdot$ $n$B$_2$X$_3$ family, magnetic interactions appear detrimental to the formation of superconductivity, likely due to a depairing effect. By establishing that pressure can drive a transition from an AFM ground state to a superconducting state in low-doped Ge$_2$Bi$_2$Te$_5$, the work adds a new magnetic member to the category of pressure-induced superconductors in this family. The authors posit that this system offers a valuable material platform for exploring topological superconductivity and correlated topological states, though they do not explicitly claim the realization of topological superconductivity itself in this specific study.